A simple explanation of how a heat pump works. Punchline: It is an air conditioner running in reverse!
There are a few difficult concepts that are glossed over in the initial explanation. Like, there are really only levels of heat. Cold is just LESS HEAT. So you cannot create or move COLD. You can only create or move HEAT. Secondly, HEAT naturally dissipates or moves from hotter areas to cooler areas to equalize temperatures between the extremes.
Air Conditioning
Most of us are lucky enough to have experienced cooling by an air conditioner. What you may not realize is that the unit is moving heat from the inside of your house to the outside. This is extremely easy to verify, though. When the A/C is running, go to the outside unit (condenser) and feel the heat coming out. It is hotter than the ambient outside temperature.
This image shows a house with an outside ambient temperature of 90 ℉. Without the A/C, we assume the inside temperature would be the same as the outside. Smart people have been able to create a device called an “Air Conditioner” that can move heat, creating a cold and a hot side. We locate the COLD part inside the house and the HOT part outside. This allows the air conditioner to cool the inside. It is also heating the outside, but we generally do not think about that.
Next let’s talk about heating.
Generating Heat
It is possible to generate heat, buy burning gas or wood, or even just use electric heating elements like space heaters. The diagram is pretty simple, but there are issues like exhausting the smoke if burning something.
For this image, we are showing an outside ambient temperature of 50 ℉. There is some device generating heat. This could be an electric space heater, a fireplace, a wood stove, or a central HVAC system burning natural gas. For this picture, the heat source is shown as 110 ℉, so heat would flow into the house to warm it up.
Fossil fuels (natural gas, coal, oil) have enormous energy potential and can create lots of heat, but they are also contributing to the increasing level of CO2 in our atmosphere. Even resistive electric heat can create high temperatures, but it is generally more expensive to heat this way.
Heat Pump Heating
We actually have already shown how heat pump heating would work! Looking back at the Air Conditioner, once you realize that it is heating the outside, we could turn this system around. So, for this next picture, I decreased the outside ambient temperature to 50 ℉, put the house on the other side, and adjusted the temperatures to be closer to the real world.
This is where it will seem a bit weird. On the outside, the Heat Pump is now pulling heat from the outside cold air, making it even COLDER. That extracted heat is the moved to the inside where it is able to HEAT the inside of the house. This may seem crazy, but when you go back and look at the Air Conditioner model, this is doing the same thing. It just moves, or PUMPs, heat.
Air Conditioners are “heat pumps,” but they are only able to work in one direction. Heat pumps made for residential use are made such that they can PUMP heat in either direction. And they literally do this by running backward! Yes, they are air conditioners that heat and cool.
Heating with a heat pump is just an air conditioner running in reverse.Ok, this article has glossed over all the details to give the highest level description of how a heat pump (and also an air conditioner) works. But, if you feel comfortable with your level of knowledge of air conditioners, you might also be happy with this high-level description of heat pumps. They are basically the same.
Some Practical Considerations in using a Heat Pump
For practical use, there are lots of considerations for using a heat pump in place of a fossil fuel-burning heater. Below we will introduce some of these topics so you can be aware and research more later.
- Cold-climate heat pumps: Before the 2010s, the most commonly used heat pumps would not work well or at all below freezing. These units are STILL widely sold. Newer units designed to work at lower temperatures, like mine, which can work down to -22 ℉, are often called “Cold-Climate Heat Pumps.” In the US, there are specific specifications to get this label.
- Coefficient of Performance (COP): Heat pumps do require energy to run. AND their performance for heating gets worse as the temperature drops. So, the amount of heat they can provide goes down as the outside temperature goes down. Fossil fuel systems do not have this problem. One measure of a heat pump’s performance is COP. This measure compares the energy to run them to the heat energy you get out of them. For example, my unit has a COP=4.3 at 47 ℉ outside, which drops to COP=1.9 at 5 ℉. Both of these are measured for heating inside to 70 ℉.
- We have been a bit spoiled burning fossil fuels, which can deliver so much energy at a low cost. Electric heating could be very expensive if you do not first insulate your house. Insulation is almost always the BEST FIRST STEP.
- Many many HVAC companies DO NOT UNDERSTAND these newer cold climate heat pumps.
WHY DID I WRITE THIS
I felt the need to write this article because I could not find a good reference to this information on the Internet. There is much detailed information about the refrigeration cycle and how heat pumps work. But, if we don’t need this level of detail for our A/Cs, we do not need it for heat pumps.