How Does a Heat Pump Work?

By Michael Thomas

How Does a Heat Pump Work?

Heat pumps are home appliances that can both heat and cool a home. Just like a furnace or a boiler, they warm up your home on a cold day. And just like an air conditioner, they keep your home cool in the summer. 

As we’ve written before, installing a heat pump is one of the best ways to save energy and money on your utility bills as a homeowner. That’s because heat pumps are 2-3 times more efficient than furnaces and traditional air conditioners. In a recent analysis, we found that the average homeowner can save $670 per year by switching to a heat pump. 

As the most energy-efficient way to heat and cool a home, and one of the few HVAC systems that can run entirely on renewable energy, heat pumps are also good for the planet

In this article we’ll answer one of the most common questions we get at Carbon Switch: How does a heat pump work? We’ll start with a simple description, then get deeper into the science. 

There are many types of heat pumps. But in this article we’ll be talking about the most common heat pumps in America, air-source heat pumps like mini-splits. In future articles we will cover ground-source, or geothermal, heat pumps. 

How does a heat pump work: Explain it to me like I’m five.

At the simplest level, heat pumps use electricity to move heat from one place to another. In cooling mode, a heat pump moves the heat inside your home to the outside, leaving your home cooler. 

Refrigerators and traditional one-way air conditioners work the same way. In fact, these appliances are examples of heat pumps, which means you’ve probably already experienced the power of this technology. 

But what makes a heat pump (such as a ductless mini-split) different from a traditional air conditioner is that this process can also be reversed. On a cold day, a heat pump can move heat energy inside and heat your home. (Yes, heat pumps work in cold climates).

If that’s a bit confusing, think about how your refrigerator works. When you put your hand inside, it’s cold. But if you stand behind it, it’s hot. Your refrigerator is pumping heat out of the insulated space inside and into your kitchen (or more likely a little-seen space behind the fridge). Now imagine you reversed this process. The inside space would now get hot. And the outside space would get cold. That’s what happens when you turn your heat pump on in the winter. 

If you aren’t interested in the mechanics of how a heat pump works, that’s really all you need to know. Heat pumps are reversible refrigerators. They’re air conditioners that also work as space heaters. And they are the most efficient way to heat and cool a home. 

But my guess is that many of you want to understand how these things actually work. So let’s dive a little deeper. 

How does a heat pump actually work?

In order to understand how a heat pump works, it’s helpful to understand the basics of the second law of thermodynamics. I know how that reads, but trust me, we can make it quick.

In simple terms, this law tells us that hot always wants to move to cold. Put a frozen bag of soup in a bowl of hot water and it will thaw. Open the door of your house in the winter and all the warm air will rush outside. In both of these examples, heat is moving from a high temperature space to a lower temperature space nearby. 

A heat pump takes advantage of this law. In cooling mode, it makes the coils inside the unit very cold, so that your home’s warmer air flows into it. That captured heat energy is then carried outside. In this way the coils are sort of like a warmth vacuum or magnet. 

How do the coils of the inside unit absorb heat like this? One way to think of it is that the heat pump is sweating for your house. Let me explain.

Within the coils, there’s something called a refrigerant. Compared to water, refrigerants have a much lower boiling point. This means it takes far less energy to change their state from a liquid to a gas. It also means they are great at absorbing and releasing heat. 

Before entering the coils, the refrigerant passes through an expansion valve. This reduces the pressure, which also reduces the boiling point. 

At this lower boiling point, some of the refrigerant vaporizes as it reaches the heat pump’s cooling coil. Another way of saying this is that some of the refrigerant evaporates. Hence why the heat pump’s cooling coil is also called an evaporator. 

Whenever something evaporates, heat is transferred into it. This is why when you sweat on a hot day, you feel cooler. (To learn about the science behind this, check out this video on Khan Academy).

The same thing happens when the low-pressure refrigerant evaporates. As the refrigerant changes states from liquid to vapor, heat from the air in your home is transferred into the refrigerant. The refrigerant has, in essence, soaked up some heat from the inside air. The refrigerant acts like perspiration for your home, minus the stinky smell of sweat.

Here’s a more accurate version of the image above, showing the refrigerant getting warmer as it absorbs energy from the air.

From here, the refrigerant travels to the outside unit, where it passes through a compressor that increases its pressure and temperature. This causes the boiling point to shoot back up again, so that the vapor refrigerant condenses back into a liquid when it reaches the heat pump’s outdoor coil. 

Now, instead of absorbing energy, the refrigerant must release energy in order to change phases. This is sped up by a fan that blows the outside air over the coils in the condenser. This is the phase that creates the heat you feel behind a refrigerator.

From here the cycle repeats until the ideal temperature inside is reached. 

So that’s how a heat pump works in cooling mode. By using a little electricity to run refrigerant through a compressor and an expansion valve, and blow some air over it to move things along, this technology can pump heat from the inside air to outside air. 

But what if we want to warm up our home instead of cooling it down? 

How does a heat pump work in the winter?

In heating mode, a heat pump works in much the same way. But the process is sort of reversed. Heat is pumped from the outside air into your home. 

At first, this is a little hard to wrap your head around. If it’s 10 degrees outside, how can a heat pump absorb any energy? 

Consider this: Even on a cold day, there is energy in the air. At a microscopic level, heat is basically just molecules bouncing around. Molecules bounce around a little slower in cold air than in hot air, but they are bouncing around nonetheless. In fact, there would be energy in the air all the way down to –459.67°F or –273.15°C, a temperature known as absolute zero. 

A heat pump absorbs energy from the outside air by offering it something even colder to flow into. It does this by pushing refrigerant through an expansion valve in order to reduce the pressure. At this lower pressure, the boiling point goes down and the refrigerant boils. 

In other words, the refrigerant changes state from liquid to gas; it vaporizes. That’s why the unit that this happens in is called the evaporator. (Careful readers will note that the evaporator and condenser are really the same thing. They’re just coils whose name depends on whether we’re in heating mode or cooling mode). 

As I mentioned in the previous section, when something evaporates, it must absorb energy. In this case, the refrigerant is much colder than the outside air (an impressive feat in the winter, but true). Because hot wants to move to cold, some energy passes from the outdoor air to the refrigerant. 

From here the refrigerant travels into the home, where it passes through a compressor that increases its pressure. This causes the boiling point to go up. As refrigerant passes through the indoor coils, it changes state from a vapor to a liquid. In other words, it condenses. 

Whenever something changes state from a gas to a liquid, energy must be released. In this case, the energy of the refrigerant passes into the air inside your home. 

And that’s it. Heat gets absorbed into the refrigerant in the outside unit and then gets released inside. In this way, the heat pump moves the heat energy from the outside air into your home. Which is all to say, yes, heat pumps work in cold climates.

Heat pumps are more efficient than traditional heaters and air conditioners

By essentially exploiting the laws of thermodynamics, a heat pump heats and cools a home more efficiently than any other HVAC system. 

The most efficient natural gas furnace, for example, might work at 98% efficiency. What that means is that for every 100 units of energy that the furnace consumes, 98 units are converted into useful heat. Baseboard heaters and electric furnaces, to give a few more examples, operate at 100% efficiency. 

Heat pumps, by comparison, typically operate at 200% to 400% efficiency. In other words, they consume 1 kWh of electricity, and then convert that into 2 to 4 kWh of energy in the form of heat. 

That’s why, for most homeowners, heat pumps are the most efficient way to heat and cool a home. That they also can run without fossil fuels, whether in the home or at the power station, is almost just a bonus.

Special thanks to Kevin Kircher at MIT who reviewed this article before publication.

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