Heat pump sizing guide: Bigger is not better

A heat pump's size—measured in British thermal units (Btu) or tons—basically tells you how much space it can heat or cool. A right-sized heat pump for your home can keep it perfectly comfortable all year and save you thousands of dollars on installation, energy, and maintenance costs.

Figuring out the right size is the tricky part. Generally, larger homes or those in more extreme climates require bigger heat pumps—but this also depends on the building's draftiness, layout, and other factors. It’s a complicated business, and even pro heat pump installers can come up with wildly different estimates, depending on the measurements they take—or more often, the guesses they make—about your home.

After speaking with HVAC experts and homeowners alike, we've narrowed it down to seven guidelines that can help ensure you're getting the just-right, Goldilocks-sized heat pump for your home. You won't find any easy shortcuts in this guide because they don't exist, but you will learn how to spot the signs of a thoughtful installer, how to pick between different quotes, and (with some work) come up with a decent heat pump sizing estimate on your own.

Heat pump size (also known as capacity) is measured in either tons or British thermal units per hour (Btu/h), which is often abbreviated to just Btu. These units are on the same scale, like feet and inches: 1 ton equals 12,000 Btu/h. 

The goal is to match the heat pump’s capacity during extreme temperatures with your building's largest heating or cooling "load." The load (also measured in Btu/h) is essentially the amount of heat your home loses or gains through walls, windows, air leaks, and other sources during the coldest or hottest weather that your local climate is likely to get.

When you get it right, your heat pump will have enough oomph to keep your home comfortable during the most unpleasant weather and the flexibility to work flawlessly in mild conditions. It'll also be highly energy efficient and may even require less maintenance over time than a mid-sized unit.

Heat pump cost per ton

*Cost and system size data include central ducted heat pumps, ductless mini-splits, and hybrid systems.

The Manual J calculation is the most thorough, reliable method for determining a home's heating and cooling load, developed by the ACCA. Here's a partial list of what goes into the equation:

Manual J is not an exact science

Most installers won't do a "perfect" Manual J calculation. For example, they can't precisely measure insulation thickness or duct sizing when it's hidden behind drywall, and blower-door tests are rare because they're relatively time-consuming.

That doesn’t mean they’re cutting corners. Real-world experience counts for a lot, and seasoned installers often have a strong sense of how homes in their region actually perform. HVAC design is part art, and high-quality installers may arrive at slightly different—but still accurate—conclusions.

The ideal size for a heat pump depends on your house, including its size, local climate, insulation, air leakage, and other factors. There’s a lot to juggle—start with these seven guidelines:

1. Simple sizing rules are basically worthless

The first page of Google is filled with terrible advice about heat pump sizing. You'll find a wide range of shortcuts and rules of thumb that’ll leave you with estimates from as low as 20,000 Btu (theoretically possible for milder climates or very well-built homes, but unlikely) up to 120,000 Btu (certainly wrong). 

Reasonable estimates need to account for more than just square footage. Ceiling heights, insulation, air leakage, duct design, or any other half-dozen factors can majorly impact your home's true heating and cooling needs. For example, tightly built modern houses might only need half as much heating and cooling power as older drafty homes, even when they're the same size and in the same climate—so just forget about these made-up shortcuts.

2. If you want to self-size, use an app

Shortcuts rarely work, but calculating on your own can give you confidence when talking to contractors—or if you plan to purchase equipment directly. Some tricks, such as using smart thermostat data, can help, but the most reliable approach is to use free load-calculation software.

One option is CoolCalc, approved by the ACCA and used by professional heat pump installers. You’ll need to register with an email, but it’s free and runs in a web browser. It automates many steps, and if you spend a couple of hours brushing up on concepts such as R-values, U-values, air infiltration, and duct placement, you can obtain a reasonably accurate estimate.

Another free option is BEOpt, developed by the Department of Energy (we haven’t tested it ourselves).

3. It's okay to leave heat pump sizing to the pros

No heat pump installer expects you to know your system size. They'll always do their own size calculation, and the good ones have the tools and experience to make a faster, more accurate estimate than an untrained homeowner could hope to. Don’t forget that sizing is just part of the job; contractors also decide unit placement and how air will circulate throughout your home.

That said, not all HVAC professionals have strong training in heat pump best practices. The technology is still relatively new in many parts of the U.S., and it's a bit more finicky than traditional HVAC equipment, such as furnaces, boilers, and central air conditioners.

