When calculating furnace size, it pays to be accurate. A home furnace is an expensive purchase that is complicated and difficult to replace. A correctly sized furnace will run more efficiently, need fewer repairs and provide years of comfortable heating.
The Importance Of Calculating Furnace Size Correctly
Although the temptation to think more is better, it is important to correctly size your furnace. A furnace that is too big for your home will turn on and off frequently-a process called short cycling. Short cycling can create condensation that will lead to corrosion inside the furnace. A furnace that is too small for your home will run for longer periods, trying to heat your home. This will lead to higher energy costs and shorten the lifespan of your furnace. In a worst-case scenario, a furnace that is too small might not even be able to heat your home properly in the depths of winter.
As you shop for furnaces, you'll be looking primarily at two numbers: the input and output ratings. These numbers will be in British Thermal Units (BTUs) a measure of the amount of energy consumed by the furnace while it's running. The input rating is the gross amount of energy used, while the output rating is based on the efficiency of the unit, which is rated as a percentage. For example, a 100,000 BTU furnace with a 92% efficiency rating would have a 92,000 BTU output rating.
The standard calculation for sizing a furnace was created by the Air Conditioning Contractors of America (ACCA) and is called the Manual J load calculation. This formula takes into account the size of your home, the amount of insulation, your homes orientation with respect to the sun and other factors.
There are several other online calculators to estimate your furnace size, including Alpine Home Air Products and Heatload.com.
If you're not comfortable with the math or technical concepts involved, you may be better off having a contractor run a calculation for you. They'll charge you for the service, but many furnace dealers will refund the fee if you purchase a furnace from them.