will teach you the steps involved in building a superior system.
It's certainly not a complete list of installation procedures. Rather it's
a list of whatís commonly overlooked and poorly executed by many
what follows will increase your odds of getting superior comfort,
quality and value.
First up is a summarized list
that includes topics in general,
air conditioners, ducts and
indoor air quality. Each
number in the list has a link that will take you to a more detailed
description below. Read the detailed
descriptions! You'll need to know these things to keep
your contractor in check. Your HVAC system is being designed and
installed by a contractor that has virtually no one watching except
you. The city or county building inspector can help a little, but his primary
concern is safety.
Most building inspectors know little about proper HVAC system
performance. A lot of contractors aren't much better.
Not every step applies to every situation. If you're replacing only
a portion of your system there's a guide at
the bottom of the page that will help break it down for your
Additionally, if you scroll down a bit on
you'll find more questions and answers regarding installation procedures.
In General (1, 2 & 3 are
1) Building permits
protect you. Pulling them is the law in virtually all jurisdictions. Yet frequently it's not even
mentioned by the contractor.
Most contractors use rules of thumb to design your system. Manual J & Manual
D calculations are the
gold standard in sizing ducts and equipment.
3) Air balancing is the only way for you to
know that you're getting what you paid for. The fact that a system
blows conditioned air says little about capacity and efficiency.
4) Don't be fooled by those who focus on brand
above all else. The brand you buy is a distant second in importance to the
contractor who's putting it in.
5) A lot of crooks
happen to be licensed and insured. Some of them even have glowing
testimonials. Checking such things is only one part of a good screening
6) The use of an oversized furnace blower
and/or a variable speed blower will cover some, but certainly not
all duct deficiencies.
Too much heating capacity can cook both you and the furnace. Be
weary of having too high a BTU input. A multi-stage furnace
can be a big help in that regard.
8) Fires have been started by flexible gas connectors
passed through the furnace sidewall. Don't let hurried installers do
that to you.
9) The exhaust pipe
on 80% efficient furnaces gets very
hot. Proper clearances must be maintained.
10) The money saved by upgrading to a 90%
or higher efficiency
furnace isn't what some claim, especially in areas with
mild winters. Consider the pros & cons carefully.
11) In a perverse sort of
balancing act, most air conditioners are
oversized but underdeliver. Learn why so you can make sure it
doesn't happen to you.
12) Extraordinarily few systems
deliver the 450 to 500 CFM per ton that is ideal for dry climates. Even systems in humid climates often lack proper flow.
13) TXV's are best for
any type of climate. TXV's can improve capacity, efficiency and
longevity. In dry climates they should be combined with oversized
Air conditioners can lose 5% or more of their capacity to
undersized refrigerant lines. Proper line size is especially
important on marginally sized systems.
15) Acid in an air conditioner?
It happens more often than you think and will slowly kill a
compressor over a period of years. Don't let it happen to you.
16) A huge percentage of air
conditioners get scale (burned metal flakes) in their lines the day they're installed.
Learn why and prevent it.
17) Checking Freon pressure
is not enough. After installation the air conditioner's refrigerant
charge should be fine tuned by the superheat and/or subcooling
18) Don't be fooled by bogus
formulas. The payback on equipment with super high SEER ratings is
realized. Demand a real calculation.
19) Wire flex ducts are infamous for how poorly they're
usually installed. A
visual inspection will reveal a lot if you know what to look for.
20) Return ducts are routinely
undersized. In general they represent the single greatest potential
for improvement of airflow in existing duct systems.
21) Most homes waste energy and
their comfort is uneven because of pressurization issues.
Extra return ducts and/or jumper ducts can be the solution.
22) Severe air leakage in
existing duct systems is
common. Duct sealing (NOT
may represent a big opportunity for improvement in performance.
23) If you
have a two story home then a zone system is almost a must. Both manual
and automated systems can work well. Unfortunately they are rarely
24) If just one contractor mentions
the need to upgrade ducts and the rest don't, that one is probably
not the crook. Consider the use of existing ducts carefully.
25) The cheap diffusers (vents,
grilles, registers) installed by most are pathetic
performers. The difference between high and low quality diffusers
can be enormous.
Indoor Air Quality
26) Most high quality filters
don't perform as they should. Either they catch less than was
advertised, impede system airflow dramatically, or both. Learn why.
27) Duct cleaning is rarely
useful and often fraudulent. It not only has virtually no potential to improve air quality,
but it will likely worsen it.
UV purifiers stand little chance of purifying a duct's air
stream. However, in the right application they can sterilize a
cooling coil. Learn to separate myth from reality.
29) Duct sealing is one of the
few truly effective methods of improving air quality. But it's a messy
business worth keeping your eye on.
whole house performance is a potentially expensive but often
very effective way to improve air quality.
actually improve air
quality you must make major and potentially expensive changes in
your home and in your behavior. You're kidding yourself otherwise.
three steps listed are ironic in that they are the most commonly skipped
yet they are perhaps the most important when installing a complete
system. Each one can sometimes
add hundreds to the cost of a system. Yet each one can save you
many times that amount in the future.
building permit must be pulled with the city or county, depending on
your jurisdiction. As far as I know there aren't any
exceptions to this rule. The permit serves a few useful purposes.
It ensures that your contractorís insurance will remain
in full effect. While the lack of a permit may or may not void
insurance coverage, why take the chance? This is especially
important when it comes to furnaces. An inferior furnace installation can be deadly. A
permit also has the potential to protect you from sloppy work.
If an installer knows his job is going to be inspected then that
knowledge may improve his workmanship. And finally a
permit could protect you from future liability. Should the work
performed not be up to code then it could be you
paying to fix it when you sell your home. Getting a contractor to correct a mistake years after the fact can be very difficult.
Of course laws vary. But locally it is the homeowner's legal
responsibility to pull a permit and have the work inspected once
the job is complete. The fact that the contractor pulled the
permit on the homeowner's behalf changes nothing. It is the
homeowner that bears ultimate responsibility for the permit.
foundation of your new system should be that of
Manual J and
Manual D calculations.
