What Is a Heat Pump System?
A heat pump doesn't generate heat; it transfers it from one place to another. It's an appliance with a bright future in an eco-conscious country.
If you aren’t familiar with heat pumps, there’s no time like the present to change that. In 2023, federal incentives kick in to make purchasing and installing one easier on the budget. And in 2030, California will begin phasing out gas furnaces.
Because heat pumps use 50 percent less energy than electric resistance furnaces according to the Environmental Protection Agency (EPA), they’re poised to become the most economical heating option for Californians, as well as people in any other states that follow California’s lead.
So what is this home appliance that “pumps heat?” The idea isn’t exotic. Turn your window air conditioner around and it could function as a rudimentary heat pump. But as you might expect, actual heat pumps are more sophisticated than that.
What Is a Heat Pump System?
A heat pump is an electric appliance that transfers heat from outdoors to indoors, and vice versa.
Most heat pumps have reversing valves that allow them to also function as air conditioners. The main working parts are a compressor, which circulates a refrigerant through a system of copper coils, and a fan to circulate air into the living space.
Heat pump systems typically consist of two units, one outdoors and one indoors, connected by copper tubing that passes through the wall of the building. The compressor is usually outside.
How Does a Heat Pump Work?
The refrigeration system consists of two coils (condenser and evaporator) separated by an expansion valve through which a refrigerant continuously circulates. In the condenser coil, the compressor pressurizes the refrigerant to condense it into a liquid. Because this process releases heat, the condenser coil is always hot.
The pressurized liquid presses against an expansion valve at the end of the coil. When the pressure is great enough, the valve opens and sprays the refrigerant into the evaporator coils, where the sudden pressure reduction turns the refrigerant into a vapor.
Vaporization draws energy from the surrounding medium (air, soil or water). This passes back into the condenser coil to be released as heat when the refrigerant re-pressurizes.
Types of Heat Pump Systems
It’s possible to categorize heat pumps according to the medium where they exchange heat.
An air-source pump gets heat from the surrounding air. A ground-source (or geothermal) pump gets it from the ground, and a water-source pump from a nearby body of water. Ground- and water-source heat pumps take advantage of the stable temperature conditions underground or in a body of water. They work better in cold climates than air-source models.
Another way to categorize heat pumps is by size. Whole-house units with powerful compressors can take the place of a furnace and air conditioner in a central heating/cooling system. The condenser coil is mounted in an air handler that also houses a blower.
Smaller heat pumps, known as ductless mini-splits, have one or more wall-mounted indoor units, each with its own fan. Mini-splits produce enough heat for a single room. There are also mini-split systems with a single compressor that feeds up to eight or nine wall-mounted units.
Where Are Heat Pumps Most Common?
You’ll find them mostly in regions with moderate winter temperatures.
According to data from the National Association of Home Builders, 75% to 77% of homes in the East South Central and South Atlantic states have them, while 20% of homes in the West South Central states do. They aren’t as common in Northern states because air temperatures are too cold, and ground- or water-source models are expensive to install.
However, the situation isn’t static. Manufacturers are working to develop more efficient models that work in cold climates. Plus, units can be equipped with auxiliary resistive heaters to provide supplemental warmth on cold days.
Buying a Heat Pump
The most important characteristics to consider are the heat output, measured in British Thermal Units (Btus), and the Seasonal Energy Efficiency Rating (SEER). Heat output is also often measured in tons, where one ton = 12,000 Btus.
There are two rules of thumb for sizing a heat pump. One is to provide 30 Btus of heat per square foot of floor space. The other is to divide the total floor area by 500 to determine the number of tons you need. It’s important for an HVAC contractor to perform tests to accurately size a unit for a particular house so it produces the right amount of heat.
SEER numbers start at 13 (low efficiency) and climb to 21 or 22 (high efficiency). A unit with a high SEER number uses less electricity, but has more moving parts and costs more to buy and maintain.
In practice, a system with a 16 SEER should be adequate in moderate climates. To maximize performance in a colder climate or minimize power usage, go with a higher SEER. You can calculate the energy a particular unit uses and its monthly cost by dividing the heat output in Btus by the SEER. You’ll get the result in watts per hour. Then multiply that by the local electricity rate.
Installing a Heat Pump
Some mini-split units, like the Mr. Cool DIY, can be installed by homeowners. If you’re not comfortable with that, hire a licensed HVAC technician.
The outdoor unit mounts on a concrete pad or footers to keep it high and dry. One or more indoor units are installed on walls or inside an air handler connected to the building’s ductwork. The two units are then connected by refrigeration tubing and electrical wires that pass through the wall.
Because most heat pumps also function as air conditioners, a drainage tube is included for condensate that drips from the indoor coil when it’s cold.
Heat Pump Maintenance
Both the indoor and outdoor units need routine maintenance, including changing air filters and cleaning dust off the coils. The outdoor unit must be kept free of snow, leaves and other debris so air can circulate.
You may have to occasionally de-ice the evaporator coil in the outdoor unit in the winter and the indoor unit in the summer. Just turn off the power and let the ice melt, rather than trying to scrape it off and possibly damaging the coil.