At What Temperature Does A Heat Pump Stop Working? 20°F?

“Heat pumps stop working at 20 degrees.”

This is a common myth about heat pump minimum outdoor temperature. At what temperature does a heat pump actually stop working? Never. A heat pump will work even in extremely cold weather (even below freezing temperatures).

We will look into the relationship between low temperatures and heat pump efficiency, and check if heat pumps can run effectively at 20°F, 10°F, 0°F and even in below-zero temperatures (-15°F).

Here is the right question we are going to tackle today:

“At what temperature does a heat pump becomes inefficient?”

Mini split heat pumps will run in low temperatures. However, at lower temperatures, the heating energy efficiency (measured by COP or Coefficient Of Performance) will be lower. At a certain temperature, the heat pump COP will fall so much (energy efficiency decrease) that it will be financially viable to switch from a heat pump to a furnace.

The average heat pump has a COP value of about 3.7 at 47°F. That means it has 370% efficiency. For comparison, electric space heaters have a COP value of 1.0 and have 100% efficiency. At temperatures below 0°F, the COP value of heat pumps can be close to 1 or even below 1. You can check COP values at low temperatures in this DOE’s study of Fujitsu and Mitsubishi heat pumps.

temperature at which you should you stop running a heat pump
At which temperature it still makes financial sense to run a heat pump depends only on the capability of the outdoor unit (indoor unit doesn’t influence COP value significantly).

Example: Let’s say we have a standard 18,000 BTU heat pump, powered by a 1,800-watt electric input (HSPF rating is 10.0) Here is why it is so difficult to say (with certainty) what is the lowest temperature for a heat pump:

  • At 47°F outdoor temperature, this heat pump will be able to produce 18,000 BTU of heating output. That is 10,000 BTU/hr for every 1 kWh of electricity it uses.
  • At 30°F outdoor temperature, this heat pump will be able to produce 14,900 BTU of heating output. That is 8,280 BTU/hr for every 1 kWh of electricity it uses.
  • At 20°F outdoor temperature, this heat pump will be able to produce 12,300 BTU of heating output. That is 6,830 BTU/hr for every 1 kWh of electricity it uses.
  • At 10°F outdoor temperature, this heat pump will be able to produce 10,600 BTU of heating output. That is 5,890 BTU/hr for every 1 kWh of electricity it uses.
  • At 0°F outdoor temperature, this heat pump will be able to produce 8,100 BTU of heating output. That is 4,500 BTU/hr for every 1 kWh of electricity it uses.
  • At -15°F outdoor temperature, this heat pump will be able to produce 4,600 BTU of heating output. That is 2,560 BTU/hr for every 1 kWh of electricity it uses.

For comparison, a 1,800W electric space heater or electric furnace will produce 6142 BTU/hr of heating output. That is 3,412 BTU/hr for every 1 kWh of electricity it uses.

Now, this is roughly how standard heat pumps withstand low temperatures. The heating output of an 18,000 BTU heat pump can easily fall below 10,000 BTU at 5°F or below, and even below 5,000 at freezing temperatures (-15°F or below). Basically, with a decrease in temperature, your heat pump becomes 20%, 30%, 40%, 50%, 60%, 70%, or even 80% less efficient.

Low-temperature heat pumps (LTHP) are more effective at lower temperatures but even those will be 50% less effective at -5°F or below.

graph of heat pump coefficient of performance to check temperature at which heat pumps stop working
COP values for low-temperature heat pumps. These HVAC units are more energy efficient at below 20-degree temperatures than standard heat pumps.

At what temperature should you stop your heat pump? At what loss of efficiency?

The heat pump’s lowest temperature that still makes sense depends on the heat pump. We cannot just say that all heat pumps stop working at 20°F, 10°F, or even 0°F.

What Is Heat Pump Lowest Temperature?

In fact, the US Department of Energy has proposed a challenge to heat pump producers titled ‘CCHP Technology Challenge‘. There are two goals here:

  • 1st goal is to create a cold climate heat pump optimized for 5°F temperatures. This would mean that the heat pump’s lowest temperature is 5 degrees.
  • 2nd goal is to create a cold climate heat pump optimized for –15°F temperatures. This would mean that the heat pump’s lowest temperature would be -15 degrees; we are talking about a below-zero heat pump.

DOE says that “The performance specifications would exceed current products on the market today and will aim to meet a 2024 commercialization timeline”.

This means that we currently don’t have a heat pump that would be optimized to operate at 5°F temperatures. We also know that current heat pumps can run efficiently at below 20-degree temperatures.

In simple terms, current heat pumps ‘stop’ working at temperatures between 5°F and 20°F. However, if you take into account that all electric heating appliances – electric furnaces or electric space heaters – have a COP 1 (100% efficiency), we can check at what temperature does an average heat pump have the same efficiency as electric furnaces, for example.

Let’s check at what temperature should you stop running your heat pump (Tip: You should definitely not run your heat pump all the time). We will compare the heating costs of running a heat pump in cold climates vs heating costs for a gas furnace, electric furnace and space heaters at:

  • 20 degrees.
  • 10 degrees.
  • 0 degrees.
  • -15 degrees.

You should get a clear picture of at what temperature it makes sense to stop a heat pump and fire up the furnace:

Efficiency At 20 Degrees

Here are the metrics that we can use to determine if it makes sense to stop a heat pump at 20 degrees:

  • At 20°F, an average heat pump has a COP value of about 2.4. That means it has 240% efficiency.
  • At 20°F, a good low-temperature heat pump has a COP value of about 2.5. That means it has 250% efficiency.
  • At 20°F, an electric furnace and electric space heater have a COP value of 1. That means they have 100% efficiency.
  • At 20°F, a 90 AFUE gas furnace will be able to convert 90% of the energy stored in natural gas into heating output.

