What is a Heat Pump? How Does it Work?

What is a Heat Pump?

There are many people who can’t tell heat pumps and conventional air conditioners apart. Few even know how these systems work to air condition the house.

So, what is heat pump and how does it work? And what are the advantages and disadvantages of heat pumps compared to other HVAC systems?

Image Credit: Open University

A heat pump is an HVAC system that cools the house during the summer and heats it during winter. It has a reversing valve which reverses the direction of flow of the refrigerant so that the high-pressure superheated refrigerant enters the house to heat the indoor air, after extracting heat from the outside air.

During summer, a heat pump cools the house by absorbing the indoor air’s heat using a refrigerant and releasing it to the outside. In winter, the heat pump extracts heat from the outside air which is then compressed (to increase temperature) and used to heat the indoor air.

Let us look at those seasons independently:

During summer, a cold liquid refrigerant enters the inside unit where it absorbs heat from the indoor air and cools it in the process. The refrigerant vaporizes and is then compressed by the compressor before releasing the heat to the surrounding inside the condenser coils.

During winter, the heat pump’s reversing valve reverses the direction of flow of the refrigerant. A very cold refrigerant enters the condenser unit where it extracts heat from the surrounding air until it vaporizes. The compressor increases the refrigerant’s temperature before it enters the inside unit where it heats the indoor air.

It is worth noting that a heat pump works just like a standard air conditioner when used to cool the house. Air conditioners are however paired with furnaces to help heat the house during winter while a heat pump extracts heat from the outside air.

Heat pumps are equipped with a secondary source of heat which supplements them when temperature falls below 35 degrees and they can’t extract enough heat from the surrounding air. The secondary heat is usually in the form of electrical resistance heating.

The main disadvantage of heat pumps is that they don’t work well when temperature dips below 35 degrees. At that point the heat pump cannot extract enough heat from the outside air, forcing the auxiliary heat to kick in. Running auxiliary heating is very expensive.

Depending on brand, type, efficiency among other factors, heat pumps stop being effective at temperatures between 25 and 40 degrees Fahrenheit. At those temperatures and below, auxiliary heat will come on automatically to supplement the heat pump.

The advantage of heat pumps is that they can cool the house in summer and heat it during winter. However, heat pumps make more sense in areas with moderate weather. If your area experiences extreme winters, heat pumps will be more expensive to run and you are better of installing a standard AC paired with a furnace.

 Types of Heat Pumps

There are 3 main types of heat pumps:

  • Air-source heat pumps
  • Ground-source/geothermal heat source
  • Water-source heat pumps

Of the three, air source heat pumps are by far the most common. Just like standard air conditioners, they have a condenser unit outside and an evaporator unit inside the house (unless you would wish to install a packaged unit).

Unless you are installing a packaged unit, most people choose between central air and ductless air-source heat pumps. If your house has ductwork already installed then installing central air-source heat pumps are the best option for you.

Ductless air-source heat pumps works just like ductless/mini-split air conditioners. The evaporator unit is wall-mounted and connected to the condenser unit outside the house. No ducts are required.

What you may not know is that even though it could be freezing cold, the ground below you (just a few feet) retains its thermal energy. In fact, the thermal energy in the ground is always the same be it summer or winter.

And that is where geothermal heat pumps come in. Coils buried deep in the ground can be used to extract that energy and use it to heat your house. During summer, excess energy from the house can also be dumped in the ground as well.


Geothermal heat pumps are more expensive to install compared to air-source heat pumps. However, these heat pumps are cheaper to run with energy.gov estimating that they can reduce energy use by up to 60%.

Geothermal heat pumps are however not for everyone. Their installation depends on the size of your property (to bury the coils), the subsoil and the landscape.

Water-source heat pumps work by extracting heat from a water body and converting it to heat that can be used to heat the house. Submerged pipes containing a refrigerant are used to absorb heat from the water bodies and transfer it indoors.

Water-source heat pumps are however not very common. Another relatively new type of heat pump is the absorption heat pump.

Absorption or gas-fired heat pumps use air, natural gas, steam, solar-heated water or geothermal-heated water as their heat source.

How Does a Heat Pump Work In Summer?

In this post, I will use an air-source heat pump to explain how a heat pump works. The principle of how these HVAC systems work is however similar.

A heat pump has 5 main components:

  • Compressor
  • Condenser coil
  • Evaporator coil
  • Expansion valve
  • Refrigerant

The condenser coil and compressor forms what is known as the condenser unit or the outside unit. Inside the house we have the evaporator unit.

Copper tubes (refrigerant lines) are used to connect the inside and outside units to form a closed loop. A refrigerant is then circulated between the compressor, condenser and evaporator where it carries heat from one component to another.

Ideally, a refrigerant will have a low boiling point and high latent heat of vaporization. It thus changes from liquid to gas and back to liquid again easily.

The following is the step by step guide of how a heat pump works during summer:

1. Inside the Evaporator Unit

Sometimes this unit is also called an air handler. It is basically a copper pipe coiled to form multiple loops around an A-frame although sometimes it can take other shapes.

The refrigerant enters the evaporator coil as a very cold liquid. Hot indoor air is then pulled by the evaporator fan and passed over the coil.

The refrigerant absorbs heat from the indoor air and that is how it cools it. Cooled air is then supplied back to the house.

As I mentioned earlier, refrigerants have a very low boiling point. After absorbing heat from the indoor air, the refrigerant evaporates (turns to a gas) and exits the evaporator coil.

2. Inside the Compressor

The heated low-pressure refrigerant gas enters the compressor where we need to compress it (increase its pressure). And why do we need to compress the refrigerant?

