A simple explanation of how a heat pump works. Punchline: It is an air conditioner running in reverse!
Heat: The difference between Hot and Cold
Let’s get the difficult concept out of the way first. What we experience as heat and cold are levels of ENERGY. If something is HOT, it has more energy. If something is COLD, it has less energy. Air with a higher TEMPERATURE has more ENERGY, and likewise, air with a lower TEMPERATURE has less ENERGY.
You change the temperature of air by adding or removing HEAT. This is done by putting it close to something HOTTER or COLDER. We cannot add COLD; we can only remove HEAT! A cup of coffee cools because it loses heat to the surroundings. HEAT naturally dissipates or moves from hotter things to cooler things to equalize temperatures between the extremes.
If you believe the above, then you might wonder if Air Conditioners cannot add cold, how do they cool? Air Conditioners are actually pretty magical and did not come into common residential use until the 1970s!
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 A/C, now called a Heat Pump, is now pulling heat from the outside cold air, making it even COLDER. That extracted heat is then 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 essentially 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 of them 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 ℉.
- Resistive heating, like any electric space heater, will have a COP=1.0. One unit of energy consumed from your electric system will result in the exact same amount of heat being produced. The magic of the Heat Pump is that it can run at a COP > 1.0! And this is what makes them an attractive option for being able to supply heat while not contributing to the CO2 levels in the atmosphere.
- 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 the newer cold-climate heat pumps. This is sad and frustrating!
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.