What Is a Heat Pump?
A heat pump is an appliance that uses a refrigerant to transfer heat from one place to another in an efficient and reversible manner. An air-conditioner transfers heat out of a room to the outside air, thus cooling the room. A heat pump does that, and in winter, transfers heat from even very cold outside air, or from the ground, to the indoors, providing warmth.
The outdoor portion of a heat pump has coils filled with a liquid refrigerant that turns to vapor at very low temperatures. When cool outside air passes over the coils, the liquid absorbs whatever heat is in the air and vaporizes. This gas moves via a pipeline into a compressor, where the pressure increases the temperature of the gas even more. It then moves to a heat exchange system inside the house, which creates warm air for home heating.
As heat is transferred into the home, the gas cools down into liquid form and is returned to the outdoor unit. The cycle is repeated until the home has reached the thermostat setting. In hot weather, the process is reversed, with warm indoor air giving up its heat to the refrigerant to be transferred outdoors.
Heat Pump Advantages Over Traditional HVAC Systems
Heat pumps operate in a clean, safe way that is uniquely efficient. As no fuel is burned to create heat, indoor air is cleaner, and dangerous gas or carbon monoxide leaks cannot occur. Heat pumps are two to three times more efficient than the best gas furnaces. The high efficiency of heat pumps means that they have much lower carbon footprints than traditional HVAC systems.
Despite their high efficiency, heat pumps usually but not always cost less to operate, because the relative prices of gas and electricity in each area are the major determinants of operating cost. However, because one appliance serves for both heating and cooling, additional savings are possible.
How can a heat pump be so much more efficient than a conventional heating/cooling system? The answer lies in the genius of refrigerants that transfer “stolen” heat from outside air to the home’s interior, or vice-versa. Because heat pumps use electricity only to drive fans, they consume less energy in the form of electricity than they produce in the form of heat energy.
An exception to that may occur when heating a home in colder climates. Despite advances in cold climate heat pumps (some newer models can handle temperatures of -22°F), auxiliary electrical heating may at times be necessary to maintain the desired indoor temperature.
Common Types of Air Source Heat Pumps
The system described above is considered an air source heat pump (ASHP) because it derives heat from the outside air. Several configurations of ASHP systems are common. Central (whole house) heat pumps are appropriate for homes that regulate indoor climate using a forced air duct system with vents throughout the house. Advantageous features are that central pumps can be configured to divide your home into different temperature areas, each controlled by its own thermostat. They easily work with humidifiers, dehumidifiers, and air-filtering equipment made by other manufacturers.
For homes built without a duct system, mini-split heat pumps are often a good option. Suitable for a circumscribed space (e.g., a room), a single zone mini-split consists of an indoor unit, usually mounted out of the way on a wall, that is directly connected to an outside unit.
Reaching a greater area, multi-split heat pumps allow for heating and cooling several zones by having, in addition to the outside unit, two to four indoor emitters. More extensive zoning in large homes is possible with multi-zone heat pumps. These have one or more junctions (“branch boxes”), each of which serves several indoor heating or cooling emitters, all connected to a single outdoor unit.
Choosing a Heat Pump System
The heat pump you select must be suitable for your home’s climate zone, as this determines the range of outdoor temperatures in which the system must be able to maintain a stable, desirable indoor “design temperature.” It must also be appropriate for the total heating and cooling load of the house (required BTUs per hour), as determined by the contractor’s “Manual J” calculation. Because a building’s insulation reduces the heating and cooling load, it is extremely important to weatherize your home before choosing a heat pump.
The contractor’s expertise is needed, too, for recommending the overall system design, including the number and arrangement of climate zones within the home. Lastly, it’s important to consider the efficiency ratings of heat pumps when you are deciding which to buy.
Once installed, heat pumps are best enjoyed when allowed to run fairly constantly, well below peak capacity, at a steady thermostat setting. This is the condition under which they operate most efficiently.
Reference: Rewiring America’s Electrification Coaching Course via Creative Commons license and verbal permission to cite (B.L.)