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What size furnace do I need?

Oct 30, 2019

The layout of a single family home on a wooden table with construction tools surrounding it.

Buying a new furnace isn’t as simple as running out to the local hardware store and picking the cheapest model. There are a variety of factors to consider before a purchase can be made. While your HVAC company can help you properly size and install your new furnace, it’s always appreciated when homeowner’s come to the table prepared for the conversation.

Here’s how to determine which furnace size is right for your home:

Why is proper furnace sizing important?

Proper furnace sizing is the key to home comfort and energy efficiency during the cold winter months. A furnace that’s too large or too small for your space can lead to a variety of issues down the line:

Read more: 4 Furnace Troubleshooting Tips for Homeowners

Problems Caused By Incorrect Furnace Size

Here are some consequences that can result from an oversized furnace:

  • Discomfort from heating too quickly
  • Reduced energy efficiency
  • Frequent repairs

You’ll also run into issues if your furnace is too small for your home, including:

  • Discomfort from inadequate heating
  • High utility bills
  • Uneven heating

In both cases, the lifespan of your furnace will be shortened significantly.

How to Calculate the Right Furnace Size

Your furnace is supposed to heat your home gradually in order to maintain efficiency, ultimately extending its shelf life. If you’ve noticed your furnace is constantly running, it’s likely too small for your space. Conversely, a furnace that starts and stops in short, frequent bursts is likely too big. In either scenario, it’s time to upgrade to a furnace designed to suit your home’s needs.

Read more: Winter Home Energy Savings Tips

Here are the factors that play into estimating furnace size:


One of the largest factors to consider when estimating furnace size is your climate. The warmer the climate you live in, the less heat output your furnace will require and vice versa. Other factors like building material and shrubbery can have an impact on furnace size, as well. For example, brick insulates a home more effectively than wood siding. Homes surrounded by dense landscaping are also better insulated and will require a lower heat output.

Home Size (Square Footage)

Simply stated, the more space you need to heat, the more heat output you’ll need from your furnace. Most homeowners will be able to determine square footage by simply looking at the original listing or lease documents. However, if you aren’t able to access that information, you’ll need to manually measure the size of each room in your home and total the values.

Square footage isn’t the only factor to take into consideration. An open or closed floor plan, the number of rooms, and the size of the rooms all make a difference when it comes to your home’s heating needs.

BTU Furnace Calculator

A furnace’s heat output is measured in British thermal units – commonly known as BTUs – which is the amount of energy required to heat one pound of water by one degree Fahrenheit. The higher a furnace’s BTU rating, the more heat it’s able to provide.

To calculate the BTU output required for your home, you’ll need to know the number of BTUs your climate requires per square foot and then multiply by your home’s square footage. For example, a warmer climate may only require around 30 BTUs per square foot for adequate heating. Therefore, a homeowner that lives in a 2,000 square-foot home in a warm climate would need a furnace with a 60,000 BTU output.

It’s also important to consider a furnace’s efficiency rating. The Environmental Protection Agency requires a minimum energy efficiency rating of 78 percent for all home heating systems. In order to heat your home efficiently, you’ll need to factor this percentage into the equation. Since a furnace can never operate at 100% efficiency, you shouldn’t buy one that maxes out at the BTU output required for your home.

Improve your heating system with CroppMetcalfe.

When in doubt, call in the pros. CroppMetcalfe’s 5-Star Technicians are here to help answer any questions you may have and find a heating solution that works for your home. Call our HVAC experts at 703-698-8855 or schedule a service appointment online today.


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