When it comes to heat pumps, bigger is not always better

A heat pump that’s too big raises costs and reduces comfort. Here are some tips for sizing your system right.
By Alison F. Takemura

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A technician works on a heat pump installed at the exterior wall of a home

Heat pumps may be a near-magical kind of technology, but they’re not one-size-fits-all. In fact, selecting the wrong size can have miserable consequences for your wallet, your comfort — even your health.

Unfortunately, installers often propose systems two or even three times bigger than needed, according to Mike Simons, training and development manager at Abode Energy Management, a home-efficiency and decarbonization consultancy. Oversizing can make heat-pump projects wildly unaffordable,” Simons told Canary Media, occasionally even leading to instances in which what should cost $15,000 is now $40,000.”

That’s a big problem that’s only getting bigger as heat pumps explode in popularity across the country, outpacing gas furnace purchases in the U.S. last year.

Before we go deeper into the oversizing problem and how to fix it, some background: The size of a heat pump is basically its maximum power output, measured in British thermal units per hour.

The smallest residential heat pumps are usually around 6,000 Btu per hour, while the largest ones can get up to 60,000 Btu/​hr. What size you select should approximately match your home’s heating and cooling needs, which are driven by a number of factors, including your local climate, home size and airtightness, and how many people live with you. (Body heat!)

Table of heat pump sizes in Btu per hour, tons and kilowatts. 12,000 Btu per hour is 1 ton, or 3.5 kilowatts.
Common heat pump sizes in Btu per hour, tons (often tossed out in conversation with installers) and kilowatts (Allison Bailes)

When people install heat pumps that are too big, they lose money upfront by needlessly buying a more expensive model.

But they also lose money on an ongoing basis because oversized systems run less efficiently.

This is because heat pumps are designed with minimum power outputs, or capacities, that they can’t operate below. They’re like galloping horses that can’t slow to a walk. So a heat pump that’s too powerful will dump large amounts of hot or cold air into a home quickly, kicking on and off repeatedly while trying to meet the set temperature. That behavior also causes greater wear and tear on the unit and reduces its lifespan.

But there’s more: On hot, humid days, oversized heat pumps have a moisture problem.

Properly sized systems operate almost continuously, which allows them to dehumidify as well as cool a space; as warm, damp air runs over a cold coil, water condenses out and is drained away.

An oversized system, though, doesn’t run long enough to dehumidify, Simons said. Instead, it can make rooms cold and clammy” — a horrible” environment that can cause rot, mold and respiratory issues.

How to size your heat pump properly

So what can you do to accurately size your heat pump? Do a little legwork to make sure your HVAC expert installs what you need.

First, get an accurate estimate of your heating and cooling loads. You can ask your contractor to assess them, hire an energy consultant, or, if you’re an ambitious DIYer ready to steep yourself in building-science concepts, calculate them yourself using free tools. Cool Calc is approved by the Air Conditioning Contractors of America for making the industry-standard Manual J” estimates for a home’s heating and cooling needs.

Carbon Switch, which provides home-electrification guides, recommends having a blower door test performed. It measures a home’s air leakage, a value that feeds into the Manual J calculation.

If you love a good data table, Simons also advises checking the specs on the heat pumps you’re considering. The Northeast Energy Efficiency Partnerships maintains a list of heat pumps that includes their minimum and maximum capacities, which vary with temperature.

A table of performance specs for a Mitsubishi heat pump
Performance specs can help you match a heat pump to your home’s needs. This table was pulled from the NEEP database for a Mitsubishi heat pump. For example, the portion circled in red shows the heat pump’s minimum cooling capacity when the thermometer shows 95˚F outside: 13,400 Btu per hour. (NEEP)

Selecting a minimum capacity that matches your home’s cooling load on the hottest days will help keep it comfortable instead of clammy.

In other words, it all boils down to a deceptively simple edict:

Match the equipment to the load,” Simons said. That’s what all manufacturers ask for.”

Now that you know how to avert a major potential problem with heat pumps, here are 10 questions to ask if you want one.

Alison F. Takemura is staff writer at Canary Media. She reports on home electrification, building decarbonization strategies and the clean energy workforce.