For homeowners, the best bet is to recognize the signs of a capable contractor and hire them to handle the job for you.

4. Installers are wrong sometimes, so get multiple quotes

Comparison shopping is a great first step in determining who's legitimate, as it exposes the most absurd estimates and obvious corner-cutting tactics.

For example, EnergySage's founder, Vikram Aggarwal, received four quotes before installing a whole-house cold-climate heat pump. Three of the estimates said he needed a 4-ton system, but the fourth suggested an 8-ton system. If that last quote had been the only one he had received, he could have ended up installing an expensive, inefficient, and uncomfortable HVAC system. 

In the best-case scenario, you'll receive a handful of quotes that all suggest the same or nearly the same size for a heat pump, and then you can choose based on other factors, such as price, timing, or equipment. The EnergySage Heat Pump Marketplace can help you get quotes from local heat pump installers.

5. More measurements make better estimates—up to a point

Heat pump sizing depends on square footage, ceiling heights, layout, insulation, air leakage, local climate, and more. It’s a complicated process, so it’s generally a good sign when an installer takes the time to inspect your home, take notes, and measure key details. 

“Someone who's only in your house for 20 minutes and doesn’t take any measurements—that’s probably not what you’re looking for,” Alex Meaney, HVAC design expert and proprietor of Mean HVAC Consulting and Design, said to EnergySage. 

Installers should measure room sizes, check your electrical panel, evaluate existing ductwork or potential spots for ductless heads, and plan a location for the outdoor condenser. Still, they rarely measure everything that could affect a heat pump’s performance—and that isn’t necessarily sloppy. 

“It’s hard for contractors to take every recommended step before you become their customer,” Meaney said. 

6. Bigger does not mean better

The goal is to match the heat pump's maximum capacity with your home's maximum heating and/or cooling needs. A higher capacity doesn’t mean better heating or cooling—it just means higher costs and, if it's significantly too large, more problems with efficiency, comfort, and potentially reliability.

Unfortunately, it’s common for HVAC equipment—including heat pumps—to be larger than necessary. One reason is that many HVAC installers don't take measurements, according to some building scientists we've talked to. 

“Getting contractors to do any kind of load calculation has been an uphill battle since I started in 2006,” said Dave Lis, director of technology and market solutions at Northeast Energy Efficiency Partnerships. Lis explained that there are more incentives for installers to go big, including a tendency for customers to request extra capacity “just in case,” so contractors may build in a cushion

If multiple quotes all recommend the same size, it’s probably correct—or correct enough. If quotes differ slightly, Lis and Meaney both suggest erring on the smaller model. The ACCA guidelines even allow undershooting the Manual J estimate by 10%, as long as the selected heat pump can handle local climate conditions.

7. There are some clear signs of a quality installer

Meaney offered some tips to help you spot installers with your best interests in mind:

• They'll present you with multiple quotes with different equipment or system configurations. There's rarely one perfect heat pump for a home, so you should have some options to choose from.

• They'll try to talk you out of getting a bigger heat pump if you ask for one. Oversized heat pumps offer no additional benefits for homeowners, but installers make a little extra money from installing bigger units. So, if they try to steer you away from a bigger heat pump, that's an encouraging sign.

• They can tell you how the heat pump will handle humid but not-so-hot days. This is a key challenge in designing cold-climate heat pump systems, and the best pros will have a plan for that.

If you're ever in doubt about the load estimates you're getting from installers, you can always hire an independent home energy auditor. You'll pay a fee for the service, but they'll take the time to run proper measurements.

Finally, there’s one big red flag to be aware of: If an installer tries to talk you out of a whole-house heat pump because it can't handle cold weather, they don't know what they're talking about. There's nothing wrong with discussing the benefits of maintaining a backup system or exploring the possibility of a hybrid heat pump-furnace setup, but plenty of heat pump models can handle 100% of a home's heating needs in any part of the continental U.S. If an HVAC pro isn't familiar with this fact, they might not be the right person to install a heat pump in your home.

A comfortable and efficient heat pump system starts with proper installation. Look for HVAC professionals with heat pump-specific experience who can provide multiple quotes and demonstrate familiarity with best practices for your climate.

For additional guidance, you can consult the EnergySage Heat Pump Marketplace, which connects homeowners with vetted installers and unbiased Energy Advisors who can help you navigate the installation process.