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An undersized furnace will work fine for the majority of the time, but every once in a while it won't get the house quite as warm as desired. How bad would this really be? You'd need to wear an extra sweatshirt sometimes. It's coldest at night, and people that have a programmable thermostat already turn the temperature down at night, so would this really be a big deal? No, probably not.
An over-sized furnace will keep the house warm no matter how cold it gets outside, but it does so at a cost. First, it will probably make the house less comfortable. When the furnace kicks on, some areas may warm up very quickly, so much so that they get uncomfortably warm before the thermostat has even been satisfied. In poorly insulated, drafty houses, this heat can be quickly dissipated, causing the furnace to turn on and off frequently. Many people run the furnace fan 24/7 to help even out the heat.
An over-sized furnace will also be less efficient. A furnace is least efficient when it first fires up, and doesn't reach its peak efficiency until the temperature of the air coming out has reached a steady value. When a furnace is over-sized, it spends a large portions of its running hours in the 'warming up' phase. Once it reaches a steady temp, the thermostat has been satisfied and the furnace shuts back off.
When a furnace constantly cycles on and off, the life of the furnace is dramatically reduced. The best analogy I can think of is a car with all city miles and no highway miles. City miles involve a lot of starting and stopping, which is what puts a lot of wear on a vehicle. Heat exchangers on furnaces fail from the metal heating and cooling repeatedly; when a furnace is over-sized, the furnace turns on and off constantly, putting all 'city miles' on the furnace. This is not a good thing, and will lead to premature failure of the furnace. Guaranteed.
Besides premature failure, an over-sized furnace will be prone to short cycling, which is a term that refers to a furnace shutting down before the thermostat has been satisfied. Furnaces are equipped with a safety feature that prevents them from overheating. When a furnace is over-sized, it will run hotter than it should, which puts it closer to the temperature at which it will shut itself off. Throw in a dirty furnace filter, and it will probably be enough to push the furnace over the edge. When a modern furnace short cycles too many times in a row, it will go into shut-down mode to help prevent further damage. This is most likely to happen when it's extremely cold outside, which is the time when you really don't want your furnace to quit working.
What about boilers?
The same stuff applies. Most boilers are oversized, and it's not a big deal if a boiler is slightly undersized. An oversized boiler will make the house less comfortable, will be less efficient, and the constant turning on and off will shorten the life of the boiler and probably cause unexpected breakdowns.
How common is this?
From my own experiences and from everything I've heard from everyone in the know, a large portion of the older furnaces out there are over-sized. HVAC contractors who are passionate about their trade love to talk about this stuff, and when they do, I listen. I've learned a lot about this topic from a couple of local, passionate HVAC contractors; one being Enviroworld USA, and another being Chris Jirak of Neighborhood Plumbing Heating and Air.
Chris estimates that at least 75% of the furnaces that he replaces are over-sized. As furnaces get more efficient, the trend should be to install furnaces with lower BTUs, not higher. Many HVAC contractors are reluctant to go smaller because they don't want to deal with homeowner complaints about the house not heating enough, but just a little bit of client education is really all it takes to prevent this. A furnace that takes a long time to heat a house is probably sized properly.
When a furnace is replaced, the HVAC contractor should perform a Manual J calculation (or the equivalent) to help them determine what size furnace the home really needs. This is a calculation that requires several of a homes variables to help determine the proper heating and cooling requirements. If a calculation isn't done, the contractor is just making a guess at the size of the new furnace, which means they'll probably go big, then go home.
Author: Reuben Saltzman, Structure Tech Home Inspections
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Reuben Saltzman is a second-generation home inspector with a passion for his work. Naturally, this blog is all about home inspections and home-related topics in the Twin Cities metro area. In addition to working at Structure Tech, he is also a licensed Truth-In-Sale of Housing Evaluator in Minneapolis, Saint Paul and several other cities.
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