Heat pump efficiency depends on outdoor temperatures. As we all know, at lower temperatures, heat pumps become less efficient. That’s why we usually use a furnace combo with heat pumps; at freezing temperatures, heat pumps become less effective, and we have to use the furnace for heating.
How much does heat pump efficiency drops in cold climates is described by the heat pump efficiency vs temperature graph (for temperatures between 0°F To 60°F).
Below you will find both a heat pump efficiency vs temperature chart and a table with heat pump COP values at low temperatures. We will also look at an example of how 24,000 BTU heat pump output falls below 15,000 BTU at low temperatures at the end.
Before we check the heat pump temperature efficiency chart and the table, let’s address the key metric we are using:
Heat pump efficiency is measured by COP or Coefficient Of Performance. Heat pump COP is defined as:
Heat Pump COP = Heating Output / Energy Input
Basically, when we are measuring heat pump efficiency, we are measuring COP. At higher temperatures (about 52°F and above), the heat pump coefficient of efficiency can be above 4. That means that a heat pump will produce 4 times as much heating output for every 1 unit f energy output. In short, a heat pump will have 400% efficiency.
All heat pumps have above 100% efficiency because they do not generate heat from electricity. They merely use electricity to pump available outdoor heat indoors.
Obviously, the efficiency of heat pumps depends on how much heat is available in outdoor air. That simply means that we have a relationship between the outdoor temperature and heat pump efficiency. In colder climates, a heat pump can extract less heat from the cold outdoor air. COP at low temperatures completely depends on the capability of the outdoor unit (indoor unit doesn’t influence the overall efficiency).
Example: An average heat pump efficiency at 45°F is about 3.7 COP. That is 370% efficiency. At much lower temperatures – say 10°F winter temperatures – an average heat pump efficiency is about 2.3 COP. That is 230% efficiency. Relatively speaking, we will see that a heat pump at 10°F will produce 38% less heat than at a higher 45°F temperature.
Let’s say that we have a 24,000 BTU heat pump. This heat capacity is usually measured at 47°F; that means that at 45°F such a heat pump will actually produce about 24,000 BTU of heating output (or a little below that). At 10°F, however, we will see a 38% decrease in heating output; a 24,000 BTU heat pump will thus produce only 14,880 BTU or even less.
How much COP drops at lower temperatures is a matter of measurement (measuring heating output for certain electricity input at various temperatures). A limited number of studies were conducted for low single-digit temperature heat pump efficiencies.
The most all-encompassing study on heat pump efficiency at lower temperatures was conducted by the US Department of Energy. The “Measured Performance of a Low Temperature Air Source Heat Pump” study was conducted in September 2013. Based on the results from that study, we can roughly chart heat pump efficiency vs temperature at low temperatures:
Heat Pump Efficiency Vs Outside Temperature Chart
This heat pump COP vs temperature chart will help you understand the efficiency of heat pumps at low temperatures. Here is the full heat pump output temperature chart, based on the measured heating outputs from the DOE study:
As we can see from the heat pump temperature efficiency chart, we are seeing very high COP 4 efficiency at higher temperatures (above 52°F. At below 32°F, we see the heat pump efficiency falling below COP 3 (300% efficiency).
At single-digit temperatures, the efficiency of heat pumps drops by almost half compared to 47°F temperature (HSPF rating, for example, is measured at 47°F). This doesn’t mean that heat pumps stop working at freezing temperatures; it just means they are less efficient and thus produce lower heating output than the one we have on the label.
Note: These are low-temperature heat pumps. The COP value of standard heat pumps will fall much quicker (with a COP rating of about 1.5 at 10°F).
Here is the promised chart of COP vs temperature for heat pumps:
Heat Pump COP Vs Temperature Table
|Outside Temperature (°F):||Coefficient Of Performance (COP):|
It needs to be noted that there is currently a challenge to create heat pumps that can heat efficiently at near-zero temperatures as well. One interesting company has already managed to increase COP ratings at very low (5°F) temperatures.
The new Cooper and Hunter Hyper Heat series gives us insight into what efficiency modern very low-temperature heat pumps are capable of. If you check the specification sheet of the Hyper Heat series here, you will see COP ratings for 5°F temperatures. Namely, you can see that:
- The small 7000 BTU HPR Series CH-HPR06F9-230VO heat pump has a 2.2 COP rating at 5°F temperatures.
- Bigger 24000 BTU HPR CH-HPR24-230VO heat pump has a 1.7 COP rating at 5°F temperatures.
What does this lower COP at low temperature actually mean in terms of heating output?
Let’s make some theoretical calculations and look at the heat pump output of a 24,000 BTU heat pump at different temperatures:
Heating Output Of 24,000 BTU Heat Pump At Different Temperatures (Example)
Let’s say that we have a 24,000 BTU (2-ton) heat pump that has about 24,000 BTU of heating output at 45°F at 3.7 COP. Based on the COP vs temperature study results above, we can calculate how many BTU will this heat pump produce at lower temperatures.
Here are the rough 24,000 BTU heat pump BTU output estimates at various temperatures:
- At 45°F, this heat pump has 3.7 COP and will thus produce 24,000 BTU of heating output.
- At 40°F, this heat pump has 3.3 COP and will thus produce 21,405 BTU of heating output.
- At 35°F, this heat pump has 3.1 COP and will thus produce 20,108 BTU of heating output.
- At 30°F, this heat pump has 2.9 COP and will thus produce 18,810 BTU of heating output.
- At 25°F, this heat pump has 2.6 COP and will thus produce 16,865 BTU of heating output.
- At 20°F, this heat pump has 2.5 COP and will thus produce 16,216 BTU of heating output.
- At 15°F, this heat pump has 2.4 COP and will thus produce 15,568 BTU of heating output.
- At 10°F, this heat pump has 2.3 COP and will thus produce 14,919 BTU of heating output.
- At 5°F, this heat pump has 2.25 COP and will thus produce 14,596 BTU of heating output.
- At 0°F, this heat pump has 2.2 COP and will thus produce 14,270 BTU of heating output.
As we can see, even with a low-temperature heat pump, the efficiency can fall so much that we lose almost half the capacity of a heat pump. That plays a major role when we have to size a heat pump. You can read more about how many BTU mini split heat pump you need here; you will see that none of the estimation tools take the drop of COP at lower temperatures into account.
All in all, it is important to understand the relationship between heat pump efficiency and outdoor temperature. In recent years, a lot of resources have been dedicated to inventing a heat pump with very good COP at low temperatures.
Essentially, because heat pumps are fuelled by electricity, they are considered ‘green’. If we can get the COP values at low temperatures us, they could be a financially viable and environmentally friendly alternative to gas furnaces.
We hope you now have an idea of how heat pump efficiency and outdoor temperatures are connected. For more in-depth and scientific studies, you can check Engineering Toolbox here.