At that point, we want to remove the heat that the refrigerant absorbed indoors to the outdoors. However, that would be during winter meaning that the outside air is hot as well.

In thermodynamics, heat is transferred from a point of high concentration to a point of low concentration. We therefore need to create a temperature differential in order for the refrigerant to release the heat to the surrounding.

Luckily, when you compress the refrigerant, it is not its pressure only that increases. Its temperature increases as well.

A compressor thereby increases the pressure and temperature of the refrigerant relative to the outside air.

3. Inside the Condenser Coil

The refrigerant enters the condenser coil as a high-pressure high-temperature gas. A condenser coil looks just like an evaporator coil only that it is larger and has a cuboid shape.

Since the temperature of the refrigerant is now higher than that of the surrounding air, the refrigerant can now release the heat to the surrounding. A fan helps to increase the rate of heat exchange.

The condenser fan blows cooler air over the coil. The cooler air absorbs heat from the refrigerant and expels it outside the unit, and hence the reason you will feel hot air being blown towards you when you stand too close to the condenser unit.

After losing most of the heat, the refrigerant exits the condenser in its liquid state. In other words, the refrigerant will have condensed.

4. The Expansion Valve

Just before the refrigerant enters the evaporator coil, it first goes through an expansion valve. An expansion valve is basically a small opening or restriction which serves 2 purposes.

First, the expansion valve controls the amount of refrigerant entering the evaporator. You see, all of the refrigerant entering the evaporator must vaporize (become a gas) before entering the compressor.

If too much of the refrigerant enters the evaporator coil, not all of it will vaporize meaning that some of it will enter the compressor in liquid form. Since a compressor is only designed to compress gases, liquids inside it will damage it.

Secondly, an expansion valve is used to lower the temperature of the refrigerant. It does that by reducing the pressure of the refrigerant.

As the refrigerant goes through the valve, its pressure is considerably reduced. A reduction in pressure also means a reduction in temperature.

And basically that is how a heat pump works in the summer to cool your house.

How a Heat Pump Works During Winter

During the months of winter, a heat pump works the same way it works during the summer, only that the refrigerant is forced to change the direction of flow. That is made possible by a device known as the reversing valve.

Instead of the refrigerant flowing from the evaporator coil to the compressor and then condenser coil, it flows from the evaporator coil to the condenser coil and then to the compressor.

Let us now see a step by step guide of how a heat pump works during winter.

1. Inside the Condenser Coil

When used to heat the house, the refrigerant in a heat pump enters the condenser coil as a cold liquid. The cold nature of the refrigerant is achieved by passing it through a series of expansion valves where its pressure is reduced, a move that also reduces its temperature.

Note: Although the outside temperature feels cold during winter, it still contains a lot of thermal energy. By lowering the temperature of the refrigerant, a temperature differential is created meaning that heat will be transferred from the outside air to the refrigerant.

And that is exactly what happens when a heat pump is used for heating. The temperature of the refrigerant is reduced way below that of the surrounding air to allow heat exchange between the 2 mediums.

The condenser fan blows the “warmer” outside air over the condenser coil, again to quicken the rate of heat exchange. Heat in the outside air is absorbed by the refrigerant which as I had mentioned has a very low boiling point.

After absorbing sufficient heat, the refrigerant turns from liquid to gas (evaporates).

2. Inside the Compressor

The compressor receives the refrigerant from the condenser coil as a low-pressure gas. Again, the function of the compressor here is to compress the refrigerant gas and in the process increase its temperature.

After compression, the refrigerant gas is ejected out of the compressor in the form of a high-pressure superheated gas.

3. Inside the Evaporator Coil

When the hot refrigerant gas enters the evaporator coil (insider the house), the evaporator fan pulls the cold indoor air from the house and passes it through the hot evaporator coil.

The indoor air absorbs heat from the refrigerant and that is how the heat pump is used to heat the house.

After losing heat to the indoor air, the refrigerant condenses (turns to liquid) and is ejected out of the evaporator. It then goes back to the condenser coil and the process is repeated over and over again.

And briefly that is how a heat pump works during winter to heat the house.

Heat Pump Auxiliary Heat vs Emergency Heat

As I had mentioned, heat pumps are designed with a secondary heat source in mind. The secondary heat source becomes important when the temperature outside drops way below the point where the heat pump is effective and also should the heat pump fail completely.

The heat pump is usually in the form of electrical resistance heating. Some heat pumps however use natural gas (in a furnace) as their secondary heat source.

One thing that confuses most homeowners is the difference between auxiliary heat and emergency heating. I will explain both of them here.

Auxiliary heating comes on automatically when the temperature of the indoor air drops by about 3 degrees lower than that of the thermostat setting. It comes in to supplement the heat pump temporarily.

Usually, auxiliary heating kicks in when the outside temperature falls below 40 degrees (or sometimes 35 degrees). At those temperatures, the heat pump cannot extract 100% of the needed heat from the outside air.

Auxiliary heat therefore comes on to help/supplement the heat pump and will turn off on its own when the temperature outside increases.

On the other hand, emergency heating must manually be turned on by the homeowner. There is a button on the thermostat for that.

As its name implies, emergency heating must only be turned on during emergencies. Such emergencies include a complete failure of the heat pump or when the condenser unit has been damaged by an object like a tree crashing into it.

Heat Pumps vs Central Air

The table below illustrates the major differences between heat pumps and central air:

[table id=4 /]

Wrap Up

And basically that is everything about heat pumps. I hope that this guide was helpful and that it was the information you were looking for.