Heat pump sizing guide: Bigger is not better

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

Figuring out the right size is the tricky part. Generally, larger homes or homes in more extreme climates will need bigger heat pumps. But it also depends on how drafty building is, how it's laid out, and other factors. 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 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.

The ideal size for a heat pump depends on your house: How large it is, the local climate, insulation, air leakage, and much more. There's a lot to juggle—and that leads us to the first guideline:

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 can't all be correct, including:

• Multiply the number of square feet of living space in your house by 30

• Multiply the square footage by 20

• Add 1,000 Btu for every 100 square feet—in other words, multiply the square footage by 10

• Multiply the square footage by somewhere between 30 to 60, depending on your local climate

• Start with 1 ton for the first 1,000 square feet, and then add an extra ton for every additional 500 square feet

So for a 2,000 square-foot house, you're left 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). And each of the estimates in between could be off by at least a half-ton.

Good 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.

If you want to self-size, use an app

So the shortcuts don't work, but you might want to calculate on your own, whether that's for a sense of confidence or peace of mind while talking to contractors or you want to purchase equipment directly without going through an installer. A few tricks seem credible, including one that relies on data from a smart thermostat. But the most reliable route is to use some free load-calculation software.

One option is CoolCalc, which is approved by the Air Conditioner Contractors Association (ACCA) for making Manual J calculations. And real-life working contractors actually use CoolCalc in the field. You'll need to register with an email address, but it's free to use, and you can run it in a web browser. We've written a little more on what it's like to use CoolCalc below, but in short: It's not designed to be a simple tool, but it does automate some steps in the process. And if you spend a couple of hours brushing up on some building-science concepts, including R-values (wall insulation), U-values (window insulation), air infiltration, duct placements, and more, you should get a pretty accurate estimate.

CoolCalc, a free program that can calculate your home's heating and cooling needs, can be tricky to use (get a load of that interface) but is far more accurate than other methods.

Another free option is BEOpt, which was developed by the Department of Energy. (We haven't tried this one out yet.)

It's OK to leave heat pump sizing to the pros

No heat pump installer will expect you to know how big of a system your home needs. 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. Plus, heat pump installations are much more complicated than just finding the right size: Somebody needs to figure out where the units will go and how they'll distribute air.

But here's the tricky part: Some contractors haven't had great training on the best practices for heat pump installations. The technology has only recently reached the mainstream in many parts of the US, and it's a little more finicky than traditional HVAC equipment like furnaces, boilers, and even central air conditioners.

So the path of least resistance for you, as a homeowner, is to recognize the signs of a capable contractor and hire them to handle the job for you.

Installers are wrong sometimes, so get multiple quotes

Comparison shopping is a great first step for determining who's legit because it'll expose the most absurd estimates and obvious corner-cutting tactics.

For example, EnergySage's CEO, Vikram Aggarwal, got four quotes before he installed 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 was the only one he had received, he could have ended up installing an expensive, inefficient, uncomfortable HVAC system. (Good thing he had some context!)

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

More measurements make better estimates—up to a point

Heat pump sizing is based on a home's square footage, ceiling heights, layout, insulation, air leakage, local climate, and a few other factors. The industry-standard manual with all the measurement procedures and notes about best practices (the ACCA's Manual J) is a whopping 627 pages long. It's a complicated business!

So it's generally a good sign when an installer spends a chunk of time looking around your house, noting details, and taking measurements. They should at least measure room sizes, look at your electrical panel, check out your existing ductwork or find spots where new ductless heads could fit, and scope out a location for the outdoor condenser. "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," says Alex Meaney, an HVAC design expert and proprietor of Mean HVAC Consulting and Design.

Small differences in a contractor's assumptions about your home can have a big effect on how big of a heat pump they'll recommend. In this example, choosing one air leakage score leads to an estimated heating load of 3 tons…

...but simply choosing the next-highest leakage score drops the heating load closer to 2.5 tons—which can mean a difference of $1,000 or more on your installation.

