What size furnace do you need? Common size calculations explained

Ordering the right-size furnace is crucial for your home’s day-to-day function and performance. If you get the wrong furnace size, you could deal with increased energy bills, costly repairs or the furnace shutting off when you need it most. 

To get the perfect fit, you must determine the required Btu output. Btu (British thermal units) are a measurement of heat energy. One Btu equals the amount of heat required to increase one pound of water by one degree Fahrenheit. This calculation factors in square footage, insulation, window efficiency and local climate.

We’ll walk you through the process of measuring for a heating, ventilation and air conditioning (HVAC) unit. In this guide, you’ll learn how to determine the best furnace output for your house based on square footage, window insulation, climate and more.

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How do you determine the right size for a furnace?

Sizing an HVAC system starts by knowing what heating unit type you need. Then, you’ll perform a simple heating load calculation. Once you do these steps, you can make your own estimation for the right-size furnace or get professional help with a Manual J calculation. 

Step 1: Determine the type of furnace you need

First, determine which furnace type you should look for to match your home’s size and ductwork. Here is a breakdown of different heaters and furnaces.

• Electric furnace: These furnaces use electricity to generate heat. They’re usually less efficient than gas but are a good option if gas isn’t available as they feature no combustion and require no ventilation.
• Gas furnaces: These use natural gas or propane as fuel. They’re currently the common furnace type and are more energy-efficient than electric furnaces. They also require gas line hookup and ventilation.
• Heat pumps: Heat pumps use electricity to move heat (rather than generate it). They’re more efficient in milder climates and typically require backup electric coils for colder weather.
• Oil furnaces: These furnaces use heating oil as fuel. They’re less common now but a good option in some rural areas without natural gas. They require an oil tank and ventilation.
• Wood/pellet furnaces: These furnaces burn wood or wood pellets for heat. They require more maintenance and cleaning than other heating methods. 

Step 2: Calculate the heating load

To ensure your home’s temperature stays comfortable, you must also determine your home’s heating load. This is the amount of heat required to maintain a comfortable temperature. To calculate the heating load, consider the following factors that determine your home’s heating capacity. 

First, assess your local climate zone to evaluate how cold it gets during winter. Colder climates need more heating capacity. 

Second, evaluate your home’s insulation quality — well-insulated homes require less heating capacity than poorly insulated ones. 

Third, consider the type and condition of your windows and doors, as this impacts heat loss. Homes with high ceilings may require more heating power. You should also consider the number of people living in the house, as more people generate more heat.

Measure the total square footage of your home, including all living spaces. This measurement will give you a starting point for the appropriate furnace size.

Step 3: Calculate the right furnace size

To calculate your furnace’s required Bty, use the following formula:

Btu = (Square Footage × Heating Load Factor) + (Number of People × 600)

The Heating Load Factor depends on your home’s insulation and climate. Typically, it ranges from 20–50 Btu per square foot. For a well-insulated home in a cold climate, consider using 35 Btu per square foot as a starting point. Multiplying the number of people by 600 determines the average heat needed for each occupant.

This table shows some common home sizes and the furnace sizes they require. 

After calculating your Btu furnace requirement, you can begin shopping around for the best furnace for your home. Furnace sizes are typically measured in Btu per hour (BtuH). This measurement uses the furnace’s Btu to determine how much heat the furnace can generate within an hour. Make sure to compare the Btu requirement to the furnace’s heating capacity. 

Step 4: Request a Manual J calculation

While these calculations provide a solid starting point, it’s always wise to consult with a professional HVAC technician. For the most accurate number of Btu your home needs, you must have a Manual J or block load calculation performed. These calculations put all the above variables into a formula to get a hyperaccurate furnace size recommendation.

Why does your furnace size matter?

The right size furnace will heat your home effectively without wasting any energy. An oversized furnace will cycle on and off frequently. This causes uneven heating and wasted energy by overheating before shutting off. An undersized furnace won’t be able to heat your home properly, leaving you cold in winter.

What’s next?

The best way to ensure you buy the correctly sized new furnace is to hire a pro. The information and steps we provided only provide a general idea of your home’s heating needs. We recommend contacting an HVAC professional to schedule a Manual J calculation to ensure your heating system matches your home’s needs and energy performance.

 A pro can also help you locate an efficient furnace with a high annual fuel utilization efficiency (AFUE) rating. AFUE ratings are a go-to measurement that determines how efficiently a furnace converts fuel to heat.