Do Heat Pumps Work When It Gets Properly Cold?

Will it still work when it is properly cold? It is the most reasonable question anyone asks about a heat pump, and it has a genuinely useful answer, though not a one-line one.

How Capacity Behaves

Output Falls as It Gets Colder, and by How Much Depends on the Unit

A heat pump collects heat from outside air. The colder that air is, the less heat there is to collect, so output drops as the temperature drops. That much is true of every unit ever built. What is not universal is the shape of that curve: how much output a specific model holds on to at a specific outdoor temperature is a property of that model, published by its manufacturer in a performance table.

About the Numbers You Will Be Quoted

A Figure Without Its Conditions Is Not a Specification

You will be shown claims of the form: this unit produces full rated output well below freezing. That may be perfectly accurate for a particular model, and on its own it means nothing, because three things are missing from it.

Ask for that table. An installer who cannot produce it is quoting a brochure, and a brochure is not a specification.

• Which model exactly, including the outdoor and indoor units as a matched pair
• What full output is being measured against, since rated capacity is defined at a stated condition rather than in the abstract
• The test conditions the figure was taken under, which are printed alongside it in the manufacturer's own table

How We Work It Out

What Gets Checked Before Anyone Answers for Your House

This is the part that turns a number in a brochure into an answer about your building.

• The design temperature actually used for your area, rather than a national average
• Heat loss across the building at that temperature
• The performance table for the specific model under consideration, at that temperature
• Whether a backup heat source is part of the plan, and what brings it on
• How the system behaves during defrost, and what you will notice when it does
• What the electrical service can carry, if that backup is electric

What We Check Before Recommending a Heat Pump