But in practice, installers rarely measure everything that could affect a heat pump's performance—and that's not necessarily a sign that they're sloppy. "It's hard for contractors to take every recommended step before you become their customer," Meaney says. It's a time-consuming process, and time is money.

For example, most pros that we've talked to don't perform blower door tests, which is the industry standard for measuring how drafty a home is. But with enough experience in a region—the quirks of its climate and housing stock—many pros argue that it's unnecessary. "You can gauge it based on the house," said one installer from Massachusetts. On top of all that, the Manual J isn't quite an exact science. So at a certain point, close enough is good enough.

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 (more on this concept below).

A higher capacity does not 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 very common for HVAC equipment (including heat pumps) to be significantly larger than needed. 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 the industry] in 2006," says Dave Lis, director of technology and market solutions at Northeast Energy Efficiency Partnerships. And in general, there are more incentives for installers to go big, including a tendency for customers to want more capacity, just in case. So they frequently build in "extra cushion," Lis says.

Even when contractors do take measurements and make accurate, un-cushioned estimates, the recommended heating loads might still be too high. "It's clear that Manual J is overstating loads," Lis says. Pages 26 and 27 of this report from the Cadmus group on real-world cold-climate heat pump performance demonstrate the point. (Cooling loads are generally more accurate, however.)

Bottom line: If you get multiple quotes that all recommend the same heat pump size, then it's probably correct (or correct enough). But, if different installers recommend slightly different sizes, Lis and Meaney both suggest erring on the side of the smaller model. The ACCA guidelines even suggest it's OK to undershoot the Manual J estimate by 10 percent. Just ensure that the particular heat pump you select and install is equipped to handle your local climate.

How to spot other 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 have no upsides 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 that 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, including that helpful blower-door test.

And 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 keeping a backup system or exploring the possibility of a hybrid heat pump-furnace setup. And certain heat pump models can't handle cold weather. But plenty of heat pump models can handle 100% of a home's heating needs in any part of the continental US. 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.

Heat pump size (aka capacity) is measured either in tons or Btu/h. Btu/h is often shortened to just Btu. One Btu (British thermal unit) is roughly the amount of heat from a burning match.

These units are on the same scale, like feet and inches: 1 ton equals 12,000 Btu/h. So, a 2-ton system has a capacity of 24,000 Btu (give or take a few hundred Btu, in practice), a 3.5-ton system equals roughly 42,000 Btu, and so on.

(Size has almost nothing to do with a heat pump's potential energy efficiency, measured in HSPF and SEER.)

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.

(Some heat pumps lose capacity when temps drop below freezing, while others are built to handle weather below 0 Fahrenheit without a hitch. In the past, inexperienced heat pump installers didn't always know to consider this. Today, trained installers typically don't make this mistake.)

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 might even need less maintenance over time than a mid-sized unit.

What happens if the size is wrong?

What happens if you don't get it right? When a heat pump is too small for your house, it won't hold a comfortable indoor temperature in harsh weather. But under sizing is uncommon, according to our research.

It's much more likely that you'll get an estimate for a heat pump that's too large. In that case, you could be throwing away thousands of dollars on the installation cost.

And when it's significantly too large, you'll have problems with short cycling, which is when a heat pump heats or cools a space too quickly and has to turn itself off and back on more often than it's designed for. That's bad for comfort, energy bills, and long-term reliability. Heat pumps work better on many levels when running for longer periods without shutting off.

In reality, heat pumps only come in half-ton increments, so you'll rarely find a perfect match for your home. It's generally OK for a heat pump to be slightly smaller or larger than the "official" estimate, even according to the Air Conditioning Contractors of America (ACCA)'s guidelines. They'll still run efficiently and keep your home comfortable—a totally satisfactory performance.

But once a heat pump is more than 15 to 30 percent oversized for your home (depending on the compressor and fan type), it will run inefficiently, struggle to keep your home comfortable, and may cost thousands of dollars extra to install.

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:

• The size of your home, including square footage and ceiling heights. Generally, bigger homes need bigger heat pumps—but not always.

