Heat Pump Efficiency vs Temperature: COP Chart, Balance Point and Defrost

Why efficiency drops in the cold

A heat pump doesn't make heat; it moves it from outdoor air into your home. The colder the outdoor air and the warmer the air you want indoors, the harder the compressor has to work to pump heat "uphill." Physics sets a ceiling on how well any heat pump can do, known as the Carnot limit:

Maximum COP = T-indoor ÷ (T-indoor − T-outdoor), with temperatures in absolute units (°F + 460)

With a 95 °F indoor coil and 47 °F outdoors, the theoretical limit is about 555 ÷ 48 ≈ 11.6. At 5 °F outdoors it falls to about 555 ÷ 90 ≈ 6.2. Real equipment achieves a fraction of these limits, but the trend is the same: halve the temperature lift and you roughly double what is possible. Capacity falls too, because cold air is less dense and carries less heat into the outdoor coil, so the compressor moves less refrigerant mass per stroke.

COP and capacity by outdoor temperature

Typical ranges for ducted and ductless equipment at full speed, based on manufacturer extended performance data. Your model's exact curve is in its engineering data or the NEEP cold-climate heat pump list.

Variable-speed models often run more efficiently at part load than these full-speed figures, which is why their seasonal ratings (HSPF2) are so much higher. To turn a COP into dollars, use the heat pump running cost calculator.

What makes a heat pump cold-climate

Cold-climate heat pumps use variable-speed inverter compressors that can spin faster when it is cold, plus refinements such as vapor injection, larger outdoor coils and refrigerants suited to low temperatures. The common benchmark, used by the NEEP specification and ENERGY STAR's cold-climate tier, is a COP of at least 1.75 at 5 °F and enough capacity at 5 °F to be useful (around 70% of the 47 °F rating). Many current models keep running down to −13 °F to −22 °F. See the heat pump temperature range guide for operating limits by type.

The balance point

As it gets colder, your home's heat loss rises while the heat pump's capacity falls. The temperature where the two lines cross is the thermal balance point. Below it, backup heat (electric strips or a furnace) must make up the difference.

Worked example: a house loses 40,000 BTU/h at a 5 °F design temperature and nothing at 65 °F, so its loss rises by about 667 BTU/h for each degree colder. A standard heat pump delivers 36,000 BTU/h at 47 °F and 22,000 BTU/h at 17 °F, losing about 467 BTU/h per degree. Setting the two equal gives a balance point near 26 °F, where both are about 26,100 BTU/h. A cold-climate unit of the same nominal size might push the balance point below 5 °F.

There is also an economic balance point for dual-fuel systems: the outdoor temperature below which gas heat is cheaper than the heat pump at your utility prices. It depends on COP, gas price and electricity price; gas vs electric heating shows the comparison. Proper heat pump sizing sets where both points fall.

Defrost cycles

When outdoor air is between roughly 20 and 40 °F and humid, moisture freezes on the outdoor coil, which is running below freezing. Frost blocks airflow, so the heat pump periodically reverses into cooling mode for a few minutes to warm the coil and melt it. During defrost:

• The outdoor fan stops and you may see a cloud of steam rise from the unit. That is normal.
• The system briefly draws heat from the house, so many units turn on backup heat to avoid blowing cool air.
• Demand-defrost controls only defrost when sensors detect frost, which wastes less energy than older fixed-timer controls.

Defrost typically costs a few percent to around 10% of heating energy in damp, near-freezing climates, less in dry cold. A unit that stays iced over for hours, or ices the top of the coil while the bottom stays clear, has a fault worth a service call.

Getting the best COP in winter

• Keep setpoints steady. Deep night setbacks force a hard recovery in the morning, when it is coldest and backup heat is most likely to kick in.
• Set auxiliary heat lockouts so strips don't run above the balance point. A smart thermostat or installer setting can do this.
• Raise the outdoor unit above expected snow depth and keep 2 ft clear around it so defrost meltwater can drain away.
• Change filters regularly. Low indoor airflow raises head pressure and lowers COP in heating just as in cooling.

Frequently asked questions