Manual J is the gold standard in determining what your heating
and cooling loads are. It can be applied as a whole house
calculation or it can be broken down to a room by room
calculation. Virtually all contractors that perform real heat load
software based on Manual J. Donít be fooled by
short forms or any other
goofball method a contractor comes up with. A true room by
room Manual J should take around an hour in the
best of circumstances. In a large house with a complex floor
plan, it can take many hours. Donít let your contractor add in
fudge factors. Manual J is
documented to oversize equipment by
some amount already.
is used to determine what size your ducts should be to deliver
the hot and cold air each room needs. Have you ever lived in a
house that didnít get enough air to the farthest bedroom? If a
true Manual D calculation had been performed and if the ducts
were installed and balanced accordingly then that would not have been the
If the new
system includes ducts then it should be air balanced.
Manuals J & D give us our specifications. But that doesn't
mean those specifications have been met. Manual D is a very good educated guess
about how a duct will perform if installed a certain way. But how a duct is installed is often
different than originally planned. Added turns and bends can
result in less airflow than expected. An
or some other airflow measuring device should be used to measure
the airflow to each room. Those measurements are then used in
conjunction with dampers to balance the system to within 10% of
Manual D and verify that output is reasonably close to Manual J.
Even if new ducts are not installed, measuring output can be
useful in determining if your new equipment is delivering what
it's supposed to.
That a new system's output is rarely
measured is astonishing. It would be like buying a car
because of its advertised MPG and then not ever verifying the actual mileage. The equipment
is rated for a certain capacity. Itís absurd that few systems are measured to see if theyíre
actually delivering that capacity.
This step alone can tell you more about your system than any
of the keys to High Performance Heating & Air is not the
brand you choose. Any
brand can deliver high performance if itís designed for your house
and installed correctly.
explains the concept in greater detail. Buying a furnace or air
conditioner based on brand would be like obsessing over the
brand of paint to use on your house and ignoring who the painter
is. The painter is many times more important than the paint. The
same goes for HVAC. Heating and air conditioning manufacturers source
compressors, fan motors and many other parts from the same part
suppliers. Often times the greatest difference between brands of
air conditioner is the shape and color of the cabinet. You should be worried most about which
contractor you choose, not which brand. An HVAC contractor
specifies and assembles your HVAC system. He is literally the
last man on your system's assembly line.
can obtain and install most any brand, including myself. In most
cases I'm utterly ambivalent about which brand my customers
choose. The only time I push one brand strongly over another is
when a customer wants a particular feature not found in other
brand. A feature has nothing to do with quality. The
fact that Carrier has their whiz bang Infinity controller with
all its readouts says nothing as to how long it will last. The
fact that Rheem has an advanced modulating furnace does not mean
it will last or that it won't.
California requires an HVAC
contractor to be licensed through the
CSLB. Other states and localities have similar requirements. A lot of
homeowners have been conditioned to believe that checking a
contractor's license and insurance along with meeting them
during a sales presentation is enough. It is not! The
Contractors State Licensing Board has precious few resources to
keep tabs on contractors. It's the same in most
parts of the country. Even when a license is revoked a lot
of hack contractors just open up shop
under a different company name with someone else's license.
Since California law doesn't require a contractor to be insured, verifying
insurance is a good measure. But of course just about anyone can
get insured if they can afford the premiums. So the fact that
they're insured is no guarantee of professionalism.
Testimonials from anonymous homeowners are obviously useless.
Every miracle diet pill sold had an anorexic blonde doing
commercials for them. The same principle applies to HVAC.
There's not a crook out there that can't fabricate testimonials
if he wants to. Even if the testimonials are legitimate, you have no way of knowing if they
represent a typical customer experience or an exception.
The answer is to be more comprehensive in your
investigation. Pick a few topics from this page that strike you as
important. Ask your prospective contractor about those things on
the phone and gauge his response. If the response is good then
ask five or ten more things when you see him in person. His
reactions will be telling. Such questions may not protect
you from the really slick hustlers. They'll tell you
anything you want to hear. But such questions will focus the
hustler on areas that he's not as comfortable with. Distract him
from his memorized lines long enough and he'll probably reveal
important role a furnace plays is that of blower.
The furnace fan blows the air through the ducts for both the
heating and air conditioning modes. The air conditioner is far
more demanding than the heater. In a dry climate itís
preferable to blow 450 to 500 cubic feet of air per minute for every
ton (12,000 BTUs) of air conditioning capacity. Most systems deliver nothing close
to that. In humid climates something in the ballpark of 350
CFM per ton is more appropriate. But even that goal is
often not reached.
This rampant airflow problem can be blamed in large part on improper duct design
and installation. It can also be blamed on restrictive high
efficiency furnace filters. And yet another factor is that
manufacturers design many of their furnace blowers based on
unrealistic assumptions. A furnace supposedly rated for 4 tons
of airflow (1600 CFM) rarely delivers that much through real
world ducts. As such most furnaces should have blowers rated
one-half to one full ton higher than their air conditioners. For
example, if forced to use existing ducts I usually match a 3 ton air
conditioner to a furnace with a 4 ton blower. The use of
blower instead of a standard blower usually has a similar
effect. Variable speed blowers are smart fans that calculate
actual airflow. They'll speed up to compensate for restrictive
ducts up to a point.
Be very careful. Oversized and/or VS blowers can't always cure
airflow problems. Even when they can help to increase overall
airflow it's not the preferred solution. Properly sized ducts
are always your best bet. Plus, in
most cases extra airflow will do nothing to fix uneven comfort. If
that far bedroom doesn't get enough air but the rest of the
house does then you need a properly
sized duct to that bedroom, not a more powerful blower.
most important role of a furnace is the obvious one of
One key here is to not oversize. Oversized furnaces will deliver
too hot an air and too much of it. As a result your furnace may short
cycle and the hot air will tend to hug the ceiling
(stratify). An oversized furnace tends
to wear out more quickly as well.
The west coast is
sometimes held hostage to an east coast engineering mentality. Cold
east coast and Midwest climates need big furnaces to handle big heating loads.
On the other hand, the southwest has fairly hot and dry summers
relatively mild winters. As such they need furnaces that can blow a lot of air in the
summer while not blasting too much heat in the winter.
Yet (for example) you simply canít buy certain 5 ton
furnaces with a BTU rating of less than 100,000. There have
been occasions where I could have used a 5 ton blower on a
furnace with a 60,000 BTU input.
But at the time no such furnace existed.
