HVAC 101: Furnace Longevity
Airflow is key.

The primary determinant of a furnace's real life span (more on that below) is how long its heat exchanger lasts. The heat exchanger is at the heart of the unit. The fuel burns inside of it and the air flows around it to bring the heat into the house. The heat exchanger is conceptually simple in that it's just a bunch of stamped metal and/or tubing, etc. Old timers referred to it as the firebox. While conceptually simple, it's also almost always the most expensive component on a furnace to replace. On very old furnaces the heat exchanger may be integral and not replaceable at all. On other furnaces it can be replaced. But after 20 years or so they're often no longer available for purchase. For those reasons and more, once the heat exchanger fails you often end up with a new furnace. The next five paragraphs will explain the process in greater detail

If left unaltered a standard furnace's gas input remains relatively constant. In other words there's a fixed amount of heat being produced inside the furnace heat exchanger. On the other hand, the airflow through the furnace tends to decrease over time as dirt accumulates on the blower & cooling coil and as the filter clogs up. In many cases airflow is bad from the day the furnace was installed because of poor duct design and installation. Reduced airflow through the furnace combined with a constant gas input causes it to run hotter. Run it too hot for too long and the metal of the heat exchanger will weaken and fail. If you're unlucky the cracks in the heat exchanger will allow exhaust gas into the house before it's finally diagnosed.

Multi-stage furnaces are growing in popularity. Their gas input varies depending on demand. However, the furnace blower's output typically varies right along with it. So except for a few notable exceptions, even multi-stage furnaces will tend to overheat when airflow is reduced too much.

There are heat sensors in the furnace that will slow (but not necessarily stop) the heat exchanger's failure in the scenario described above. The sensor will shut the flame down once the furnace gets too hot. On most furnaces the blower will keep blowing to cool the furnace down internally. Then once the temperature is merely warm and not hot the sensor will reset and the flame will come back on. On older furnaces this process can repeat hundreds of times without the knowledge of the homeowner. Since the sensor is not designed for constant duty it will typically fail at some point and you'll finally notice the problem. On more modern furnaces the circuit board will typically lock the furnace out after the sensor has tripped a number of times. If the sensor only trips once in a while then it may not lock the furnace out, leaving you with no idea that there's an ongoing problem.

Alternatively it may be that the installer or a subsequent service man noticed that the furnace was overheating. If it's bad enough then the correct response is for the technician to recommend repairs that will address the low airflow. That could be duct upgrades, coil cleaning, blower cleaning or even just filter replacement. But if a dirty filter isn't the problem then the quick and dirty solution is to lower the gas pressure. Doing so will obviously reduce the furnace's internal temperature. But when taken to extremes the furnace will run too cold internally. The damage that results can best be understood by looking at your car's tail pipe in the morning.

When you burn fuel you create water vapor. When either your car or your furnace is cold the water vapor in the exhaust gas will condense to liquid and coat the internal surfaces. That process doesn't usually go on for very long. Your car's exhaust system quickly heats up and the water vapor then stays in vapor form. The same is normally true of furnaces. But if the gas pressure is turned down too far then it can take a very long time for the furnace exhaust system to heat up enough to stop the condensation. The increased wet time can potentially rust out your furnace very quickly.

Even if the increased wet time doesn't cause immediate damage, the reduced temperature can potentially reduce energy efficiency. Continuing with the car analogy: Racing engines run much hotter than standard car engines. The extra heat makes it easier to completely burn the fuel and get every last bit of power from it. Try running a racing engine at street car temperatures and it simply wouldn't perform. A similar thing can happen in furnaces. Make the heat exchanger too cold and you may turn less of the fuel into usable heat. At best you'll simply spend a little more on fuel. At worst the incomplete combustion process will cause soot to form and foul the furnace. There are certain types of furnaces that if they get fouled bad enough you're left with no other choice but to replace the furnace.

Since the heat exchanger is at the heart of the system I linked it to the furnace's "real lifespan". Most everything else on a furnace can be fixed indefinitely. But the reality is that HVAC shop owners are not usually that patient. A lot of heat exchangers will last 20 to 30 years. Given the profitability of new furnace installations it's becoming increasingly common for some HVAC shops to push new furnaces when the old one is as little as ten to fifteen years old. You must be cautious if you're not to be taken.

A furnace that's only ten years old doesn't usually need to be replaced unless it has a cracked heat exchanger. Even then it's usually fixable. But what about a 16 year old furnace that needs $500 in repairs? It can become a fuzzy issue. In parts of the country that have mild winters that furnace may have 5 to 15 years left. In areas with severe winters it may be on its proverbial death bed. You have to consider the cost of the new furnace vs. the cost of repair, how long you plan on living in the house, the potential energy savings, the potential increase in comfort, the potential increase in safety, etc. Suffice it to say that the new equipment hustlers will tell you that the answer is obvious. But quite often it is not. If the heat exchanger is sound then an old furnace can almost always be fixed. You must determine if it's worth it to you. 

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