What Exactly Is A Heat Pump?
Traditionally, homes use a furnace for heating and an air conditioning unit for cooling. A heat pump works as both a heating and cooling system for your home. It does not burn fossil fuels and is more energy efficient than a furnace or air conditioner. This provides you monetary savings and increased comfort year-round, while also reducing your carbon footprint. In addition, a single heat pump may be able to provide both cooling and heating for your whole house without the use of separate systems.
How Does a Heat Pump Work?
Much like a refrigerator, a heat pump uses the process of heat exchange to either cool or heat your home. Through the use of a compressor and expansion valve, the refrigerant is run through a set of coils as either a gas or liquid to absorb heat. A heat pump contains an outdoor unit and an indoor unit. Both units contain a coil and a fan. The outdoor unit facilitates the heat exchange by blowing air over the outdoor coils. The indoor unit moves air across the indoor coils and throughout the ducts of your home.
Heat pumps do not generate heat like a furnace, rather, they extract heat from a source, amplify, and transfer it elsewhere. Modern heat pumps can heat a home 3-5 times more efficiently than a gas boiler. In cooling mode, a heat pump functions identically to an air conditioner.
For a more in depth understanding of how a heat pump transfers heat, This Old House has a great video worth watching.
Types of Heat Pumps
If a heat pump does not generate heat, where does that heat come from? Heat pumps transfer heat from the air or the ground. Some heat pumps can also use water as a source, or in conjunction with other sources. Let’s look at the two main types of heat pumps: air-source, and ground-source.
Air: Ducted vs Ductless
Heat pumps that use air as the source of heat will transfer heat between the air inside your home and the air outside your home. In heating mode, the outdoor unit pulls in cold air from outside. Wait, cold air?! Yes, scientifically speaking, there is heat present in cold air as long as it is above zero degrees Fahrenheit. The heat inside this air is absorbed by the refrigerant, which then passes through a compressor to pressurize and amplify the heat, and is then pumped to the indoor unit where a fan blows the heated air through the ductwork of the home. Finally, the refrigerant goes through an evaporator to de-pressurize so the cycle can start anew.
When in cooling mode, the process is reversed from how the pump transfers heat to the inside of your home, working just like an air conditioner. The indoor unit pulls the hot air from inside your home, the refrigerant transfers the heat, and the cooled air is blown back through the ductwork while the heat taken from that air is blown outside your home.
If your home does not have ducts, a mini-split heat pump can be retrofitted. Instead of a single indoor unit, as many as four smaller units are placed in rooms throughout the home and are all connected to a single outdoor unit. Each unit has its own thermostat which allows for single room, or “zone,” heating and cooling, much like systems that use dampers to heat or cool only specific or occupied spaces. Additionally, a special type of heat pump is available that uses hot and cold water, rather than air. This method works well for radiant heat systems.
Ground-source heat pumps are more commonly referred to as geothermal heat pumps. They work by transferring heat from the ground or a nearby water source. Though these systems are more expensive than air-source pumps, they have higher efficiency ratings. The temperature underground is more constant than the temperature above ground, requiring the system to use less energy by 30%-60%. Other benefits include better control over humidity, increased reliability, and they can fit in a wide variety of homes. They also perform better in colder climates. The compatibility of your home for geothermal will depend on the lot size of your property, the subsoil, and the general landscape.
Absorption heat pumps, sometimes called gas-fired heat pumps, are a newer type of heat pump. Instead of using electricity as the power source, the absorption heat pump is operated by a heat source such as natural gas, propane, solar-heated water, or geothermal-heated water. Natural gas is the most common heat source for this set-up, thus why they are also called gas-fired heat pumps.
Instead of pumping a singular refrigerant through the system, these heat pumps use a combination of ammonia and water. The ammonia-water solution cycles through the system, absorbing and transferring heat like any other heat pump. The key difference, however, is the lack of a compressor in these systems: “Instead, the ammonia is absorbed into water where a relatively low-power pump can then pump the solution to a higher pressure.” The natural gas, or other heat source, then boils the water to remove the ammonia so the cycle can continue.
A component of gas-fired heat pumps is the ability to recapture the lost heat released by the absorption of the ammonia into water. Other technological advances enable these heat pumps to operate at higher efficiencies. Although more common in commercial or industrial buildings, there are models available for large residential homes (4,000+ sqft). These pumps also only make sense for homes without an electricity source.
Heat pumps can also work as a hybrid system in conjunction with a gas furnace. The furnace supplements the heat pump when temperatures outside are too cold for the heat pump to operate as efficiently. The colder it is outside in the winter (especially in the negative temperatures), the harder and more frequent the heat pump system has to work to heat your home. Using a heat pump alongside a gas furnace reduces the amount of electricity used by a heat pump in more extreme weather, as the gas furnace takes over when the temperature is too low. This duel-fuel system can also be less costly and provide higher efficiency than either system alone, depending on the climate and energy needs of your home.
In addition, here are some other cool features being used in some heat pump models:
- Two-Speed Compressors: This type compressor reduces the wear on the compressor by reducing how often the heat pump turns on and off. They do this by operating more efficiently to the heating/cooling capacity needed relative to any outdoor temperature.
- Variable-Speed/Dual-Speed Motors: These speed-controlled motors are found on the outdoor unit blower (fan), indoor unit blower, or both. By keeping air moving at a comfortable pace, they minimize cool drafts and increase energy savings. They are also quieter compared to their single-speed predecessors.
- Desuperheater: This device utilizes captured heat from the heat pump’s cooling mode in order to heat water. Having one of these equipped on a heat pump can heat water 2-3 times more efficiently than a standard electric water heater.
- Scroll Compressor: Traditional compressors operate using pistons to pressurize the refrigerant in order to amplify the contained heat. A scroll compressor is comprised of two spiral-shaped scrolls; one is stationary while the other rotates around it. The rotating scroll compresses the refrigerant by forcing it into smaller and smaller areas. Some studies have shown these compressors can increase the temperature of the heated air by 10 -15 degrees.
Heat pumps have come a long way. They can offer greater comfort over a gas boiler, while using less energy to provide you savings. If you are looking for more ways to save energy (and future dollars), check out our article on solar panels.