HVAC Sizing Guide

If you’re always either too hot or too cold at home, you may have had the wrong-sized heating or cooling system installed. But it doesn’t have to be that way.

There are two main rules to knowing how to size a furnace, boiler, or air conditioner:

1. Don’t do it yourself (unless you’re a trained pro)
2. Make sure the trained pro you hire isn’t just guessing

Keep reading to learn what it takes to properly size your heating and cooling system. If you want a rough idea now, go straight to our HVAC sizing calculator.

Whole-House AC and Furnace Sizing

Heating and cooling equipment sizing for an entire home is best done using a professional Manual J Calculation. This official calculation is put out by the Air Conditioning Contractors of America (ACCA).

An ACCA Manual J Calculation is the only trusted way to get the right-sized heating and cooling equipment for your home.

Any AC or furnace size calculator you find online won’t give you anything more than a guess of your home’s heating and cooling load.

Interestingly, that’s the same thing some contractors do. They use rules of thumb based on your home’s square footage to pick equipment that they think will work. Oftentimes, to cover themselves, they’ll oversize the equipment, which wastes energy and money down the road.

There are four main factors a Manual J Load Calculation considers that many rules of thumb and cheap calculators don’t:

1. The Local Climate: A building located in a hot and humid climate will require a larger air conditioner than an identical building located in an area with cool, dry summers. Similarly, a building in South Dakota will require a larger furnace or boiler than an identical building in South Carolina since the winters are colder in South Dakota.
   
   
2. Desired Temperature: How warm or cool do you want it inside your home? As the difference between the outdoor and indoor temperature increases, there will be more heat transfer through the walls of the building, as nature always seeks equilibrium. Therefore, you need even more energy than you might think to heat a building to 75° F compared to heating it to 68° F on the same day.
   
   
3. Building Envelope: The term “building envelope” refers to how well your home is insulated from outdoor air. This includes not only the pink stuff inside your walls, but also how energy efficient your windows and doors are. The tighter your building envelope, the less hot or cold air you will lose to the outdoors.
   
   
4. Exterior Surfaces: Different rooms in your home will change temperature differently depending on how many exterior walls they have. For example, an interior bathroom without any exterior walls will lose less heat than a living room surrounded by three exterior walls and a ceiling.

“Rule of Thumb” guesses don’t take all these variables into account. That’s why, when it comes to furnace and AC sizing for you whole house, go with a Manual J Calculation.

Single-Room HVAC Calculator

If you don’t want a central heating or cooling system and are instead installing a mini split or PTAC unit, the calculation becomes much simpler. Essentially, you are now sizing for either a single-room, or multiple rooms separately. Our calculator below can help with that.

Use that calculator only as a guide, though. For best comfort results, you should still get a Manual J Load Calculation done, even if just for a single-room. Although you’re less likely to get a single-room application “wrong,” it can happen, so why not protect yourself?

Size Matters

Get the right size HVAC equipment and say goodbye to chills during winter and sweating during summer while saving energy and money on your heating and cooling equipment. 

By  Ken D.
Product Expert

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