The answer to the
southwest's high airflow / low heat predicament can be the
multi-stage furnace. The highest stage of a multi-stage furnace
is ďfull blastĒ just like a regular furnace. But the first stage(s)
can start as low as 40% of the furnace's rated input. The lower
output can result in more even heating and better comfort. Itís quieter as well.
Just donít be misled. In terms of energy efficiency a multi-stage furnace
has no advantage over a single stage
If you happen to live in an area with cold winters then
furnace oversizing may not be as big an issue. But don't let your guard
down too much. Some hack installers have dazzled with their
ability to screw things up. Oversized
furnaces are installed in all climates.
that the furnaceís flexible gas connector does not penetrate the
furnaceís sidewall. The
flexible connector should sit outside
the furnace and hard pipe should penetrate the sidewall. In some jurisdictions theyíre beginning to
require excess gas shutoff valves right at the appliance as well.
It may seem like a small issue compared to the larger concepts
explored on this page. But locally at least this problem is
rampant. Since on rare occasion it can lead to death and
destruction, it's worth a mention.
sure that your furnace vent pipe (exhaust pipe) has proper clearance. A
single wall vent should have six inches clearance to any
combustibles. A double wall vent should have one inch clearance
to any combustibles. Your contractor can point out what single
wall and double wall pipes are. Just like the flex connector
issue, it's a relatively minor thing to deal with that has
potentially dangerous results if it is not.
The decision to
buy an 80% efficient furnace versus buying a 90%
efficient furnace should be considered carefully. 90+ furnaces are more complex, are
more expensive to work on and are more likely to fail than their 80+
counterparts. The ten to fifteen percent savings on your gas bill may never
actually pay you back the added capital, maintenance and repair
costs. If your salesman is pitching energy savings then take a
look at this related note
Of course there are reasons to buy a 90+ beyond that of energy
savings. 90+ furnaces tend to be safer, have better warranties and run quieter than
their 80+ counterparts. California Title 24
some Californians added incentive to purchase 90+ furnaces in certain climate zones.
In a perverse
sort of balancing act a huge percentage of air
conditioners are oversized but underdeliver. A
study done by
Proctor Engineering Group found that an air conditioner sized by
Manual J is on average 24% too big. Add in the fudge factors that
fearful contractors are wont to do and you could end up with
an air conditioner that's 50% too big. The study refers to
the air conditioner's rated capacity. Many air conditioners donít deliver anything near what
theyíre rated for.
reasons for this. Some of them are covered
below. But one very important one is
customer perception. Itís sometimes difficult to convince a customer
that bigger is not better. While an air conditioner that can turn a
house into a meat locker inside of ten minutes may be pleasing on a
hot day, itís a bad thing in many respects. In humid climates such
rapid cooling will leave you feeling cold and clammy and
possibly lead to mold growth. An oversized air conditioner costs
to run and may have a shorter service life. A properly sized air
conditioner wonít take the house from 90 to 70 in ten minutes.
But when used properly it will keep you comfortable, be more
energy efficient and may last longer.
step to high performance air conditioning is proper
airflow. I've mentioned it above already.
another mention. In a dry climate you should get that airflow up
to 400 CFM per ton at an absolute minimum. 450 to 500 CFM per
ideal. Even in humid climates where an airflow of 350 CFM can be
appropriate, many systems fail to deliver that. I once measured a system that delivered only 200
CFM per ton! That system would do poorly in any climate.
Such amazingly low airflow is more common than you might think.
Maintaining proper airflow will
improve performance for reasons that you probably arenít aware.
An air conditioner has two functions. First it lowers the
temperature you read on your thermometer. We call that sensible
capacity because both you and the thermometer sense it. Roughly
70% of a typical air conditionerís capacity will be spent on
sensible heat removal (temperature reduction), though that
number varies considerably. Second it removes
moisture from the air. We call that latent capacity. Roughly 30%
of your air conditioner's capacity is spent on latent heat
removal (moisture removal).
In a dry climate latent heat removal is a waste of energy.
Unfortunately that waste is often compounded by low airflow. Low
airflow can cause that 70/30 ratio to drop down to 60/40. But
with high enough airflow and a big enough coil you can get that number up to 85/15. That 25% difference translates into 25% more
capacity from the same exact air conditioner for nearly the same
cost in electricity. For humid climates latent heat
removal is desperately needed. But reduce airflow too much (like
in the 200 CFM example above) and the reduced airflow is no
longer enhancing moisture removal. It's just wasting energy and
step to high performance air conditioning is the use of a
(thermostatic expansion valves) as the metering device. An air
conditioning system's metering device controls the flow of
refrigerant to the cooling coil. Many cheap
cooling coils have fixed metering devices that feed refrigerant
to the coil at a fairly constant rate. In
contrast the TXV varies the flow of refrigerant depending on
heat load. The result is better energy efficiency, a potential
increase in cooling capacity and potentially longer compressor
life. A TXV can be added to most coils. You can see a TXV
next to the alternative
High efficiency coils typically come with a TXV already
installed. High efficiency coils are usually bigger than
their cheaper counterparts. Their large size allows for better
airflow. And because of their size the refrigerant is spread
out over a larger area allowing for more effective cooling.
In dry climates we can amplify the effect by oversizing the high efficiency coil.
That combined with the blower
recommendations above means (for example) that a two ton air conditioner could be
matched to a three ton high efficiency coil and a three ton furnace. Such a
combination is not only approved by the manufacturer, itís the
best way to design a system for a dry climate.
and oversized coils cost more their cheaper
counterparts. But they're well worth it IF they're right for
your application. In some instances the
results can be unbelievable. I increased one customer's sensible heat
removing capacity by 34% by simply replacing their compact cheap
coil with an oversized high efficiency coil.
See the pictures here and
If you're in a very humid climate then oversized and / or
high efficiency coils can be exactly the wrong thing to do. The
bigger the coil the more it will favor sensible heat removal over
humidity removal. High SEER air conditioners come with high
efficiency coils by default. In other words, a super high
efficiency air conditioner is more likely to have trouble
keeping your house dry if you live in a humid area. If you live
in a humid area then you must be diligent in picking a
contractor that understands how to balance energy efficiency
with moisture removal.
step is to properly size the refrigerant lines.