• The layout of your home. Heat rises, so homes that sprawl horizontally tend to have a higher load than multi-story homes on a smaller footprint.

• Windows. More and larger windows increase your load because they leak heat faster than walls. Single-pane windows do, too—they have a lower U value than double-pane windows. And the more sunlight that your windows let in, the higher your cooling load—but the lower your heating load.

• Insulation. More or higher-quality insulation generally means your home has a higher R-value, which measures heat retention. Higher R values in your walls and attic or roof lead to lower loads—sometimes substantially so. Modern insulation can easily shave a half-ton or more off the load compared to older standards.

• Air leakage (aka air infiltration). Ideally, you'll get a blower-door test, which tells you how much air leaks through all the little gaps and cracks around your home. But most calculations rely on estimates that are pre-built into the Manual J procedure (more on those later), and each step up in "tightness" can reduce the building's load by a few thousand Btu.

• Local climate. On the 1% coldest days in your town, what's the temperature? What about on the hottest days? And the most humid? Your load—and the way you should pick a heat pump model—is based on these extremes rather than averages.

• Ductwork (for ducted heat pumps—not mini-splits). Duct placement matters; your load will be higher if they're in an unfinished basement or attic. Better duct insulation and air sealing will bring the load down, among other benefits. (Then there's the separate issue of duct sizing and whether they can handle the airflow your system needs to keep every room comfortable.)

• Body heat and heat-generating appliances. The more people in your house, the more body heat they'll give off (typically a few hundred Btu/h per person at rest). Appliances and electronics warm up your home, too.

• Preferred temperature. Every degree you turn the thermostat up in the winter or down in the summer can change the load by a few hundred Btu.

Small tweaks to the insulation values, air infiltration scores, and window types can add up to a difference of more than 1 ton in your building's load. If you miscalculate those factors, your estimate will be way off. (This should start to illustrate why simple shortcuts are a waste of your time.)

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 (and time is money).

This doesn't necessarily mean that they're cutting corners, either. Real-world experience counts for a lot, and after a few years in the field, installers might have a pretty good idea of how buildings tend to perform in their home region.

Installers might not even perform an "official" Manual J calculation, though it's tough to understand why: At least some of the approved software makes it pretty easy to get a decent estimate even without precise data. For example, most programs let you enter either a precise air-infiltration score or pick a qualitative score from a drop-down menu.

Can you just look at the capacity of your existing air conditioner, furnace, or boiler and use that as your target for a heat pump? The experts we've talked to don't recommend it. According to experts we've talked to, HVAC equipment tends to be massively oversized because most installers haven't traditionally taken many measurements. (It's also not as big of a deal when furnaces or boilers are oversized because the price doesn't increase much with the capacity.) And if you're in a climate with cold winters, the cooling capacity of your central AC probably won't translate to match your heating needs.

Manual J is not an exact science

Like any design discipline, HVAC is part art, and there's rarely one right answer to heat pump sizing. High-quality installers can come to slightly different conclusions, and that's normal—within a certain range, anyway. And even the ACCA (the authors of Manual J) acknowledge that there's some wiggle room: You'll still be comfortable even with equipment that's 10 percent undersized or up to 30 percent oversized (with a variable-speed compressor; fixed-speed systems don't have quite as much wiggle room).

Let's say that an installer runs a complete, rigorous Manual J calculation based on perfect information about a single-family house and finds that the load is 2.6 tons. Heat pumps only come in half-ton increments—the closest options are either 2.5 tons or 3 tons.

So is it best to round up to 3 tons to be safe? Well…maybe. Some so-called 3-ton systems actually work more like 2.9-ton systems, so that might be a good fit. Certain 3-ton systems might struggle to dehumidify a house on a muggy but relatively mild day, while other 3-ton systems will have no problem with that. And in some situations, some pros could recommend a 2.5-ton heat pump since Manual J (arguably) overestimates heating loads.

