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

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!)

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.