Many are undersized. While the effect isnít nearly
as dramatic as that of undersized ducts, it can become critical
when thereís a big distance between the furnace and air
conditioner and/or when your system is barely big enough.
example, the recommended suction line (the Freonís return line
back to the compressor) size for most 5 ton R-22 air conditioners
is 1 1/8 inches (outside diameter). Yet most five ton
systems have 7/8 inch suction lines. The larger suction
line is more expensive and much more difficult to work with. On
a twenty-five foot run the 7/8 inch line results in a loss of
only 1.4% of cooling capacity. But on a hundred foot
run the 7/8 inch line results in a loss of 5.5% of cooling
capacity. (Those numbers are taken from one famous OEM's
refrigerant piping guide.)
It may seem unlikely that your furnace and air conditioner are 100
feet apart. But weíre actually talking
about total equivalent length or TEL. For example, every short radius 90
degree copper fitting used in your refrigerant line has a TEL of about 8 feet.
A 60 foot refrigerant line with 5 of those 90 degree fittings
would have the same resistance to refrigerant flow as a
perfectly straight 100 foot line. Given the turns and bends that
most refrigerant lines take, a 100 foot
(equivalent) line is not as uncommon as you might think.
recommends the following suction line sizes for R-410A and
R-22 systems. The first number is R-410A, the second R-22 in
-For a 1 ton system it's 1/2 and 1/2.
-For 1.5 ton systems it's 1/2 and 5/8.
-For 2 and 2.5 ton systems it's 5/8 and 3/4.
-For 3 and 3.5
ton units itís 3/4 and 7/8.
-For 4 and 5 ton units itís 7/8 and 1 1/8
Smaller sizes are approved, though as outlined
above it results in reduced capacity. Larger sizes are recommended on especially long runs. Since R-410A is more dense the
recommended suction line sizes are smaller. Other manufacturers
have similar recommendations.
While we're on the topic, make sure they insulate the ends of
the refrigerant lines. Too many hurried installers leave them
exposed. In some cases the condensation that forms on the
exposed line will drip down onto the furnace and damage
gauges are important but seldom used. Trade schools teach
the use of micron gauges as a matter of course. Doing so can prevent the formation of acids. Yet most
installers donít even own one. You can get a detailed explanation and pictures
refrigerant lines are connected is another area that often gets
botched. There are two methods.
The copper lines can be brazed
together. Brazing means heating the copper lines to around 1200
degrees and then applying a filler material to the joint. At
such high temperatures the copper oxidizes. Scale (burned metal
form on and in your copper lines. The scale will float around the
refrigerant circuit and contaminate the compressorís oil sump.
It could even clog the metering device as seen on point 16 of
this page. To prevent the scale from forming your installer
is supposed to purge the lines with nitrogen. Nitrogen is inert
and purging with it will keep the inside of the copper lines
clean. But quite frequently that step is skipped.
An alternative to brazing is
a silver solder like Stay-Brite 8. Stay-Brite 8 is one of the
few solders available thatís
approved for air conditioning. Its advantage is that
the copper pipe has to be heated to
only around 400 degrees. At that temperature virtually no scale
is formed. Stay-Brite 8 is considerably more expensive than brazing alloy.
In the right application itís well worth it.
that the refrigerant charge in your new air conditioner is
fine tuned. Too many installers skip
this step. The factory puts a base charge of refrigerant in
their new air conditioners. On split systems the factory has no way of
knowing how long the refrigerant lines will be or what type of
evaporator coil will be installed. As such the refrigerant
charge should be fine tuned. Checking pressures is not enough.
Refrigerant must be adjusted by either the subcooling, the superheat
or the manufacturer's otherwise specified method.
The exception to this rule might be package units. If your
heater and air conditioner are combined into a single machine
then adjusting the refrigerant charge may not be needed.
However, on rare occasions a new package unit will be low on
refrigerant due to a refrigerant leak. So even a package unit
might need to be checked.
Energy efficiency is a hot topic
that's steeped heavily in propaganda. Since 1992 the minimum
efficiency for newly manufactured air conditioners was 10
SEER (seasonal energy efficiency ratio). The minimum efficiency
as of 1/23/06 is
13 SEER. For mild climates like ours in the SF Bay area it
doesn't often make sense to upgrade to anything higher than a 13 SEER model. Even
in more extreme climates it's not a given that higher SEER
equipment is worth the extra cost.
Some salesmen use formulas based on
heating degree days and/or cooling hours to determine your energy
savings and convince you that a high SEER or high AFUE model makes
sense. Those numbers are meteorological in nature. They are based on statistical averages that may bear
no resemblance to how you use your system. The real way to determine how much
you will save is to look
at your past utility bills, determine baseline and peak usage
and calculate from there. Itís not hard to do. If your salesman
is pitching energy savings then demand that he prove it by
working with your past utility bills and not some trumped up
That does not mean there arenít good reasons to buy high SEER
and high AFUE
equipment. High SEER air
conditioners are often quieter, sometimes have better warranties and
may even look nicer. High AFUE furnaces are often quieter and
safer. And of course there are some
circumstances where a customer will indeed save big money by
buying high SEER and high AFUE equipment. Just don't assume
so. Crunch real numbers to prove it.
deficits found in most systems are not surprisingly found in the
part unseen by the customer. Few customers inspect crawl spaces to
look at their ducts. And even if they did few would know what to
look for. Read on and you'll have an idea of what to look for.
The first key
to a high performance duct system has already been mentioned. Make
sure that a Manual D calculation is performed and that the ducts are
Next make sure that the ducts
are installed correctly. Most commonly
in our area
wire flex duct is installed.
Imagine a giant
slinky with plastic bonded to it. Wrap that with a layer of
insulation and wrap it again with an outer plastic shell and you
have wire flex duct. The most common problems with wire flex
ducts are in how they're attached and how they're hung.
If the ducts are hung they should be supported every four feet
with two inch strap. There should be little up and down
sagging. Excessively sharp turns and kinks will reduce
airflow. Unnecessarily long runs will also reduce airflow. Some
installers will loop a long duct around rather than be bothered
with cutting it short. If in an attic, laying them on top of the
ceiling is perfectly acceptable for most local jurisdictions. In
fact, per California Title 24 burying them with blown insulation
after they're installed is allowed as a way to make them more energy efficient. If under
the house they should not lie on or even touch the ground. That
can be quite the challenge in crawl spaces with minimal space. Metal
ducts are required to have four inches clearance to the ground.
