HVAC 101: An HVAC System's Cooling Capacity
The AC, blower, ducts, lineset & cooling coil all play important roles.

An air conditioner's cooling capacity is rated in tons. The term "ton" relates to ice. For example, running a two ton air conditioner at full capacity for 24 hours would provide the same amount of cooling as melting two tons of ice. It's important to understand that the air conditioner sitting outside doesn't work alone. For the air conditioner to deliver its rated capacity it must be properly matched to other system components. The blower, ducts, evaporator coil and refrigerant lines (lineset) must all be capable of doing their part. A weakness in any link of that chain will cause the system to deliver less cooling than it would otherwise be capable of.

For each ton of air conditioning capacity the blower in the furnace or air handler must deliver 400 cubic feet of air per minute. For humid areas of the country it's not uncommon to reduce airflow to 350 CFM per ton. The lower airflow helps to remove more humidity (and consequently less sensible heat) from the air. In dry climates it's a good idea to increase airflow to 450 CFM per ton or more. The increased airflow wastes less energy on unneeded humidity removal.

For the blower to push 350 to 450 CFM per ton the ducts must be sized correctly. It's a common misconception that if you were to block a standard blower that it will work harder to compensate. The exact opposite is true. Starve a standard blower for air by undersizing the ducts and it simply blows less air. To put it in semi-technical terms: The increased static pressure robs disproportionately from velocity pressure. The exception to this rule is blowers that use variable speed motors. Though sometimes much more expensive than a standard blower, VS motors are gaining in popularity. They can speed up in response to restricted ducts. But they can only do so to a point and should not be relied on as a cure all for undersized ducts. The bottom line is that duct sizing is critical no matter what type of blower you have.

Related to proper duct sizing is duct leakage. You can have every other point on this page covered and if the ducts leak 30%, which happens more often than you might think, then you'll still have a sub par system. Title 24 rules require a maximum of 6% leakage on entirely new duct systems and a maximum of 15% leakage on the rest. Not all of California's climate zones are affected. There are some loop holes. And of course Title 24 is strictly a California requirement. But such standards are entirely reasonable goals for any system, especially new systems. If a contractor can't get duct leakage down to 6% on an entirely new system that he's installing then he doesn't belong in the business.

In addition, for the blower to push 350 to 450 CFM per ton the blower itself must be sized correctly. That's a little trickier than you might think. When a contractor refers to a two ton (or any tonnage) furnace what he's actually referring to is the furnace blower's ability to push air. The heating capacity of a furnace is a separate matter entirely. The critical thing to understand about blower ratings is that they're usually based on having textbook perfect (or nearly so) ducts installed, which almost never happens. In other words, few furnaces actually deliver the airflow that they're ostensibly rated to deliver when installed on a typical undersized duct system. Smart contractors will often oversize standard blowers by one-half to one full ton to compensate, even more in some cases. Oversizing the blower is not the textbook perfect solution. There are drawbacks including that of noise. If the ducts are really restrictive, it's not a solution at all. However, used judiciously it can be a reasonable thing to do.

Now even if you have a two ton (or any other size) air conditioner with a blower and duct system that actually delivers two tons worth of air, you'll still not get two tons of air conditioning capacity if the copper lines that carry the refrigerant (sometimes called Freon) are too small. Just as a blower can't push its rated airflow if the ducts are too small, so too an air conditioner will not push its rated amount of Freon if the Freon lines are too small. Generally speaking undersized refrigerant lines aren't as problematic as airflow problems. But nevertheless it's not hard for an air conditioner to lose as much as 5% of its capacity to undersized refrigerant lines.

Finally we come to the evaporator coil (cooling coil). An air conditioning system's capacity can be broken down into two functions: heat removal and humidity removal. (Humidity removal is actually a form of heat removal. But that distinction is not important for this discussion.) Cooling coil selection doesn't affect total capacity as much as it does the ratio of those two functions. Just as lowering the airflow will cause more humidity removal and less heat removal, reducing the coil size will do the same. Either action causes the coil to get colder and condense more water from the air. Conversely, increasing the coil size and airflow will cause the coil surface to be a bit warmer. That may sound bad. But in dry climates it's good because the system will remove more heat and less humidity. Further explanation can be found here and here. The bottom line is that your cooling coil needs to be selected in order to provide you with the correct ratio of heat and humidity removal. In some areas of the country too much of one at the expense of the other will cause an otherwise properly sized air conditioner to not cool well.

Hopefully you now understand that the relationship of the air conditioner, blower, ducts, refrigerant lines and evaporator coil is critical. Break any link of that five link chain and your air conditioner will not deliver the cooling capacity that it's rated for and that you need. Unfortunately a broken chain, so to speak, is the rule rather than the exception. Learn these things for yourself. Present your contractor with them. And you'll increase your chances of getting an unbroken chain. You may even teach him a thing or two.

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