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, computer 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.
Earlier this year in a blog post warning consumers about cheap AC tune-ups, I mentioned how a hack service tech / salesman 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 over-sized furnaces.
A perfectly sized furnace will probably run almost all day long 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 will 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 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.