The ends of each section of wire flex duct will be attached to
the metal collar of a boot, wye, plenum, etc. Each end should be
secured with a mechanical connector like a Panduit strap and
also sealed with tape or mastic. A
Panduit strap looks like a giant wire tie that you secure with
a tensioning tool. The mechanical connector is what keeps the
duct attached. Tape doesn't work well for this
purpose. Many tapes will dry out over time and loosen. Tape is
only to be used for making certain connections airtight.
Old fashioned duct tape is no longer considered proper for air sealing. There is a new
type of tape that looks like packing tape that is specifically
designed for ducts. And even this new tape is only useful for
certain connections. Little gaps and other types of air leaks should
be sealed with a duct mastic. Big gaps should
first be covered with secured sheet metal before being sealed.
exposed metal of the duct system that resides in
unconditioned space must be insulated. In some cases the return
duct is exempted from this requirement.
Return ducts are very often
undersized. If you are planning on replacing your
ducts then size the return duct as big as space allows, within
reason of course. You
literally can't oversize a return. If you can fit a 20" return
then it may be a good idea to put a 20 inch return in
even if conventional wisdom says you shouldn't. If your air conditioner is rated for 5
tons then even a 20 inch return may not be enough. You may need to
add a second return. An oversized return will compensate to some
for undersized supply ducts.
One thing you should not worry too much about when it
comes to your return duct is placement or how many you have.
Some contractors erroneously think that large homes must
have more than one return. That is not the case. Returns can
help to alleviate stagnation in areas that lack supply vents.
But the primary means by which comfort is achieved is through
proper air mixing provided by the strategic selection and
placement of supply diffusers. Generally speaking returns act as little more
than drains - an important function, but not one that enhances
Pressurization issues (point 21) can be addressed with
added returns. However, addressing pressurization does not
necessarily require adding returns.
- Point 20 assumes that you have a
typical home with just one or perhaps two return ducts. If you
can afford it then put a return or jumper duct in every room
that has a door which is frequently closed. That usually means
every bedroom and possibly other rooms.
Homes are made terribly
inefficient and air quality is negatively affected by
pressurization issues. If a duct is trying to dump 100 cubic
feet of air per minute into a bedroom through a duct that has an
area of 50 square inches, what chance does that air have of
escaping back to the return duct in the main part of the house through a crack under the door
of less than a tenth that size? It has no chance whatsoever.
Your bedroom may look fairly airtight. But on older homes
especially there are
numerous cracks through which air can escape to the crawl spaces
and outdoors. A
typical HVAC system running with the bedroom doors closed will put
those rooms under a positive pressure and the central portion of
the house under a negative pressure. The bedrooms will leak the
air you paid to condition to the outdoors. That will cause the center of the house
to be negatively pressurized. It will
draw in air from outdoors to make up the difference. Some of
that unplanned makeup air will come from crawlspaces that may
not have very good air quality.
Leaving most of the
doors open most of the time will solve the
problem. Or a ducted return can be installed in each room that has an
oft-closed door. Or jumper ducts can be installed. Jumper ducts
don't connect directly to the HVAC system. They're very short
ducts that simply jump the wall separating a closed room from
the central portion of the house.
Leaky ducts have a similar effect on
house pressurization as described in point 21. Most systems leak more air on their supply ducts
than they gain through leaks on the return duct. As a result
they'll negatively pressurize the house and force outside air in
through whatever cracks the air can find. Some of the cracks may
lead to a damp crawl space under the house or to an attic full
of insulation and rat droppings.
California Title 24 rules require (under certain
circumstances) new duct systems to leak less than 6% and old
duct systems to leak less than 15%. That may sound like a lot of
leakage. But many existing duct systems leak that much and more. Even
if your home isn't affected by those rules the maximum leakage
that they call for is reasonable for any home.
If you own a typical two story home then I don't need to
tell you about uneven comfort. You've lived it. Two story homes need most of the
cold air delivered
upstairs in the summer and most of the warm air downstairs in the winter.
But it doesn't happen because there's
simply no way for a fixed duct system to adjust itself for the
different seasons. Even a large single story home can have
There are many potential solutions to uneven comfort. Improved
insulation, upgraded windows and many other factors all play
their part. But as it relates to heating and air conditioning a zone system can be the
answer. For a two story home a zone system is often the only
answer. The simplest zone system might be comprised of manually
controlled dampers that the homeowner adjusts twice per
year. The ultimate zone system would be completely separate
HVAC systems serving different parts of the home. Such a setup is easily the most comfortable, the most
energy efficient and also the most expensive to install. The next best solution is to split a single HVAC
system into multiple automated zones. A single system with
automated zones will be the focus of
the rest of this section.
The Problem With Zone Systems
Unfortunately zone systems are notoriously problematic
because they are so rarely designed and installed correctly. Before
we get into
the nuts and bolts of the matter, there are a few
fundamentals you might want to know so you'll get the big
picture. Alternatively you may wish to read the rest
of this section on zone systems and then refer to the links if
you want more information.
An HVAC System's Cooling Capacity
Airflow's Effect On Compressor Longevity
While they touch on other
issues, all three links above describe why maintaining proper
airflow is so incredibly important. Low airflow causes a
loss of cooling capacity, increased energy usage, decreased
compressor and / or heat exchanger life, short cycling of the equipment, etc. And low
airflow is precisely what you get on most zone systems. It is the
fundamental problem with zone systems: When only one zone comes
on your home needs only a fraction of the system's cooling or
heating capacity. And with only a portion of the ducts open they
can only accommodate a fraction of the airflow. Yet most
furnaces and air conditioners attempt to run at 100% capacity
any time they're on even when just one zone calls. Most furnaces
and air conditioners are single stage units, meaning that they
attempt to deliver a constant output of 100%. They need 100% airflow or at least
something close to it at all times.
The hypothetical I routinely tell customers is this:
Imagine that you have ten ducts
supplying air to the house. And imagine they all have the exact
same airflow. If you were to take duct tape and seal off five of
those ducts what you'd find is that the other five would not
have double the airflow. They'd have considerably more to be
sure. But the increased static pressure caused by the choked off
ducts robs disproportionately from velocity pressure. So the total airflow running
through the equipment drops.
