Will Heat Pumps Work in Subzero Temperatures


heat pump subzero

More and more homes today are built with electric air sourced heat pumps because of how remarkably energy efficient these systems are longterm. However, a common thought is that heat pumps do not work well in cold climates. While this was true in years past, modern technology has improved heat pump efficiency in colder climates.

When considering a heat pump for your home, you may be wondering if a heat pump will work efficiently if temperatures drop below zero? Modern heat pumps can work efficiently in sub-zero temperatures for a short time period. If you live in a cold-weather climate that has extended periods of subzero temperatures, a gas furnace system or a dual fuel heat pump system with a gas-powered backup heat source is the best option.

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According to Trane (one the leading manufacturers of heat pump technology), “A heat pump is most effective in temperatures of 30 degrees Fahrenheit and above. A heat pump is most effective when supplemented by a secondary heat source at 25-30 degrees Fahrenheit and below.” 1

Obviously, there are limitations. If you live in the areas that have brutally cold winters, a gas furnace heating system, or a dual fuel heat pump system would be a more practical choice. Consult with your licensed heating and cooling contractor as every home is different and they can perform proper heat gain calculations to ensure you are getting the most from your investment.

The image chart from Trane shows which states are good candidates for air sourced heat pumps and which should have a gas furnace heat source.

To better understand how modern heat pumps can extract heat from subzero temperatures, it helps to understand how heat pumps work.

Many posts will confuse the heat pump’s ability to work in temperatures below zero with the heat pump’s methods of reaching maximum efficiency. So while you may have previously heard that a heat pump will not work if it gets too cold, what this really means is that the heat pump will switch methods of heating your home by using an external heating system (aka emergency heat) rather than the standard transference of external to internal hot air via the coil and fan.

How Heat Pumps Work in Cold Weather

All heat pumps have a heating and a cooling mode. It is important to note that heat pumps work to transfer the hot air from the outside inward or vice versa. Heat pumps operating in standard functioning do not create heat but provide the capacity for heat energy to be transferred from the hot location to the cold location per the laws of physics. 

In cold weather, the heat pump will trap heat energy (that is above 40 degrees Fahrenheit) from the outside air using a liquid refrigerant, pressurize this cool gas through a compressor to create a hot gas, and cool the hot gas through the cooling airflow. This will cause the internal air temperature to rise (aka your home to become warm) and the hot gas to turn to a warm liquid which will then pass to the outside unit with less pressure resulting in a cool liquid to repeat the process. 

If there is not enough heat energy outside (typically in weather below 40 degrees Fahrenheit), the heat pump will use emergency heat (through electric or gas) to supply the needed bursts of heat energy to maintain the temperature set in your home. It will use its standard method only so long as the heat energy is being transferred efficiently; when it has to “work too hard” to produce heat, emergency heat will then fill in the gaps so that the heat pump is not being over-labored. 

Thus, the heat pump has automatic settings in which to operate efficiently and using the best method of heating your home. Do note, though, that you should not switch your heat pump to use emergency heat unless you are suspicious that your heat pump is not working for some reason. The manual adjustment is not necessary here and could cause your heat to run entirely on emergency heat which would result in high energy consumption and thus a high energy bill as well.

If your heat pump is not heating, see our article 16 Reasons Your Heat Pump Doesn’t Blow Hot Air

How Cold Can You Run a Heat Pump

You can run a heat pump at all temperatures as it is designed to switch to emergency heat when it reaches below 25-30 degrees Fahrenheit. Therefore, there is no need to manually switch an air heat pump off, even in the coldest temperatures, because it was created with efficiency in mind. Your heat pump will simply recognize that it is not functioning efficiently if it gets too cold, and will switch to its alternative emergency heat setting.

Many people confuse the emergency heat setting on their thermostat with what the heat pump recognizes as “emergency”. The emergency heat setting on your thermostat manually overrides the heat pump’s standard mode of operation. Thus, it will require your home to run entirely off of “emergency heat” through electric or gas supply. 

The heat pump’s “emergency heat” mode, on the other hand, only uses these methods of heating the home when it cannot efficiently maintain your desired temperature within the home on its own. By manually intervening, you will likely decrease your home’s heating system’s efficiency, thus resulting in a high energy bill unnecessarily. If you simply leave your device to operate as it was designed to do, you should not have any issues.

