How Heat Pumps Work, and How They Actually Perform in the Cold

By Max Fisher · Published 2026-10-22 · Updated 2026-10-24

Short answer. A heat pump does not burn anything. It moves heat that already exists from one place to another, the way a refrigerator moves heat out of its box. Because moving heat takes less energy than making it, ENERGY STAR says an air-source heat pump “can deliver up to three times more heat energy to a home than the electrical energy it consumes.” Newer cold-climate models keep heating at outdoor temperatures that older units could not handle, but performance is model-specific, so good heat pump installers show you the manufacturer’s capacity data instead of a slogan.

The cycle, in plain terms

Every heat pump has the same four parts working in a loop: an outdoor coil, a compressor, an indoor coil, and a valve that lets the refrigerant expand. Refrigerant runs through all of them.

In heating mode:

  1. The refrigerant reaches the outdoor coil colder than the outdoor air, so it picks up heat from that air, even on a cold day.
  2. The compressor squeezes the refrigerant, which raises its temperature well above room temperature.
  3. At the indoor coil, that hot refrigerant gives its heat to the air your blower sends through the house, or to the room directly in a ductless unit.
  4. The refrigerant passes through the expansion valve, gets cold again and returns outside to repeat.

In cooling mode, a reversing valve sends the refrigerant the other way. Heat is picked up inside and released outside, exactly like a central air conditioner. ENERGY STAR describes a ducted air-source heat pump as “a central air conditioner that also works in reverse.” That is why one system can replace both a furnace and an air conditioner.

Why efficiency is described differently

Furnaces and boilers are rated by what share of the fuel’s energy becomes heat, so their ratings top out below 100 percent. A heat pump is not converting fuel. It is moving heat, so the heat delivered can be larger than the electricity used. The ratio of heat delivered to electricity consumed is called the coefficient of performance (COP). A COP of 3 means three units of heat for one unit of electricity.

COP is not fixed. It falls as the outdoor temperature drops, because there is less heat in the air to collect and a larger gap between outdoors and indoors to push against. That single fact explains most of the cold-weather debate.

Where the “doesn’t work in the cold” reputation came from

The standard heating rating for air-source heat pumps, HSPF, has a gap. The Northeast Energy Efficiency Partnerships (NEEP) points out that the HSPF test does not include any test point below 17°F, assumes electric resistance backup, and tests at steady state instead of letting variable-speed equipment modulate. A buyer in a cold place had no standard way to see how a unit performed when it mattered most. Single-speed units that lost capacity quickly in cold weather and leaned on resistance strips earned the bad reputation.

NEEP’s response was the Cold Climate Air Source Heat Pump (ccASHP) Specification and Product List. The specification is aimed at heat pumps best suited to heat efficiently in IECC climate zone 4 and colder. Version 4.0 took effect January 1, 2023, and covers central ducted heat pumps, VRF multi-split systems and some packaged units. Listed products report standardized low-temperature performance data, so a contractor or homeowner can compare models at the temperatures that matter. NEEP also notes that some small packaged units with variable-speed compressors now operate at 5°F and below without supplementary electric heat.

ENERGY STAR makes the same point in consumer terms: many new certified air-source heat pumps “excel at providing space heating even in the coldest of climates” because of advanced compressors and refrigerants, and you should ask your contractor for an ENERGY STAR certified cold-climate model if winter temperatures regularly dip below freezing.

What decides how yours performs

Equipment is only part of it. Four things decide whether a heat pump keeps a DMV house comfortable in January:

1. The house’s heat loss. A leaky, under-insulated house needs more heat on the coldest day. Air sealing and insulation shrink that number, which can let a smaller, cheaper system do the job. See Is your home ready.

2. A real load calculation. ACCA’s Manual J is the national ANSI-recognized standard for residential heating and cooling loads. ENERGY STAR tells buyers their contractor should verify the right size “using a tool called Manual J.” Sizing from the old furnace’s label skips this.

3. Equipment chosen against that load. ACCA’s Manual S is the guide for selecting equipment that meets the calculated load. For heat pumps, that means checking the model’s capacity at your design temperature, not only its nominal size.

4. Installation quality. NIST’s study of heat pump installation faults found that duct leakage, refrigerant undercharge, low indoor airflow from undersized ductwork and refrigerant overcharge do the most damage to performance, and that some faults together are worse than each alone. A well-chosen unit installed badly will not deliver its ratings.

Backup heat and the balance point

Every air-source heat pump has a balance point: the outdoor temperature below which it can no longer supply all the heat the house loses. Below it, something else has to help. That might be electric resistance strips in the air handler, or an existing furnace in a dual-fuel setup. ENERGY STAR describes dual fuel as a way to pair a new heat pump with a relatively new furnace and use each when it makes the most sense.

A cold-climate model with a low balance point, sized from a Manual J for a sealed house, needs backup less often. Your installer should be able to tell you roughly where your balance point falls and what runs below it.

Ducted or ductless

ENERGY STAR notes that a ducted air-source heat pump can connect to the forced-air ductwork most American homes already have, and can often replace a central air conditioner or furnace when either needs replacing. Homes without ducts, or additions where running ducts is hard, can use ductless mini-split heat pumps, mounted on a wall or ceiling with an outdoor unit. The DOE’s Energy Saver heat pump page lists air-source, geothermal, ductless mini-split and absorption heat pumps as the main types. Geothermal is compared in its own pillar: air-source vs. geothermal.

What to ask an installer

An installer who answers with documents is easier to trust than one who answers with reassurance. Before you rely on anyone’s technical claims, check the license too: see contractor licensing.

How to verify this yourself

Tell us if a source changes. To find installers who list heat pump work, start with your city on the DMV directory, or read the sizing and installation guide.

Sources

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