The airflow problem with
zone systems is enhanced by the fact that most ducts are
undersized. Yet a zone system's ducts should be oversized by 25%
The final nail in the coffin is the fact that
even on a two zone system the split is usually not 50/50. Quite
often the larger floor (usually downstairs) accounts for perhaps
sixty to seventy percent of the ducts. When a two zone system is
trying to cool only the smaller floor you often end up asking the
blower to push 100% of the air through 35% of the ducts. It
isn't going to happen. You'll get drastically reduced airflow
and the problems that are described in those three links above
will manifest. Three and four zone systems are even worse.
Trying to maintain anything remotely close to 100% airflow
through one zone of a four zone system is a near impossibility.
Obviously the HVAC industry has done its best to come up
with solutions to this airflow problem. Some of them work
magnificently and some very poorly.
The most commonly employed solution is not much of a solution at all. Most contractors
will install a bypass damper. The bypass is a short duct that
connects the output (supply air) of the equipment to the input
(return air). When only one zone of a multi-zone
system is operational the static pressure in the ducts will
increase. A bypass damper will sense the increase in static
pressure and open up. This has the marginally beneficial
effect of restoring some of the lost airflow through the
equipment. But it also causes return air
temperatures to climb radically in winter and drop fast in
summer. Normally the air entering the equipment is house temperature. But
during the winter a bypass will circulate
very hot air directly from the furnace's output right back into the furnace. As a result the furnace may quickly overheat and
shutdown. During summer you'll circulate ice cold air directly
into the cooling coil. The cooling coil will get much too cold
and may ice up and/or shut down on its safety. Even when a bypass
damper is installed according to instructions the problems just
described often exist.
Another solution is to implement a dump zone. A dump zone might
be an area of the house that is generally unoccupied. When the
zone system senses the need to increase airflow it will open the
damper(s) to the dump zone.
This sort of solution would most logically be applied to systems
with three zones or more. If applied to a two zone system your
dump zone would end up being the zone that's not calling for
conditioned air. With the dump zone open your two zone system
would be operating as though it didn't have zoning at all.
Yet another solution is to employ controlled pressure relief
dampers. That's a fancy way of saying that it turns the entire
house into a dump zone. But it does so in a more controlled
fashion. For example: If you had a three zone system where only
zone A was calling, the controller
would inevitably sense the need to increase airflow. Instead of
opening up zone B fully and dumping all the excess there, a
controlled pressure relief damper system would open both zones B
and C a small amount. Studies have shown that most people
don't notice temperature swings of a degree or two. By dumping
just a small amount to all the other zones the theory is that we'll
get the extra airflow we want without anyone noticing. How it
actually works out in practice is hard to predict.
The absolute best solution is to limit yourself to two fairly
even zones and
to install multi-stage equipment. Multi-stage furnaces are rather common place these days. The price
premium is not huge. On the other hand multi-stage air conditioners are prohibitively expensive for
a lot of people. But if you can afford to install both a multi-stage
furnace and air conditioner with
properly sized ducts then it becomes a very elegant solution. Airflow
problems become much easier to deal with because now your
furnace and air conditioner don't require 100% airflow in first
(low) stage. Such a setup can rival
the efficiency and longevity of having two entirely separate
The solutions above represent a mere fraction of what you'll
find out there. There are many hybrids of these solutions as
well as completely unique ideas. For example: The advent of
variable speed motors presents us with yet another possible
solution. A variable speed blower will speed up and slow down in
response to changes in static pressure. But its ability to do so
is limited. Slapping in a zone system with a variable speed blower
without upgrading the ducts is usually a bad idea. A variable
speed blower will most likely try and fail to maintain 100%
airflow when only one zone calls. But if the ducts are made big
enough then 100% airflow could be maintained at all times
without the need for multi-staged equipment, dump zones or
bypass dampers. The first challenge in implementing this
solution is being able to install big enough ducts. In some
cases they'd have to be so big that they wouldn't fit. The
second challenge is picking the right diffusers (vents, grilles,
registers). If the diffusers are sized to maintain proper throw
at all times then they may be very noisy some of the time.
When The Solutions Fail
With the most commonly utilized method of dealing with poor
airflow in zone systems also being the worst method, you may
wonder how it is that many customers are happy with their
zone systems. The first answer is the fact that the equipment is fairly
tough and takes the abuse for quite some time. The second answer is simply that
customers don't know any better.
The lost energy
efficiency isn't usually noticed because customers have no way
of knowing what their bill would have been otherwise. The lost
equipment longevity isn't noticed because often the failures
don't manifest themselves until after the original purchaser has
moved out. Even if a failure does occur most homeowners blame
the equipment maker before they blame the contractor. The thought that the
design of the zone system is causing
the problem doesn't occur to them. They assume it must be poor
quality on the part of the manufacturere. In addition, problems with overheated
heat exchangers and iced cooling coils can go unnoticed by the customer for a very long time.
In the case of a furnace the safeties will kick in repeatedly to
save the day, at least until the safeties breaks. And in some climates the
summers are mild enough and/or short enough that any frost or
ice formation on the cooling coil quickly melts away between
cycles. In addition the zone system itself often has
safeties built in that will short cycle the equipment and mask
the airflow problem.
Does all of this mean that you must replace
your entire heating and air conditioning system in order to have zone system? Of course
not. But what it does mean is that we're often left with
choosing between very expensive upgrades or unpredictable results. Warranties as well as personal
assurances that a system will run well assume proper airflow is
maintained. If it isn't then how can any tradesman predict the
results? There is no manual that tells us how much we can hack a
project and still have happy customers. It's a reality that most homeowners don't want to
hear. And it puts the ethical tradesman at a disadvantage. There are plenty of
contractors that are willing to hack a zone system onto existing
ducts while giving you any assurance you ask for. They can get
away with it for the reasons I gave in the paragraph above. Make
yourself well informed so that they don't.
What to do?
Get measured results. If you have a zone system installed then insist that at least
275 CFM per ton of airflow be maintained even when just the
zone is calling. That number is based on what a couple of
OEM's have been known to allow on their zone systems. Even
that is pushing the limits. If the contractor balks then ask him for
documentation showing that anything less than that is allowed.
When all zones are calling then at least 350 CFM per ton must be
maintained. If you live in a dry area it should be more than
that (as outlined above). The only way you'll know that they've achieved that
result is if
they pull an airflow hood out and actually measure the airflow.