As we can see, heat pumps have an above 100% higher efficiency than electric furnaces at 20°F. That is because heat pumps don’t ‘burn’ electricity to create heating output as electric furnaces. They use electricity to pump available heat from the outdoors inside.

A 3-ton average heat pump with a 9 HSPF rating will run on 4,000W max. wattage. At 47°F, it will be able to produce 36,000 BTU heating output (COP = 3.7). At 20°F, it will use that 4 kWh of electricity per hour to produce 23,351 BTU of heating output (COP = 2.4).

Basically, a heat pump will produce about 5,838 BTU per 1 kWh of electricity. If we take the US national price of electricity ($0.1319/kWh), you are getting 44,260 BTU per every US dollar of electricity.

Let’s compare that with a 90 AFUE 3-ton gas furnace. Such a furnace will produce 36,000 BTU of heating output regardless of the outdoor temperature. Because it burns gas at 90% efficiency, you will have to burn an equivalent of 40,000 BTU of natural gas to get 36,000 BTU of heating output.

How much natural gas is that? Well, according to EIA, burning 1,000 cubic feet of natural gas produces 1,037,000 BTU. Again, according to the EIA annual natural gas price database, 1,000 cubic feet of natural gas had an average cost of $12.24 in 2011.

If we are using a gas furnace that burns natural gas with 90% efficiency, we are effectively paying $12.24 for 933,300 BTU of heating output. That means that we are getting 76,250 BTU per every US dollar.

What does all of this mean?

That even at 20 degrees, a gas furnace is more energy efficient (dollar per dollar) than an average heat pump. However, a heat pump is still more energy efficient than an electric furnace or using a space heater.

Let’s look at the same calculations at 10, 0, and -15 degrees:

Efficiency At 10 Degrees

Here are the metrics that we can use to determine if it makes sense to stop a heat pump at 10 degrees:

  • At 10°F, an average heat pump has a COP value of about 1.9. That means it has 190% efficiency.
  • At 10°F, a good low-temperature heat pump has a COP value of about 2.3. That means it has 230% efficiency.
  • At 10°F, an electric furnace and electric space heater have a COP value of 1. That means they have 100% efficiency.
  • At 10°F, a 90 AFUE gas furnace will be able to convert 90% of the energy stored in natural gas into heating output.

Again, here we see that we should not stop a heat pump and start running a space heater or electric furnace.

Using a heat pump, we are now getting only 18,386 BTU from a 36,000 BTU heat pump (that’s about 50% of the rated heating output). That is 4,621 BTU per 1 kWh of electric input. That means we are getting 35,039 BTU per every US dollar.

A 90% AFUE gas furnace will produce 76,250 BTU per every US dollar worth of natural gas.

That means you should switch a heat pump to a gas furnace, but you should still not switch a heat pump to space heaters.

Efficiency At 0 Degrees

Here are the metrics that we can use to determine if it makes sense to stop a heat pump at 0 degrees:

  • At 0°F, an average heat pump has a COP value of about 1.2. That means it has 120% efficiency.
  • At 0°F, a good low-temperature heat pump has a COP value of about 2.2. That means it has 220% efficiency.
  • At 0°F, an electric furnace and electric space heater have a COP value of 1. That means they have 100% efficiency.
  • At 0°F, a 90 AFUE gas furnace will be able to convert 90% of the energy stored in natural gas into heating output.

Here we see that the efficiency of a heat pump is about the same as the efficiency of electric furnaces and space heaters.

Using a heat pump, we are not getting only 11,676 BTU from a 36,000 BTU heat pump. That is 2,919 BTU per 1 kWh of electric input. That means we are getting 22,130 BTU per every US dollar.

A 90% AFUE gas furnace will produce 76,250 BTU per every US dollar worth of natural gas.

That means you should switch a heat pump to a gas furnace at 0 degrees, but you still don’t have to switch a heat pump to space heaters or electric furnaces.

Efficiency At -15 Degrees

Here are the metrics that we can use to determine if it makes sense to stop a heat pump at -15 degrees:

  • At -15°F, an average heat pump has a COP value of about 0.5. That means it has 50% efficiency.
  • At -15°F, a good low-temperature heat pump has a COP value of about 0.8. That means it has 80% efficiency.
  • At -15°F, an electric furnace and electric space heater have a COP value of 1. That means they have 100% efficiency.
  • At -15°F, a 90 AFUE gas furnace will be able to convert 90% of the energy stored in natural gas into heating output.

At below zero temperature, the COP value of heat pumps falls below 1. That means not that it would be better to use a gas furnace; it also means that electric furnaces and space heaters become cheaper to run than heat pumps.


Bottomline On At What Temp To Stop Your Heat Pump

As we have seen, given the US national electricity and natural gas prices, it makes sense to switch from a heat pump to a gas furnace at about 20°F. A lot of this depends on what the electricity and natural gas prices are, of course, as well as on if you have a standard heat pump or low-temperature heat pumps.

If, however, electric furnace or space heaters are your alternative for heating, you should only stop a heat pump at about 0°F. Around that temperature, the heating BTU output per dollar is the same for heat pump and electric furnace.

At below freezing temperatures, the COP value of heat pumps falls below 1 (that means less than 100% efficiency; from an average of 370% efficient at 47°F) and a heat pump becomes inefficient compared to all heating sources.

Leave a comment