Short answerA cold-climate heat pump uses an inverter-driven variable-speed compressor and enhanced vapor injection to keep delivering most of its rated heat at 5°F, and useful heat down to minus 13 or lower. For Southeast Michigan, look at the published capacity at 5°F and 17°F, not the nameplate tonnage, size it to the heating load at the local design temperature, and keep backup heat for polar-vortex nights.
What "cold-climate" actually means
Any heat pump will run in cold weather; the question is how much heat it makes. A conventional single-stage heat pump that produces 36,000 BTU at 47°F might produce 20,000 at 17°F and 12,000 at 5°F, with efficiency falling just as fast, which is why those units have an electric strip heater or a furnace standing behind them.
A cold-climate heat pump is engineered to hold capacity as the temperature falls. The important pieces are an inverter-driven compressor that can speed up when it is cold, a larger outdoor coil, and on most models enhanced vapor injection (EVI), which injects partly expanded refrigerant into the compressor mid-cycle to boost output at low temperatures. The practical result is a unit that still delivers 70 to 100 percent of its rated capacity at 5°F and keeps running, at reduced output, to minus 13 or minus 22°F depending on the model.
The industry benchmark is the NEEP cold-climate specification, which requires a certain COP at 5°F and publishes capacity at 47, 17 and 5°F for listed models. The federal ENERGY STAR cold-climate designation is similar. When a salesperson says "cold-climate," ask for the 5°F capacity number; that is the one that matters in Washtenaw County.
Reading the performance table
Every real cold-climate unit publishes an expanded performance table. For a nominal 3-ton unit you might see something like 36,000 BTU at 47°F, 30,000 at 17°F, 26,000 at 5°F, and 20,000 at minus 13, with a COP that falls from around 3.5 at 47°F to about 2 at 5°F. Those are the numbers that get compared against the house's load.
A 1,800-square-foot 1970s colonial in Plymouth with decent insulation might have a heating load of about 45,000 BTU per hour at our design temperature of roughly 1 to 5°F. The nominal 3-ton unit above delivers 26,000 at 5°F, so it cannot carry the house on its own at design; it would need backup, or a 4- or 5-ton unit, or insulation work to cut the load. That is the arithmetic that decides whether a heat pump is a primary heater or a shoulder-season heater in a given house, and it has to be done, not guessed.
Sizing for Michigan: heating governs
In most of the country a heat pump is sized for cooling and happens to heat. In Southeast Michigan the heating load is usually two to three times the cooling load, so a heat pump sized for cooling is badly undersized for January. Sizing it for heating means a larger unit than the cooling load wants, and a larger unit that cannot modulate low enough will short cycle and dehumidify poorly in July.
This is where the inverter compressor earns its keep. A variable-speed cold-climate unit might modulate from 30 percent to 110 percent of rated capacity, so a 4-ton unit sized for winter can run at the equivalent of 1.5 tons on a humid August afternoon. The sizing conversation becomes: pick a unit whose 5°F capacity covers most of the design heating load, confirm its minimum capacity is low enough for summer, and decide how to handle the few hours a year below design. What size air conditioner do I need explains the cooling side of that balance.
Defrost, and why the vents go cool for a few minutes
Below about 40°F, moisture in the outdoor air freezes on the outdoor coil. The unit periodically reverses into cooling mode for a few minutes to melt it, during which the indoor coil gets cold and the supply air drops. On a dual-fuel system the furnace fires during defrost; on an all-electric system the strip heat does. You will see steam rising from the outdoor unit and hear a whoosh as the valve shifts. Defrost is normal, and it happens more often in the damp 25-to-35°F weather that Southeast Michigan has for weeks at a time than in the dry deep cold. A unit that defrosts constantly, or that ices into a solid block, has a sensor or charge problem and is a heat pump repair call.
Outdoor unit placement matters here. The coil sheds water during defrost, and if the unit sits on a pad at grade on the north side of a Canton house, that water freezes under the unit and lifts it. Cold-climate units should sit on a stand 12 to 18 inches above grade, clear of roof drip lines, with room for the meltwater to go.
Backup heat: what kind and how much
Even a well-sized cold-climate heat pump needs something behind it for the handful of minus-15 nights and for defrost. The options:
- Gas furnace (dual fuel). The best backup in a gas-connected home. The thermostat switches over at a set outdoor temperature, and the heat pump shuts off entirely below it. Cheapest to run in deep cold on DTE rates. Heat pump vs furnace in Michigan covers the economics.
- Electric resistance strips in the air handler, 5 to 20 kilowatts. Simple and standard in all-electric installs. Expensive to run and hard on a 100-amp panel; a 15 kW strip pulls over 60 amps on its own.
- Propane furnace in rural townships, where the heat pump will almost always be cheaper than propane above about 10°F, so the switchover is set low.
Lock out the strips above about 30 to 35°F so they do not run during defrost on mild days, and set the thermostat to prevent "auxiliary" heat from kicking in on every temperature setback. Those two settings account for most of the shocking electric bills people blame on the heat pump.
The homes where a cold-climate heat pump truly works
- Propane homes in Augusta, Sumpter, Salem, Lodi and York townships. Propane per BTU costs far more than natural gas, so a cold-climate heat pump wins on cost at nearly any temperature above zero.
- Tight newer construction in Saline, Pittsfield and Northville, where the heating load is low enough that a 3- or 4-ton unit covers design and the strips rarely run.
- Homes with solar or on DTE's time-of-day rates, where shifting heating to electricity has an advantage.
- Houses with new AC due anyway and a working furnace, where the dual-fuel premium is modest and the heat pump does the bulk of the shoulder-season work.
And the homes where it is harder: a 1920s Ypsilanti two-story with a 90,000 BTU load and 100-amp service, where carrying the house electrically would need a 5-ton unit plus 20 kW of strips and a panel upgrade. Insulation and air sealing first, then a right-sized heat pump, is the better order there.
A note on refrigerant and the 2025 equipment change. New heat pumps since 2025 use R-454B or R-32 rather than R-410A. Both are mildly flammable (A2L) and require leak sensors and specific installation practices, which is one reason the installer's training matters more than it did. Neither affects performance in cold weather in any way a homeowner would notice. R-454B: the new refrigerant in 2025 has the details.
Incentives and getting it right
Federal 25C tax credits apply to heat pumps meeting the current efficiency tiers, and DTE Energy has offered rebates on qualifying heat pump installations, with larger amounts for cold-climate models in some program years. Those shift year to year, and we confirm what applies when we quote.
A heat pump installation estimate from us starts with a Manual J heating load at the local design temperature, then matches a unit's 5°F capacity against it, then checks the electrical panel, the duct capacity and the outdoor placement. If the numbers say the house needs insulation before a heat pump can carry it, we will tell you that instead of selling a bigger unit.