Of course, that happens so rarely that making such a request may
leave you hard pressed to find a contractor capable of
- The sale of
new equipment tends to be where a contractor makes his
biggest profits. The profit margin on new
equipment combined with the relatively low commitment of man
hours to put it in makes for a nice P&L statement. The
installation of new duct systems on the other hand is quite the
opposite. New duct systems require comparatively inexpensive
materials and a lot of man hours. When business is booming and
staffing is short many contractors have little interest in
looking at your ducts. Ironically it's extraordinarily common
for an existing HVAC system's ducts to be its single greatest
weakness. So if you get several bids and only one suggests that
your ducts need improvement, don't automatically assume he's
just trying to pad the bill. It may be that he's the only one
trying to do the job right.
most homes have are the HVAC equivalent of Yugos. They're made
with one goal in mind: cheapness. So few contractors ask for
high quality diffusers that my local HVAC wholesalers don't even stock
them. The cheap diffuser that
Ace Hardware sells is the same thing most
local HVAC wholesalers stock.
High quality diffusers will improve airflow, air mixing and do
so more quietly. For example, a cheap 4x14 inch floor diffuser (350
series - page 16) can handle up to 84 cubic feet of air per minute
10). But the high quality equivalent (375
series - page 16) can handle up to 128 cubic feet of air per minute silently
11). That's 52% more air at the same silent noise level.
Furthermore, the cheap diffuser has to drop the air velocity down to
400 feet per minute to remain silent. The quality diffuser can deliver its
higher volume silently at 500 feet per minute, which is the
minimum velocity we want to ensure proper air mixing.
example above was based on silent air delivery. But our quality
4x14 inch diffuser can deliver up to 230 cubic feet of air per
minute at acceptable noise levels. Force 230 CFM of air through
the cheap diffuser and the resulting noise would be more suited
to a warehouse, not a home. At 230 CFM the air velocity from the
high quality diffuser is 900 feet per minute. Such high velocity
serves a very useful purpose. Proper air mixing is critical to
even comfort. While it's true that hot air rises, if you deliver
that air at a high enough velocity it will tend to wrap around
the room and create circular currents. Those circular currents
will help to push hot air down and scoop cold air off the floor.
document and you'll see pictures of how this phenomenon
Indoor Air Quality
Indoor Air Quality or IAQ has become
the darling of HVAC business gurus. Their ship has come in, so to
speak. As a result there is an abundance of both excellent products
and expensive snake oil vying for your hard earned dollar. Read
carefully and make an informed decision.
- The most common
failure of air filtration design has to do with the
size of the filters.
HVAC system designers tend to specify filters that are much too
small. Trying to jam too much air through a filter causes it to
become an air restriction which results in less airflow. We've
covered the importance of maintaining good airflow above. All filter manufacturers have maximum allowed velocities beyond
which their filters and/or the HVAC system won't work correctly. In the case of
this standard one inch filter you can see a few paragraphs
down that its rated ability to catch dirt is based on a
relatively slow 300 feet per minute. Bump that velocity up to a
more common 500 FPM and they don't even publish the results.
Most electronic air cleaners and similar whiz bang filters
have an added challenge. Not only do some of them become a restriction
when too much air is jammed through them, but they also catch
less dirt. Most electronic air cleaners will advertise that
they catch ninety-something percent of the dirt that passes
through them. Installed on a 2.5 ton air conditioner
this electronic air cleaner (page 3) will do exactly that.
But put that same air cleaner on a 5 ton air
conditioner and 95% arrestance drops down to less than 70%. And
that's only when the air cleaner has just been cleaned.
As the plates collect dirt the arrestance drops further.
bottom line is that for truly effective filtration and
proper system performance the air
velocity through most filters should be around 300 feet per
minute or less. To achieve that might mean installing more than
one filter. And to maintain that you must replace or clean the
cleaners routinely imply or outright state that they can
improve air quality and enhance an HVAC system's performance. The cold
hard truth is that they routinely do the exact opposite of both of those things. I spell that out in great detail
- Ultraviolet purifiers are all the rage these days.
Some say that by
putting high intensity ultraviolet light in a duct the air
passing through it will be purified. Others focus on its ability
to sterilize a cooling coil.
UV light will destroy an airborne bug if exposed long enough. But simple math suggests
that that will never happen in a duct system. Let's
be real generous and assume that the lamp of a UV purifier has a two foot effective radius. Air
velocity through a duct commonly reaches 600 feet per minute.
That's 10 feet per second. That means that the airborne bug will
be dosed for one-fifth of a second. Even under the unrealistic
conditions suggested by one manufacturer, the
airborne bug will still only get a half second of exposure.
That is simply not enough time. The idea that a
UV lamp can purify the air stream is totally unproven and defies
What a UV light can do is sterilize
stationary surfaces near it. Since UV
purifiers can be installed right above or below the cooling
coil they have the potential of killing anything that's
growing there. The number of residential systems that can
benefit from that in very dry climates are an extreme minority
in my opinion. Things that grow need a consistent source of
moisture. If you live in a humid climate where the cooling coil
stays wet for weeks at a time then it may be beneficial. Think
of it as a potential solution to a known problem, if
there is a known problem such as dirty sock syndrome. Install a
UV purifier for no particular reason other than it seems like a
good idea and you may well regret it for the reasons below.
Strong ultraviolet light will breakdown oil
based products. Wire flex ducts, air filters, blower motors and
cooling coil drain pans all have materials that are negatively affected by UV light.
If those components are not meticulously protected then you
stand a very good chance of damaging them in the long term.
Plus, many UV lamps create ozone. Some have promoted ozone as
beneficial, but it is in fact pollution. In addition, few UV
lamps last more than a couple years. Most OEMs recommend
replacing them yearly.
If there is a known and specific need that UV can actually
help with; and if you protect your equipment properly; and if
you replace the bulbs at the recommended interval; and if you
can tolerate the ozone, then it is possible to benefit
from a UV purifier.
sealing is uniquely effective in improving air quality.
Ducts are typically located above or below the house in a crawl
space. Considerable air leakage is not uncommon. If a furnace is
pushing 1,600 cubic feet of air through the ducts every minute;
and if the ducts leak at a rather pedestrian rate of 15%; then a
house can lose 240 cubic feet of its air every minute to the
crawlspaces. That's the
entire air volume of a 2,000 square foot house being flushed out
every 67 minutes, which is three times the normal infiltration
rate of an average home. That lost air has to be made up
by entering the house through cracks in the walls, floor and
ceiling. An awful lot of those cracks lead to areas of the house
where you'd never dream of breathing deeply. Yet that's where
much of a house's air is coming from when the HVAC system runs.
