A perfectly sized boiler would run continuously on the coldest day, adding heat to the home at exactly the same rate that it is being lost from the home. If the boiler is just slightly undersized, the house will get cold, and heating contractors will get complaints. Heating contractors don't like complaints. Therefore, most heating systems are oversized. A slightly larger boiler rarely costs significantly more money, and the risk of having a boiler that is slightly undersized simply isn't worth it. Most boilers are designed to keep the house warm on the coldest day of the year. Consequently, the boiler is oversized every other day of the year. It's unusual to find a boiler that is too small for a house. This can happen, for example, when an addition is put on a house.
The size of the boiler depends on several factors including:
the size of the house
how quickly the house loses heat (insulation and weather-tightness)
the output capacity of the heating system (steady state efficiency)
lifestyle of the homeowners and amount of other heat generated in the home
In the middle part of North America, we find that modern homes need roughly 20 to 30 BTUs per square foot. Older homes (typically poorly insulated and not very weathertight) need 30 to 60 BTUs per square foot.
Like all good guidelines, these should be used with extreme caution. Remember that we are talking about the output capacity of the boiler rather than the input capacity or firing rate.
Most of the comfort complaints people have with home heating systems have more to do with uneven or inadequate distribution than inadequate capacity.
A new 1600-square-foot home that is reasonably well insulated and weathertight may require about 25 BTUs per square foot. On its design day, it would need 40,000 BTUs . If the boiler had a steady state efficiency of 80%, it would have to have an input capacity of 50,000 BTUs per hour.
If you look at an older 1600-square-foot home that is not well insulated, you may need 50 BTUs per square foot to keep it warm. This means, on its design day it would lose 80,000 BTUs per hour and we need a boiler with a steady state output of 80,000 BTUs per hour. Assuming a steady-state efficiency of 80%, the boiler would have an input capacity of 100,000 BTUs per hour. These are common residential boiler sizes.