Calculating Furnace Size

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.

Furnace Ratings

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.

Crunching Numbers

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.

Related Life123 Articles

How does a furnace work its magic on a cold winter day? 

It's not magic, but how does an induction furnace work? Michael Faraday first discovered the process of electromagnetic induction in 1831, but it wasn't until almost one hundred years later that the first induction furnace was put into production.

Frequently Asked Questions on Ask.com
More Related Life123 Articles

It's not magic, but how does an induction furnace work? Michael Faraday first discovered the process of electromagnetic induction in 1831, but it wasn't until almost one hundred years later that the first induction furnace was put into production.

Read articles about how does a furnace work, calculating furnace size, how does an induction furnace work and replacing furnace thermocoupler.

Read articles about how does a furnace work, calculating furnace size, how does an induction furnace work and replacing furnace thermocoupler.

© 2014 Life123, Inc. All rights reserved. An IAC Company