If you elect to have duct sealing performed then choose your
contractor carefully. Duct sealing can be labor intensive and often
difficult. The duct's insulation and vapor barrier must be
partially or completely removed to apply duct mastic. It's not uncommon for
the mastic to splatter all over the place. And unfortunately
it's also not uncommon for the insulation to
be reinstalled in shoddy fashion. A simple visual inspection of
the crawl space can verify your duct sealer's professionalism.
Taking before and after pictures is not a bad idea either. And the use of a Duct Blaster can measure how much your ducts
are leaking after the sealing is performed.
- A growing industry is that of
home performance. Home
performance specialists address the house as a system rather
than just focusing on a specific trade. Whole house remediation
might involve insulation upgrades. Upgrading attic insulation is
often a slam dunk in terms of payback. Upgrading wall insulation
may or may not be. And surprisingly upgrading the windows
makes little real financial sense despite what the propagandists
have been advertising. Obviously insulation and windows don't
have huge impacts on air quality. But an area that does is moisture intrusion
and air infiltration. Even a seemingly dry
crawlspace can add gallons of water to a home's air every day
and degrade air quality. That moisture is carried in by means of
air infiltration. As it relates to air quality, air infiltration is perhaps the
most important problem a home performance specialist addresses.
A home can be thought of as one big chimney. Warm air rises out
of the house through dozens and sometimes hundreds of little
holes and cracks in the upper portion of the house. Cooler and
makeup air enters the house through holes and cracks in the
lower portion of the house. As a result a typical home might
change out its air with the outdoors eight times a day or more. Old ceiling lights are some
of the most notorious sources of leaks. Plumbing penetrations
under sinks and baths are also great sources of unwanted air.
Even electrical outlets will leak considerable air into and out
of a home. A blower door test can determine where these leaks
The home performance industry has its share of drawbacks just
like any other. First realize that even though some companies
advertise themselves as home performance contractors, the
California Contractors State License Board has no such classification.
The same is probably true in the rest of the country. In some cases general contractors will
perform the work. In doing so they dabble in many different
trades. In other cases specialty contractors like insulation or
HVAC contractors will do the same. Second, the price can be
high. Bills of ten grand that don't include new HVAC components
are not uncommon. Throw in new HVAC and a few other things and
bills can top out at twenty grand or more. Third you're still
faced with the challenge of weeding out the shysters from the
craftsmen. There are plenty of home performance contractors
who'll be more than happy to sell you
cleaning and other bogus services if you
- If you've read the entire page
up to this point then you should be congratulated.
Either that or your sanity should be questioned. ;^) Having
done so you now realize that there's a great many forces at work
in a home that all intermingle. This is especially true of air
A home will naturally change out its air with the outdoors as
much as eight times per day or more. This process of
infiltration goes on 24 hours a day. That $1,500 electronic
air cleaner isn't doing much when you consider that your furnace may run for
only one or two hours per day, even less during mild
answer some have come up with is to run the furnace blower 24
hours a day. Variable speed blowers in particular cost little to
run and are very quiet.
But if your ducts are leaking badly then running the blower forces
outside air in at a faster rate than natural
infiltration does. Will the filter remove particles faster than
the artificially increased infiltration brings them in? It's
anyone's guess. And to top it all off, all of these measures are
made moot by homeowners who like to open the house up during
Real improvements in air quality can be elusive.
Much of the money spent to that end is more placebo than
anything else. A real effort involves
sealing leaky ducts, installing improved filtration, cleaning or
replacing filters regularly, running the
furnace blower 24/7 (or in the circulate mode that some
thermostats have), sealing up major leaks in the house shell
and keeping the house closed up all the time. If you're willing
to do all of that then indeed you may improve air quality
in the home. Anything less and you can easily fall into the
A Guide to Partial System Replacement
This is where it
can get messy. While every step above may need to be addressed when installing a completely
new system, the matter is less clear when replacing only some
components thereof. I can give you more refined recommendations
over the phone or by e-mail. In the mean time the following is a
is a must. The Manual J portion of
2 can still be quite useful
in preventing the installation of a furnace with too high a BTU
input. If no ducts are being replaced then the Manual D portion of
step 2 as well as all of 3 are of little
practical value. Though measuring the furnace's output without the
air balancing aspect of step 3 will satisfy a natural curiosity to
know the furnace's delivered output. 4 through 10
most definitely apply. 19
should be considered. Even if you're not replacing your ducts you
may want to know if there are any gaping holes and disconnects.
26 through 31
can be relevant in that many salesmen will attempt to bundle IAQ
products with a new furnace.
Air Conditioner Only
The manual J
portion of 2
is strongly recommended. If you're happy with your old air
conditioner's performance then it's understandable if you don't want
to pay for a Manual J heat load calculation. But for those with the
goal of reducing their utility bill by just a little more a heat
load calculation may reveal an opportunity to downsize the air
conditioner. Conversely, if you're unhappy with your old air
conditioner's performance than a Manual J may help determine the
true cause of the problem. If no ducts are being replaced then the
Manual D portion of step 2 as well as all of 3
are of little practical value. Though measuring the air
conditioner's output without the air balancing aspect of 3 will
satisfy a natural curiosity to know the air conditioner's delivered
output. 4 and 5
are relevant. 11 through
18 are required reading, though you
have little control over 12 when replacing
just the air conditioner. 13 and
14 apply only if you're changing
out the metering device, evaporator coil or refrigerant lines.
Replacing the air conditioner without replacing the evaporator coil
possible, but not generally a good idea. 19
should be considered. Even if you're not replacing your ducts you
may want to know if there are any gaping holes and disconnects.
26 through 31
can be relevant in that many salesmen will attempt to bundle IAQ
products with a new air conditioner.
2 and 3
are absolute musts if you're looking to improve the performance of
your old duct system. There's no other way to properly design a duct
and 19 through 25
remain unchanged. 26 through
31 can be relevant in that many salesmen will
attempt to bundle IAQ products with new ducts.
The first system
in which I was able to employ literally every one of these
techniques can be seen
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