Old-school folks will say , 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. 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. 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. 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. Not only does this affect how your home feels, but it also has an impact on the ability of your thermostat to effectively control room temperature the way it should.
Have you noticed that your furnace seems to turn on and off relatively quickly? Although it may seem normal, shortened cycles may mean that the furnace is not operating efficiently and is using more energy than it should be. Because a furnace uses most of its energy to start and heat up, the pattern of short cycles mentioned above means that the unit must use more energy at a greater frequency. Have you noticed more noise than usual when your furnace cranks up? Although some clamor is to be expected as air flows through the vents, a lot of noise is cause for concern.
If your furnace is circulating more air than your ducts are designed to handle, you can expect to hear more noise during operation; this can actually signal a need to have both your furnace and your air ducts inspected for proper size. The issues mentioned above, such as shortened run times and greater energy use, inevitably take a toll on equipment. The furnace then ends up turning on and off more times than is necessary. A few areas get toasty, and then the system turns off.
Then, as that heat spreads out, the unit has to warm up again and start all over again. This makes over-sized furnaces really inefficient. A furnace that is appropriately sized for your home and needs will turn on and heat your space gradually. This makes the process much more energy-efficient and puts less strain on your equipment. On the coldest night of the year, your furnace may not be able to keep up.
This is actually normal.
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