There are several factors to consider when sizing a furnace for a house, including the square footage of the building and the climate it occupies. A rule of thumb is to allow 25 to 30 BTU per square foot for moderate climates and up to 45 BTU per square foot in harsher climates. For a 2,000 square foot home, the ideal furnace size therefore varies from 60,000 to 90,000 BTU.
Efficiency is an important consideration in selecting the correct size of furnace for a home. Multiplying the BTU output by the rated efficiency provides the effective BTU. A difference of as little as 10 percent in efficiency can have a big impact on the actual output of the unit. Choosing a higher efficiency unit allows an owner to reduce the size of furnace required, according to The Nest.
The correct size of furnace improves comfort levels in the home and ensures a longer furnace life. Furnaces that are too small are often overworked to keep a home comfortable, causing additional wear on the components. When outside temperatures drop, an undersized unit can lose its ability to maintain the desired comfort level. However, selecting an oversized unit also causes problems. The unit will then heat the home quickly and shut off. Such short-cycling is hard on the mechanical parts of the furnace and often leads to premature failure.