Cold climate heat pumps keep working in freezing weather

Heat pumps are an efficient and clean way to heat a home with electricity, but their efficiency drops as the air temperature gets colder. Heat pumps don’t stop working when it’s really cold, but once the outdoor temperature drops below freezing, conventional heat pumps may need to switch to a backup heating method to keep a home warm.

That’s when cold climate heat pumps (CCHPs) are a good choice. They continue to work even when the temperature drops well below freezing, eliminating the need to use backup heat in all but the most extreme temperatures. In fact, the countries that are the biggest users of heat pumps are Finland, Norway, and Sweden¹.

That increased efficiency in cold weather means that cold climate heat pumps will produce more heat with less electricity, but the tradeoff is that they cost more. Whether that’s worth it is something you will need to discuss with your HVAC installer.

How do air-source heat pumps work?

Heat pumps work like your refrigerator: a heat exchanger takes heat from one location to warm up a refrigerant, which is then moved to another heat exchanger where the heat is expelled.

In a refrigerator, one heat exchanger is located inside the food compartment while the other is outside, either at the rear or bottom of the appliance. In a heat pump, there’s an outdoor heat exchanger and one or more indoor exchangers that transfer the heated refrigerant to an air exchanger or water (in the case of hydronic systems, which are popular in Europe).

A refrigerator can extract heat from the air, even inside the freezer compartment. This is also true of a heat pump: even when there’s snow outside, a heat pump will still continue to work.

This is because the freezing point of water isn’t significant to a heat pump. A heat pump uses a refrigerant (commonly R-410A or newer refrigerants with less global warming potential such as R-454B or even propane), which doesn’t freeze at 32°F.

(That said, ice buildup can block the operation of the outdoor unit, but that’s a mechanical issue that’s different from the physics of why heat pumps don’t work as well in the cold. Outdoor heat exchangers have a defrost cycle to deal with ice buildup.)

The efficiency of conventional heat pumps becomes noticeably worse when it’s freezing outside not because the heat pump itself “freezes”, but because there’s less heat energy available in the air.

Heat pumps don’t suddenly stop working when it’s really cold outside. They will still continue to extract heat from the cold outdoor air, but the issue is that they may not be able to generate enough heat to maintain your desired indoor temperature.

When should you use a cold climate heat pump?

A conventional heat pump will be configured to switch to backup heat (more about that later) around 25°F (-4°C) or so. If your climate drops below this temperature only occasionally, you probably don’t need a cold climate model.

However, if you experience temperatures like this several weeks a year, it may be a good choice to choose a cold climate model, which will have lower operating costs.

Why cold climate heat pumps work better in extreme cold

Cold climate heat pumps use a few technologies that make them more efficient even in extreme cold, such as inverter-driven compressor motors, variable-speed compressor fans, and electronic expansion valves that can more precisely control the refrigerant flow.

With these improvements, a cold climate heat pump can work well even when it gets down to 5°F.

In the United States, “cold climate heat pump” is a certification that the unit meets a set of specifications by ENERGY STAR and the Northeast Energy Efficiency Partnerships².

The specifications are different for ducted and ductless heat pumps. If you’re not familiar with terms like HSPF and COP, you can read my primer on heat pump terminology to learn more.

One challenge is that finding the HSPF2 and SEER2 ratings for a heat pump is usually easy, but the 5°F COP rating can be difficult to find. The cold climate label is helpful for consumers because it guarantees that the unit meets the minimum standard.

Performance differences between cold climate and conventional heat pumps

When discussing heat pump performance, it’s useful to know more about COP.

The term Coefficient of Performance (COP) is often used to describe the heat output of a heat pump. It’s always specified at a particular temperature because the efficiency of a heat pump is better in warmer weather (and better in cold weather in cooling mode).

COP is measured relative to the heat output of an electric resistance coil. Resistance coils are found in common household appliances like toasters and hair dryers, and they work by passing electricity through a wire with high electrical resistance, which causes the wire to heat up.

The conversion rate is 3,412 BTUs of heat for every 1 kilowatt-hour of electricity consumed. This is the same for any electric resistance coil.

COP is a factor based on this rate. This means that a toaster or hair dryer has a COP of 1. If a heat pump has double this efficiency – that is, it generates 6,824 BTUs of heat for every 1 kWh of electricity – it would have a COP of 2.

COPs of heat pumps varies quite a bit by manufacturer and model, but a cold climate heat pump may have approximately 30% better efficiency than a conventional model. The graph below provided by the Heat Pump & Thermal Storage Technology center of Japan is based on heat pumps manufactured by Daikin:

Just keep in mind that these numbers are based on one manufacturer, so if you want to compare models, you’ll need to look up their specs. Fortunately, HSPF2 is also a ratio based on the BTU output of the unit. (HSPF2 differs because it’s an estimate of the average heat output of the unit over a typical winter season, rather than an estimate a specific temperature.) Specifically, a heat pump with an HSPF2 rating of 9 has an average heat output of 9,000 BTUs/kWh, while one with an HSPF2 rating of 12 has an output of 12,000 BTUs/kWh, and so on.

Given that a conventional heat pump might have an HSPF2 rating of 16 and cold climate models are usually 20 and above, the 30% estimate is a pretty reasonable approximation. To compare two heat pumps, just divide their HSPF2 numbers. For example, a 22 HSPF2 unit is 37.5% more efficient than a 16 HSPF2 model (22 divided by 16).

What is backup heating?

As mentioned earlier, a heat pump doesn’t suddenly stop working at low temperatures. Instead, the heat output of a heat pump drops as temperatures get lower. If the temperature is low enough, the heat that your home loses through the walls and roof will be greater than the heat output of the heat pump, causing your home to slowly get colder.

In this situation, the thermostat will ask for a backup heat source to kick in. The temperature at which this happens is called the balance point, and it’s configured in your thermostat. Your heat pump installer will select a balance point that’s appropriate for your setup.

There are different options for backup heat. For ducted heat pumps, this is often an electric resistance coil that is added to the air handler. This is often called “strip heating”, and it has a few advantages: the equipment cost is low, and the heat output is limited only by the capacity of the electrical circuit. This means that if you have a large enough circuit, strip heating will be able to keep your house warm even in the coldest weather.

The disadvantage of strip heating is poor efficiency. Electric resistance heaters have a COP of 1, which means that a cold climate heat pump operating at 5°F will be at least 75% more efficient. This is why if you have very cold winters where you live, it’s often more economical in the long run to choose a cold climate heat pump over a conventional model because you’ll use less backup heat.

Another option is to use a furnace for your backup. This is what I did in my home. When I upgraded my central air conditioner to a heat pump, I kept my existing gas furnace as backup heat. When the temperature drops below the balance point set in my thermostat, my furnace is used to heat my house instead of my heat pump. This happens perhaps for two weeks out of the year, so it’s not a significant amount of the overall heating season for me.

Bottom line: heat pumps work very well even in the freezing cold, but be sure to get the right model for your climate

Contrary to what you might expect, heat pumps can still efficiently heat your home even when it’s bone-chillingly cold outside. The popularity of heat pumps in Nordic countries demonstrate their practicality in subarctic and even arctic climates.

The ENERGY STAR “cold climate” designation ensures that a heat pump has the performance specifications to perform well in the cold. Cold climate heat pumps can be around 30% more efficient than conventional models.

But even non-cold climate heat pumps can still be used in cold climates. The heat pump will take care of heating during all of the the shoulder season and during average cold temperatures. Backup heating will take over only for the coldest days of the year. This means that a heat pump can benefit you for most of the heating season as well as all of the cooling season.

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