Capacity can change with the weather—sometimes

On top of that, capacity changes with the weather, so apples-to-apples comparisons can be tough. A heat pump's advertised size is based on its heating capacity at a very mild 47 degrees. But heat pump performance changes with the weather, and many lose their heating ability as temperatures drop below freezing. (It's a myth that heat pumps can't work in cold weather, but many models will indeed struggle.)

For example, a 3-ton single-stage heat pump that's built for a climate like Miami might act more like a 1.5-ton heat pump on a chilly winter night in New York City and a one heat pump during a cold snap in Chicago—just when your heating load would be at its greatest.

On the other hand, a 3-ton "cold climate" heat pump with a variable speed compressor can work like a 3-ton system all winter in NYC and Chicago. (NEEP maintains a database of heat pumps that can maintain at least some of their heating capacities in cold climates.)

Another complication: Loads of heat pumps fall somewhere in between that "warm climate" and "cold climate" distinction, and so do many parts of the US, including cities like Charlotte, Atlanta, and Dallas. They can drop to 20 degrees Fahrenheit, which is tough for a super-basic to handle. But an expensive cold-climate system would be overkill. (Ditto for Miami, from the previous example.) The top-line capacity specs aren't useful here, so installers must look deeper.

It's entirely possible in an in-between situation like this that you'll get one quote for a so-called 3-ton system and another for a 3.5-ton system. But when the temperature drops to the expected local low of 17 degrees, they both perform like 2.8-ton heat pumps—which might be exactly what both installers were aiming for.

Those installers must also consider how those same heat pumps will handle cooling and dehumidification. It gets a little bit complicated in climates with cold winters, moderately hot summers, and stretches with significant humidity. We won't peel back that onion here. But it's one more example of how great installers need to thread the needle and find smart compromises—and why that might lead to sizing inconsistencies between quotes.

CoolCalc is a free-to-use, web-based program for estimating heating and cooling loads. We've talked to pros who use it, and it's on the ACCA's list of approved software for Manual J calculations. There are paid tiers for CoolCalc, but if you're just looking to estimate the right heat pump size for your home, all you'll need to do is register with an email address.

By default, CoolCalc automatically fills in huge parts of the Manual J calculation based on your address and the local building codes during the year the home was built or most recently renovated. Then it walks you through a long list of inputs (similar to what we outlined above) and allows you to correct any details that it got wrong.

I personally played around with CoolCalc to get an estimate of my house's heating and cooling loads, assuming that I was installing a ducted, whole-home heat pump. The interface isn't polished, but it's adequate. It took me about 15 minutes to initially work my way through all the input pages. I'm reasonably confident the estimate I got was in the ballpark of what a pro installer would find—probably within a half-ton.

That said, I'm a well-educated consumer, and I was able to correct a lot (but probably not all) of the errors that the auto-fill feature made. I still had to Google some details, like what a window's U-value means; the R-value of the fiberglass batts in my roof; and whether the air-infiltration score for my 100-year-old house with blown-in cellulose insulation should be scored as Average, Semi-Tight, Semi-Loose, or something else.

In all of those cases, the details that CoolCalc auto-filled turned out to be incorrect (I think) and swung the final heating load by a couple of thousand Btu total. Most homeowners won't catch those errors.

It was striking to see how much the estimate changed when I did adjust certain inputs. At first, I indicated that my ductwork was in an unconditioned space, which gave me a heating load of around 42,000 Btu/h. When I corrected that single mistake (my entire house is conditioned, actually), the estimate dropped to about 37,000/h—almost a half-ton. That difference alone could raise the price of a heat pump installation by a few thousand dollars.

A comfortable, efficient heat pump system starts with the installation. EnergySage now has a Heat Pump Marketplace where you can receive quotes from experienced, vetted heat pump contractors in select areas. Have more questions about heat pumps? When you sign-up (free of charge!) for the marketplace, we'll connect you with an EnergySage Energy Advisor—one of our in-house heat pump experts who can help guide you through the installation process.

And don't forget: When heat pumps run off of renewable, zero-emissions electricity from solar panels, they're even better for the environment and can even eliminate your heating and cooling bills entirely. The EnergySage Marketplace can help you find a top-quality solar installer in your area.