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.

Geothermal

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

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.