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Science of Heating: How a Heat Pump Works

Girl with Thermostat

A heat pump is a mechanical device designed to move thermal energy (heat) from one location to another. Chances are you already have a heat pump in your home: a refrigerator is a heat pump system designed to remove heat from a box so your food and drinks stay cold inside.

Air conditioners are also heat pump systems. They remove hot air from the inside of a building and pump it outside, where it is dispersed. Both air conditioners and refrigerators work the same way, using the technology of vapor-compression refrigeration.

What you might not know is that the process of vapor compression can be reversed, so that the same system that cools you off in the summer can be used in the winter to keep you warm, often times for substantially less energy cost than traditional heating. In the HVAC industry, the term heat pump is specifically used for a single unit that can be used for both heating and cooling, while an air conditioner provides only cooling.

So how does it work? What are the different types of heat pumps available? And is there an advantage to using a heat pump over a traditional heat source for your home? We'll discuss all these questions in this article, so it's time to decide if a heat pump is the right choice for your heating and cooling needs.

The Refrigeration Cycle

Vapor-compression refrigeration (or the refrigeration cycle) works by using a liquid heat sink to absorb heat from a cold space and release it into a warmer one. The heat sink is called a refrigerant; there are various types of refrigerant in use (such as Freon), but the common property is that it boils at very low temperatures, which means it transforms very easily from liquid to gas and back again.

Refrigeration Cycle

The refrigerant circulates through a closed conduit as part of the heat pump system, which has four basic parts:

The compressor pressurizes the refrigerant, turning it into a hot, high-pressure gas.
In the condenser, there is a long coiled tube like on the back of your refrigerator, heat from the refrigerant is dissipated into the environment (usually with the help of a fan). As the refrigerant cools, it starts to turn back into a liquid.
Expansion valve
As the refrigerant passes through the expansion valve, the pressure drops dramatically, and the temperature also drops.
As it passes through the evaporator, which is another long coil, the now-cooled refrigerant begins to absorb heat from the environment. When it absorbs enough heat, it starts to boil, so by the time it reaches the compressor again, it has returned to its hot, gaseous state.

In an air conditioner, refrigerant can flow in only one direction through the system. The heat-dissipating condenser will always be outside.

In a proper heat pump, however, the addition of a reversing valve on the compressor makes it possible to reverse the flow of the refrigerant. The outside condenser now becomes an evaporator, capturing heat from the surrounding air, and the interior evaporator becomes the new condenser, spreading heat indoors.

Advantages and Limitations of Heat Pumps

Heat pumps are powered by electricity, but only to run the compressor motor and fans. The actual heat comes from the external environment, not from burning electricity (or oil or gas) as a fuel. That means that it uses far less electricity than a conventional resistance heater to generate the same amount of heat. In fact, while a resistance heater is roughly 100% efficient, producing 1 joule of heat for each joule of electrical energy it uses, a heat pump can be three to four times as productive.

But the colder it gets outside, the less efficient a heat pump becomes. When the refrigerant passes through the condenser, it must be hotter than the indoor temperature in order to release heat into the air. Then, when it passes through the evaporator, it must be colder than the outside temperature in order to absorb more heat. If there is a wide difference between indoor and outdoor temperatures, then the pressure needed to make these temperature shifts increases. This means the compressor has to work harder, and more electricity is required to operate the system.

Heating Cost Comparison

For this reason, heat pumps are not recommended for whole-house heating in climates that regularly see temperatures below freezing. The colder it gets, the more money it will cost to use the heat pump. However, as the technology continues to improve, new refrigerants are being developed that can compensate for temperature conditions in different regions. If this type of heating system interests you, it's worth your time to consult with a local HVAC specialist who can make recommendations based on where you live.

Another thing to look out for with heat pumps is that moisture in the outdoor air may condense and freeze on the outdoor heat exchanger. The system must shut down briefly to defrost itself, causing an interruption in heat output.

Different Types of Heat Pumps

Geothermal Heat Pumps: The most common type of heat pump is an air-source heat pump, they use air as a source for heat. Ground-source heat pumps are growing in popularity, however, because they take advantage of the ample, consistent heat provided by the earth itself. In this type of system, the evaporator is buried underground, so they can be used in colder climates than air-sourced heat pumps. They can be expensive to install, but the payoff in efficiency, which make them worth the cost. Because they often use water instead of chemical coolants to move heat, they are a favorite for environmentally-conscious builders.

Mini-split heat pumps: If your home doesn't have ductwork, a mini-split heat pump allows you to install both heating and air conditioning without expensive renovations. The heat pump sits outside your home and is connected to air handlers in different parts of your home via a narrow conduit that carries power cable, refrigerant tubing, suction tubing, and a conduit drain. Only a small hole has to be drilled in your wall for installation. These systems are energy efficient and allow a lot of flexibility in maintaining the temperature in different rooms in your house.

Space Heating Comparison

Portable Heat Pumps: Many portable air conditioners offer a heating function at little or no additional cost. They can provide substantial supplemental heat to a home or office. A portable air conditioner must be vented (out of a window or wall) whether it is providing cooling or heating, as it provides much more heat for the energy it costs to operate than a regular space heater. Plus, it saves you the trouble of having to switch out one appliance for the other as the seasons change.

Heat pumps can also be used for water heaters, to help cut costs on what is the second largest energy expense (after heating) for most home owners.