What Size Furnace Do I Need?

Woman Staying Warm with Heating

When it’s time to replace the furnace in your O’Fallon, MO, home, it’s important to choose one that’s the right size. A furnace with insufficient capacity will run all the time and quickly wear itself out. On the other hand, a furnace with too much capacity will waste energy and could lead to parts of your home that are too warm or too cold. At Hoff Heating & AC, we’re the trusted installation team for new furnaces, and our skilled technicians will ensure that you get a new furnace that’s the right size for your home. Here are some of the factors that we use when determining the size or capacity of a furnace you need.

Determine Your Home’s Heating Factor

Your home’s heating factor is a measure of how much heat energy is required to maintain a consistent indoor temperature of 72 degrees Fahrenheit. Climate is the primary consideration in a home’s heating factor, but there are a few other aspects of this measure that are important to keep in mind. If your home was built in 1950 with 2-by-4 lumber and still has the original single-pane window with aluminum storm windows, your home will need a furnace with a higher capacity than a home of the same size that was built in 2020 with double 2-by-4 walls or 2-by-6 walls with 2 inches of polystyrene insulation and triple-pane windows.

Learn Your Climate Zone

Your home’s heating factor is related to its geographic location. The United States Department of Energy created heating zone maps to indicate how many British thermal units (BTUs) of heat it will take to warm your living space. O’Fallon is in heating zone three. This area needs 40 to 45 BTUs per square foot. This is a general recommendation, and your particular home might need a few more if it’s older, inefficient or has other issues, such as leaky air ducts or insufficient insulation.

Calculate Your Home’s Square Footage

To know what size of furnace your home requires, you also need to know its square footage. The square footage of your home can be calculated by multiplying its length by its width. If your home has an irregular shape, you may need to measure the length and width of each room. Keep in mind that your property tax assessment, county assessor or auditor, mortgage documents, blueprints, or other materials related to your home may already have its square footage listed. If you don’t have a way to measure your home’s square footage, one of our technicians can do this for you while planning the installation of your new furnace.

Multiply Square Footage By BTU Requirements

To know how many BTUs your home needs from a furnace, multiply the square footage by its BTU requirements from the climate zone. For example, if you have a 2,000-square-foot home in O’Fallon, you’ll need a furnace with 80,000 to 90,000 BTUs.

Consider the Furnace Output

Most furnace manufacturers rate their furnaces in BTUs per hour or BTUH. If your home’s requirement is 90,000 BTUH, it’s better to choose a furnace with a 100,000 BTUH capacity than one with an 80,000 BTUH capacity. Furnace manufacturers list this information in their owner’s manual and on their websites in the descriptions of each model of furnace.

Inspect Your Insulation

The size of furnace your home needs will also depend on factors that are specific to your home. If two homes are both located on the same street, were built at the same time, and are the same size, they might still have different BTU requirements. One of these factors is the amount of insulation your home has. If your home doesn’t meet the Department of Energy’s recommended R-value of insulation, it’s a good idea to add supplemental insulation to reach the recommended amount before you invest in a new furnace. You might not need as large of a furnace if your home has adequate insulation.

Understand the Glass Factor

The glass factor is another issue to consider when it comes to furnace capacity. Thinking back to the two houses that are the same size and age and located on the same street, the house with more windows or more total glass will need a higher furnace capacity. Windows, patio doors, glass French doors, and skylights all allow heat to escape to the outdoors. Homes with more of these or with large windows will need a furnace with a higher BTUH capacity.

Think About Your Home’s Construction Material

The materials used to construct your home are also an important consideration in choosing the right size of furnace. Brick homes retain more heat than homes made of two-by-four lumber. If your home isn’t well-insulated, it will also need more BTUs of heat from the furnace. Houses with a large basement or crawl space may need more BTUs because some heat will be lost to these areas. If your house has a two-story great room or high ceilings, you’ll need a furnace with a higher BTU output.

Factor Your Home’s Location

Your home’s geographic location is one of the top considerations in determining what size of furnace you need. Newly built homes that are close together in an urban environment will have a slightly lower BTU requirement than a home of the same age in a rural area without any nearby structures. If your home is located at the top of a hill, it will need more BTUs than a home that’s located in a valley that traps more warmth overnight. Nearby trees that serve as a windbreak may slightly lower the amount of BTUs your home needs. Our NATE-certified technicians take these factors into consideration when calculating your home’s heating load requirement.

Know the Furnace’s Energy Efficiency Rating

You’ll need to consider the furnace’s annual fuel utilization efficiency (AFUE) rating. If your home currently has an old furnace that’s only 65% efficient, replacing it with a new furnace that’s 98% efficient might allow you to get one with a lower capacity. For example, if your home needs 95,000 BTUs, a furnace with a 100,000 BTUH capacity and a 95% AFUE rating will produce 95,000 BTUs. However, a furnace with a 100,000 BTUH capacity that has an 80% AFUE rating will only produce 80,000 BTUs, which isn’t enough to heat your home.

Observe the Behavior of Your Current Furnace

Consider the behavior of your current furnace. If it runs all the time, its capacity might be too low. It may also be inefficient. If your current furnace rarely cycles or only performs short cycles when the weather is frigid, it might be too big for your home. Our heating load calculations are accurate, but this type of information will help our technicians with the installation of a properly sized furnace for your home.

At Hoff Heating & AC, we’re ready to install a new furnace in your home. You can also turn to us for air conditioner installations. Our heating and cooling system maintenance and repairs keep your home comfortable and energy-efficient year-round. If you’re ready to lower your home’s carbon footprint, we offer geothermal heating and cooling systems. To learn more about what size of furnace your home needs, get in touch with us at Hoff Heating & AC today.