The furnace size your home needs depends on a variety of factors—so many that we actually can’t give you an accurate estimate without visiting your home.

In this article, we’ll go over:

  • The importance of correct furnace sizing
  • How furnace size is measured
  • What goes into sizing a furnace

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The importance of correct furnace sizing

A lot of homeowners think that when buying a furnace for your home, bigger is better or smaller means less money.

In both cases, you’ll end up paying more money in the long run. We’ll explain.

When a furnace is too large, it will short cycle, which is when the system repeatedly turns on and turns off after running for a short time.

A furnace that’s too small will run constantly, trying (and failing) to keep up with your home’s heating demands.

Both situations can lead to problems like:

  • Hot and cold spots. Because an oversized furnace starts and stops so much, it doesn’t have time to heat every inch of your home, which means certain areas of your home are noticeably warmer than other rooms. Undersized furnaces, on the other hand, will run more, but won’t be able to properly heat your entire home.
  • More frequent repairs. Oversized furnaces frequently starting and stopping puts a lot of stress on the system, causing furnace parts to wear out quicker. Meanwhile, an undersized furnace has to run constantly, which means its components rarely get a break and the furnace won’t last as long as a properly sized furnace. The cost of furnace repairs can quickly add up, likely costing you more money in the long run.
  • High energy bills. Furnaces use a lot of energy starting up. An oversized furnace short cycles and frequently has to turn on, increasing your energy consumption and your utility bill than a properly sized furnace would. An undersized furnace has to run nonstop, which also increases your energy consumption.

How furnace size is measured

Furnaces are sized based on BTUs (British Thermal Units), which measure how much heat the furnace can put out. One BTU is roughly the amount of heat produced by a single match.

Most residential furnaces range from 40,000 to 120,000 BTUs. The more BTUs a furnace has, the more heat the furnace can produce.

While bigger homes usually need furnaces with more BTU output, a lot of factors go into sizing a furnace (see section below), which means you can’t buy a furnace only based on the square footage of your home.

Furnace efficiency matters too

A furnace’s efficiency also impacts its size. Efficiency is measured by the AFUE (Annual Fuel Utilization Efficiency) rating, which determines how well the furnace can convert fuel to heat.

Generally, the more efficient the furnace, the more BTUs it can deliver to your home.

For example, a high-efficiency furnace (AFUE rating of 90 or higher) that can output 100,000 BTUs can actually deliver—depending on other factors like ductwork quality—about 90,000 BTUs of heat into your home.

We know this is a lot of technical information to take in, which is why we recommend leaving furnace sizing to the pros.

What goes into sizing a furnace

If a furnace technician gives you a recommendation based on square footage alone, you’ll want to get a second opinion.

Sizing a furnace on square footage alone can lead to undersizing or oversizing, which can lead to the problems we mentioned above.

To properly size a furnace for your home, a furnace technician needs to perform a “heat load calculation,” which determines how much heat your home loses.

Many factors go into calculating heat loss, including:

  • Your home’s square footage
  • The number and size of your home’s rooms, doors and windows
  • Your home’s orientation (to determine whether it gets direct sunlight)
  • How much shade your home gets
  • How well your home is insulated
  • And more

After calculating the heat load, your technician can give you the correct furnace size for your home.

Need a new furnace? Call Roy’s for a free estimate.

Roy’s furnace specialists have been serving the Buffalo community for nearly 50 years. Since 1974, we’ve sized and installed thousands of furnaces, providing each and every customer with first-class service (just look at our thousands of 5-star reviews), upfront pricing, and great financing options.

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