Do heat pumps work in cold climates?

A heat pump heats a home by moving heat from the outdoor air indoors. There is heat to move even in very cold air, but the colder it gets outside, the harder the compressor works and the less heat it delivers. Whether a heat pump “works” in a cold climate is therefore not a yes-or-no question about the technology. The question is whether a particular unit still delivers enough heat for a particular house on the coldest days where that house is.

What makes a heat pump “cold climate”?

In the United States, the most widely used definition comes from ENERGY STAR. Version 6.2 of its specification for central air conditioners and heat pumps lets a heat pump carry the Cold Climate designation only if it passes a low-temperature test at 5 °F (−15 °C) as well as the standard ratings.

Requirement	Ductless split	Ducted split	Single package
SEER2 (cooling)	≥ 15.2	≥ 15.2	≥ 15.2
HSPF2 (seasonal heating)	≥ 8.5	≥ 8.1	≥ 8.1
COP at 5 °F (−15 °C)	≥ 1.75	≥ 1.75	≥ 1.75
Heating capacity at 5 °F, share of capacity at 47 °F (8.3 °C)	≥ 70%	≥ 70%	≥ 70%

The specification adds a third condition that is easy to miss: a controls verification procedure. The unit has to reach those 5 °F figures under its own controls, “operating as they would in a customer’s home”, not only with the compressor locked at a test speed. Gas/electric package heat pumps are not eligible for the Cold Climate designation at all.

Two of the rows need a word of explanation:

• The coefficient of performance (COP) is heat delivered divided by electricity used. A COP of 1.75 means 1.75 units of heat for every unit of electricity. Electric resistance heat, such as baseboard heaters or furnace strips, has a COP of 1.
• HSPF2 is a seasonal heating rating under the federal test procedure. It summarizes a whole heating season, while the 5 °F figures describe one cold hour.

The Northeast Energy Efficiency Partnerships (NEEP) keeps a separate cold-climate specification and product list. Its Version 4.0 has been in effect since January 1, 2023, and a Version 5.0 is in development. If a rebate program names one of these lists, check that the exact model combination appears on it.

How much heat does a heat pump lose in the cold?

Every air-source heat pump delivers less heat as the outdoor temperature falls. The ENERGY STAR threshold allows a certified cold-climate unit to lose up to 30% of its 47 °F capacity by the time it reaches 5 °F. Some units do better. In the DOE’s Residential Cold Climate Heat Pump Challenge, launched in 2021, ducted units had to deliver at 5 °F at least as much heat as at 47 °F, a capacity ratio of 100%. They also had to reach a COP of 2.4 at 5 °F for units up to 48,000 Btu/h and 2.1 for larger ones.

Test point	°F	°C	What it tells you
Standard heating rating	47	8.3	Nameplate capacity and efficiency in mild weather
Low-temperature rating	17	−8.3	Capacity left in a typical cold spell
Cold-climate test	5	−15	The point ENERGY STAR and NEEP report for cold-climate units
DOE Challenge compressor cut-out, at the latest	−10	−23	Challenge units had to keep the compressor running at least this low

Eight manufacturers committed to the Challenge: Bosch, Carrier, Daikin, Johnson Controls, Lennox, Midea, Rheem and Trane Technologies. The Challenge set targets for the prototypes. It is not a label, so a product carries no “Challenge” rating. Check the ENERGY STAR or NEEP listing and the manufacturer’s extended performance data for the model you are quoted.

How do I know if a heat pump is big enough for my house?

Compare two numbers at the same outdoor temperature:

1. Your home’s heat loss at the local design temperature. The design temperature is not the coldest temperature ever recorded. It is the 99% heating value: the outdoor temperature your location stays above for 99% of the hours in a year. That leaves about 88 hours a year colder than design. The home’s heat loss at that temperature comes from a room-by-room load calculation such as ACCA Manual J.
2. The heat pump’s capacity at that same temperature. It is listed in the manufacturer’s extended performance data, not on the nameplate.