If it is above 40 degrees Fahrenheit, though, and you notice that your heat pump is constantly blowing, it may be time to call a repairman to check out the system. Your heat pump can be turned on at all times, but should not require a constant stream to maintain heat within your home. In other words, just because your heat pump is “on” does not mean that it should be constantly “working” to transfer heat.

If your heat pump is not producing heat like you think it should, we have 16 Reasons Your Heat Pump Doesn’t Blow Hot Air which may help shed some light on why you’re not getting any heat.

What is the Lowest Temperature a Heat Pump Will Work

While some standard air heat pumps can operate as low as -4 degrees Fahrenheit, the device will have to work extremely hard to pull heat from outside into your home if the external temperature is below 35- to 40- degrees Fahrenheit. When temperatures reach below freezing, the heat pump will then use emergency heat to supplement the heat energy needed to warm your home.

If you consider how the physics of a standard air heat pump system works, you will recognize that the transference of heat energy is the most critical component to pay attention to. This is the component that ultimately is the cause for the answer to the lowest temperature a heat pump will work is. 

Heat energy is transferred from hot to cold, thus following the laws of thermodynamics. A heat pump is designed to promote this transfer in order to heat your home (in heat mode) or cool your home (in air conditioning mode). As the heat pump recognizes which environment is hotter (inside or outside), it is able to move the heat energy from the hot location to the cooler location.

Thus, if the temperature outside is below freezing, the air heat pump will have limited heat energy to pull from, resulting in a significantly less efficient heat transferring process. You could imagine trying to warm your entire home through an electric blanket- the heat transfer may be possible but would take an extreme amount of energy.

Recognizing this, the heat pump will save energy by switching to an external heat source- emergency heat- to provide the heat needed to warm your home.

If you live in an area that gets routine snowfall, building a snow shelter to enclose or at least cover your heat pump is recommended. See our article How to Correctly Enclose a Heat Pump for tips on protecting your outside condensing unit.

How to Use a Heat Pump in the Winter

Using a heat pump in the winter looks different than using it in other seasons for the simple reason of what the device was created to do: transfer heat energy from outside to inside or vice versa. As the winter approaches, temperatures outside lower. Thus, your heat pump will have less heat energy readily available to transfer indoors. This creates an environment in which you will need to keep a few things in mind.

To use a heat pump in the winter, consider moderation, trusting the device to work as it was designed, and limit manual involvement. First, regarding the device itself, you do not need to (and should not) cover a heat pump without clearance of a close 1.5 to 2 feet. If debris or snow cover it, you may brush it off carefully or allow the defrost method to melt frozen liquids.

Next, in using moderation, you will want to consider a temperature that is comfortable within your home without “heating the neighborhood”. During warmer months, you will want to keep your home as warm as possible without being uncomfortable to reduce the need for much of a heat transfer. During colder months, try sticking with a range of 66- to 70- degrees Fahrenheit and use alternative heating options such as warmer clothes, blankets, and energy-efficient space heaters.

In trusting the device to operate as it was designed, do not use the emergency heat setting on your thermostat unless you are confident that the heat pump is not working. The heat pump will automatically switch to emergency heat if it is not able to efficiently transfer heat from outside due to low temperatures.

By changing your thermostat to “emergency heat”, you are actually turning off the heat pump’s standard mode of operation and overriding to only use emergency heat. This will be incredibly inefficient if operating in emergency heat at all times.

In Closing

Finally, if you have any questions about your heat pump’s efficiency, method of operation, or effectiveness, it is best to ask an HVAC repairman rather than mess up your home’s heating unit entirely.

While there are many ways to fix a faulty part or system, it is best to invest your time in someone who has been trained to efficiently and effectively do so. Those who have been trained and certified in HVAC repair will know the solutions you may need and should be able to provide them with ease.

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Hubert Miles

I've been conducting home inspections for 17 years. I'm a licensed Home Inspector, Certified Master Inspector (CMI), and FHA 203k Consultant. I started HomeInspectionInsider.com to help people better understand the home inspection process and answer questions about homeownership and home maintenance.

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