Don’t oversize your furnace

At my house, we say “Go Big or Go Home.”  Bigger is better when it comes to a lot of stuff: snow forts, leaf piles, power tools, PC monitors, towers made out of Duplos… but not furnaces. When it comes to sizing a furnace, there are a lot of reasons to not go bigger.

Many years ago I wrote a blog post warning consumers about cheap AC tune-ups. I shared the story of a hack salesman who tried to convince me that a bigger furnace would be good for my house because it would “heat my house better”. Now that we’ve officially reached the heating season in Minnesota, it’s time to take a closer look at oversized furnaces.

A perfectly sized furnace will probably run almost all day on the coldest day of the year, and it might not get the house up to 70 degrees about 1% of the time during the winter. No joke. That would be a perfect system. For 1% of the time during winter, you might need to put on an extra sweatshirt. No big deal. For us in the Twin Cities metro area, that would be any time the temperature drops below -11°. Old-school folks might say -20°, but that’s the old number.

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 who use programmable thermostats already turn the temperature down at night, so would this really be a big deal? No, probably not.

An oversized 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 oversized 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 oversized, it spends a large portion 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 oversized, 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.

Besides premature failure, an oversized 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 oversized, 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.

How common is this?

From my own experiences and from everything I’ve heard from everyone in the know, a large portion of older furnaces are oversized. 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.

The one exception to this is for dual-stage or multi-stage furnaces, which will initially fire on a lower heat setting, acting like a smaller furnace. I’m a fan of those.

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 home’s 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.