The DOE Building America guidance for cold-climate heat pumps puts it bluntly: “Equipment should not be selected based on nameplate capacity. Instead, the capacity at design conditions must be determined.” It recommends plotting the home’s heating load line against the heat pump’s maximum and minimum capacity across outdoor temperatures. The temperature where the load line crosses the maximum-capacity curve is the balance point. Below it, the heat pump alone can no longer keep up.

The same guidance warns against rounding up. When the Manual J process is followed, “additional upsizing or rounding up is not needed”. An oversized unit spends mild days switching on and off, which costs efficiency and comfort.

Should the heat pump be sized for heating or for cooling?

In a cold climate the heating load is usually larger than the cooling load, so the two goals pull in different directions. The Building America guide lays out four approaches:

Approach	Sized to	Backup heat
1	The design cooling load; a cold-climate unit is not necessary	Needed for much of the winter; balance point around 40 °F (4 °C)
2	Cooling, with a variable-capacity unit chosen so its mid-range meets the cooling load	Needed in colder weather
3	Somewhat undersized for heating, for example around 80% of the heating load	Needed on the coldest days
4	The full design heating load at the local design temperature	Only below the balance point, which can be −15 °F (−26 °C) or colder

Sizing for heating usually means the unit is larger than the cooling load requires. That is why the guide points to variable-capacity heat pumps. Their compressors can turn down to a low output in summer and on mild winter days instead of cycling on and off.

What happens below the balance point?

Below the balance point something else has to supply the missing heat. ENERGY STAR’s own advice is that a cold-climate heat pump “will continue working at temperatures below 5°F, but pairing it with a back-up energy source will heat your home the most efficiently when temperatures are even lower.”

There are two common arrangements:

• Auxiliary heat strips inside the air handler. They are electric resistance heaters with a COP of 1, simple and cheap to install but expensive to run for long periods. The Building America guidance stresses keeping resistance backup to a minimum, especially in climate zone 4 and colder.
• A dual-fuel system, in which a gas or oil furnace takes over below a set outdoor temperature.

Which one costs less to run depends on local electricity and fuel prices.

How do I read extended performance data?

Manufacturers publish extended performance tables for each combination of outdoor and indoor unit. For heating, a table usually lists, for a range of outdoor temperatures, the minimum and maximum capacity in Btu/h, the power input and the COP. Three things decide whether the numbers apply to your system:

• Use the capacity the unit can sustain. For the DOE Challenge, maximum capacity meant the output at “the highest compressor speed allowed for extended periods of operation (i.e. not including temporary ‘boost’ or ‘turbo’ modes)”. A figure reached only in a boost mode is not the one to size a house on.
• Look at the minimum as well as the maximum. When the home’s heat loss on a mild day falls below the unit’s minimum output, the heat pump cycles on and off. The Building America guide asks for the heat pump’s modulating range to overlap the home’s load line as much as possible.
• Check the exact pairing. ENERGY STAR requires manufacturers to state that split systems, other than ductless ones, “must be matched with appropriate coil components to meet ENERGY STAR criteria”. A rating belongs to a matched outdoor and indoor unit, not to the outdoor unit alone.

To compare with metric figures: one kilowatt is about 3,412 Btu/h, so 12,000 Btu/h, one ton, is about 3.5 kW.

What should I ask a contractor?

• Which ENERGY STAR or NEEP listing does the model appear on, and under which exact indoor and outdoor model numbers?
• What is the unit’s heating capacity at our design temperature, from the extended performance data?
• What heat loss did the load calculation find at that temperature? Can I see the Manual J report?
• Where is the balance point, and what supplies heat below it?
• At what outdoor temperature does the compressor cut out, and when does it restart?
• How is the outdoor unit protected from snow and drifting, and where will defrost water drain?

A contractor who answers these with numbers from the load calculation and the extended performance tables is sizing the system for your house.