# 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? At what temperature is a heart pump not effective?”

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.

For comparison, electric space heaters have 100% efficiency (equivalent to a 1.0 COP value). As we will see in a chart further on, good heat pumps have a 220% efficiency (2.2 COP value) at 0°F. That means that even at 0°F they have more than double the efficiency any electric space heater has.

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.

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 furnaces 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 the heat pump and electric furnace.

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

### 13 thoughts on “At What Temperature Does A Heat Pump Stop Working? 20°F?”

1. The final paragraph of your article above should state: at below 0 temperatures, the COP value of heat pumps falls below 1. Currently it says ‘below freezing’. Its a great article and I will use it. Thanks.

• Hi Jim, thank you for the correction. Was probably thinking in degrees Celsius; 0 degrees Celsius = freezing point. We have corrected the mistake; again, thanks a lot for pointing it out.

2. Nice long answer to dodge the actual question.

I’m sure the actual question being asked is “how cold will it be when my heat pump STOPS EFFECTIVELY HEATING MY HOUSE?”.

Just because you’re producing 4500 BTU/h at 0 degrees, doesn’t mean it’s blowing much more than 10-15C air out of the vent. You’re “making heat” but you’re losing more heat out of your house than your heat pump is putting in it. Take the outside air in freedom units and add 40 freedom degrees. That is roughly what your duct temperature will be with a heat pump. I suppose 40 degree air is “warm” when it’s 0, but you aren’t heating anything.

• Hi there, that’s exactly the point: heat pump ‘working’ doesn’t have a binary answer; is working vs isn’t working. The key understanding we should have is that a heat pump works all the time; what changes with temperature is the efficiency. 4500 BTU/h is still 4500 BTU/h; it’s more than 0 BTU/h but, admittedly, it’s not full 100% specified heating output.

• Correct but incorrect. See as a graduate from hvac/r technical school I had heat pumps one and two. And what is left out is the safeties on the outdoor and indoor system. In theses safeties to protect your compressor, which has a harder time starting in colder weather, it shuts down the outdoor unit and switches to electric strip heat or whatever your secondary heat is. Here in Michigan that’s usually a central furnace. But in other cases the electric strip heat installed in the duct plenum just after the indoor coil. This after a certain. Temp your glorious heat pump in that situation becomes a central space heater using up electricity to heat your home. Heat pumps are viable in south but almost useless up north.

• Hi Chuck, a valid point. However, the new heat pumps can withstand cold climates. A good read would be this Canadian technology challenge here. It’s useful to read the challenge specification; as an HVAC/R graduate, you will understand more than almost anybody else the existing and new heat pump tech and how it can achieve to keep the COP high even in low temperatures.

Just the fact that Canada (a country with a rather cold climate) is actively pursuing the production of heat pumps that would work in Canada effectively is quite telling as far as how well do modern heat pumps work in cold climates.

• Not really dodging. The mechanical device will still function. Functionality isn’t the issue. Most heat pumps have safeties that protect the unit. These safeties shut down the compressor and outdoor coil at a certain temperature and trigger emergency or secondary heat. In most homes I’ve seen in Michigan that was a 90-96% afue central furnace. In other cases electric strip heat is installed in the ductwork and the blower continues to pump heat from what is now a giant central space heater. Meaning you’re getting 100% efficiency in the btu/hr you’re getting. But you’re also going to see the electric bill shoot higher. Al because whether heat pump is still working or it’s switched to electric strip heat secondary heat your still using electricity to energize the outdoor coil and blower for the indoor coil. And electric strip heat isn’t more efficient or cheap to run.

• I’ve been searching this whole article for the statement you made. I live in a poorly insulated rental duplex. And when it get to be less than 27 degrees outside my heat pump can’t keep the house warm. Thanks for your very valid “reality check”.

• In other words the heat pump can only put out X amount of BTUs. If you had a gas heater that put out the same amount of BTUs it would also struggle?

3. Good information. But since efficiency is a function of the temperature difference between inside and out, all of the efficiency measures are based on an assumed room temperature. What temperature is that? If it’s mentioned somewhere, maybe a good idea to mention it a few more times. I heat to 62 degrees at night but some people go for 72 degrees.