Time for a new furnace and you’re wondering what capacity unit you’ll need? You’ve come to the right place. While you’ll want to consult with a qualified HVAC professional before making your final decision, our furnace size calculation guidelines can give you a good idea of what you’ll want to be shopping for.
The most accurate and recommended method of determining your furnace size is to have professional perform a heat loss calculation following ACCA or ASHRAE guidelines. Most heating contractors have the software to perform such a calculation and they can work with a set of plans or perform the calculations on-site. If you’d like to get a rough idea of your heating needs there are some rule-of-thumb numbers you can use.
Of course, heating capacity needs vary by climate, so the following rules-of thumb values are specific to the Upper Midwest.
The heating factor is a value that’s based on climate and expressed in BTUs, or British Thermal Units, a standard unit of heat. Across the U.S., the heating factor can vary from 5-45 BTUs. It should be no surprise that the heating factor will be highest in the Upper Midwest, and it is. Here, the factor is 15–45 BTUs per square foot of interior space. The variation is due to the wider range of the way our homes are built. From the old 1955 home with 2 x4 walls with 3-1/2” of fiberglass insulation with single pane windows and add-on “storms” that leak with a light breeze, to newer 2 x 6 or double 2 x 4 walls with 6+” of polystyrene insulation and triple pane windows that makes the home almost air tight even in a heavy wind.
Manufacturers often rate their furnaces in terms of BTUH capacity, such as 100,000 BTUHs, which indicates a heating output of 100,000 BTUs per hour. Furnace outputs are typically expressed in increments of 20,000 BTUs: 80,000 BTUs, 100,000 BTUs, 120,000 BTUs and so on.
Calculating Your Basic Heating Needs
Calculating your basic heating needs is pretty straightforward. You’ll want to multiply your home’s square footage by the recommended heating factor to obtain the furnace output rating you’ll need. For example:
- If your home is 2,400 square feet and you figure 35 BTUs per square foot, you’ll need a 84,000 BTUH-capacity furnace.
- That same home, using a heating factor of 15 BTUHs, would require an output capacity of 36,000 BTUHs.
That’s a pretty significant difference, so it’s important to take into consideration some variables when calculating furnace capacity.
Variables to Consider
Here are some of the most common variables to keep in mind when deciding the right size furnace for your home.
- Insulation — If your home is well insulated, start by using the 15 BTU heating factor. But if your home is older, not well insulated, or you’re not sure how well insulated it is, go with the higher, 35 BTU factor.
- The Glass Factor — If your home has a lot of windows, patio doors, or skylights, you may need a higher capacity furnace to compensate for heat loss through the glass.
- Efficiency Ratings — The output (BTU) capacity of a furnace is not the same as its efficiency rating. For example, take a furnace with 100,000 output rating with 95% efficiency. The actual output of that furnace is 95,000 BTUs. Not bad. But compare that to a 100,000 BTU furnace of 80% efficiency — only 80,000 BTU output. Make sure you factor in efficiency ratings in your calculations.
- Current Heater Behavior — How does your current furnace work? Does it run near-continuous in cold weather? If so, maybe you have a furnace that is sized right on the money or slightly undersized. Does it cycle rapidly or run very little in subzero weather? If so, it's probably oversized. Professional load calculations are the most accurate, but know what your existing furnace does in the extreme conditions is also very helpful to getting the next one sized right.
Making a Decision: Check With a Qualified HVAC Pro
Calculating your expected furnace capacity is a great idea while you’re shopping around and comparing prices and manufacturers. However, you’ll want to consult with a qualified HVAC professional before you make your final decision because it’s easy to over or under calculate what your specific home needs. Buying a unit that’s too big for your home can result in higher, unnecessary heating bills. One that’s too small can cycle on and off too frequently, resulting in damage and high repair or replacement costs. Choosing the right size furnace for your home is a decision you’ll live with for a long time, so make sure it’s the right decision the first time around.