Heat pumps

The idea is the same one your kitchen uses. Like your refrigerator, a heat pump uses electricity to move heat from a cool space to a warm space, making the cool space cooler and the warm space warmer. Unlike a furnace or an air conditioner, a heat pump provides both heating and cooling from the same piece of equipment. The part that makes this possible is the reversing valve, which changes the direction the refrigerant flows. In heating, the outdoor coil gathers heat from outside air and the indoor coil gives it to your rooms.

Ducted air-source heat pumps use your home's existing ductwork to deliver heating and cooling. In most climate zones, an air-source heat pump can be installed as a drop-in replacement when either a central air conditioner or a furnace needs replacing. Dual-fuel, or hybrid, systems pair the efficiency of a heat pump with the reliability of a gas furnace. So the time to ask about one is often when old equipment is due for replacement.

Source: U.S. Department of Energy · Pacific Northwest National Laboratory · ENERGY STAR

Cold weather is the hard test for any heat pump. During especially cold weather, a heat pump cannot pull enough heat from the outdoors to keep the inside warm. Newer technology lets some models run efficiently, at near-full capacity, in very cold weather, and these cold-climate air-source heat pumps are now a workable heat source in any U.S. climate zone. ENERGY STAR certification requires third-party verified performance at low temperatures, testing air-source heat pumps down to 5°F.

A cold-climate unit keeps working below 5°F, but pairing it with a backup heat source heats your home most efficiently when it gets colder still. Heat pumps optimized for peak heating may carry the ENERGY STAR Cold Climate mark if they are certified to meet its criteria. One guideline for cold climates is to check that a unit keeps at least 80% of its rated 47°F heating capacity at 5°F. When you compare models, look at how much heat each one makes at low temperatures, not just its headline rating.

Source: U.S. Department of Energy · ENERGY STAR · Northwest Energy Efficiency Alliance

Most heat pumps in cold places have a second source of heat behind them. Heat pumps have traditionally been installed with backup heat, usually an electric resistance element, so the home stays warm at cooler outdoor temperatures (for example, below 35°F). If the heat pump's heating capacity is too small, the backup heat makes up the difference, but at a much lower efficiency. A properly sized cold-climate heat pump can meet 100% of a home's heating load in nearly any location in the United States.

The balance point is a useful idea to know. It is the outdoor temperature at which your home's heating need matches what the heat pump can put out. Above it, the heat pump provides all the heating. Below it, the heat pump cannot fully meet your home's needs without a backup heat source. How the heat pump is sized shifts that balance point warmer or colder.

Dual-fuel systems handle the switch in a set order. With a gas furnace as backup, the thermostat first brings on the heat pump to answer a call for heat, and only turns it off and brings on the furnace if the heat pump cannot satisfy that call. The outdoor temperature at which it switches can be set by comparing the cost of heat from the heat pump with the cost from the backup. Ask your installer where your switch-over point is set and why.

Source: U.S. Department of Energy · Natural Resources Canada

Here is what happens when frost builds up outside. Frost forms on the outdoor coil while a heat pump runs. The outdoor coil can gather frost or ice in certain conditions of cold air and humidity while it is heating. Under certain conditions, the heat pump must enter a defrost cycle to melt that ice. Defrosting usually works by reversing the refrigerant cycle, running the heat pump as an air conditioner for a while.

During defrost, the reversing valve switches the heat pump from heating to cooling for a short time. The compressor pumps hot refrigerant through the outdoor coil, melting the ice on it. Once a control board finds the defrost is complete, usually from time or temperature sensors, it switches the valve back to heating. In one government lab test of two heat pumps, the indoor fan stopped and the unit gave no heat during defrost.

Melting frost has to go somewhere. Outdoor units release water when they defrost in winter, and that water can form icy patches. Defrost also costs some efficiency: one example cited by the Department of Energy puts the seasonal efficiency loss from frost and its removal at 1.5% to 12.7%. If the outdoor unit stays covered in thick ice, tell us what you see.

Source: U.S. Department of Energy · ENERGY STAR

A common complaint about heat pumps is air that feels cooler than you expect. Settings play a part. In general, heat pumps are most efficient with the indoor fan speed set to auto, and fixing the fan at one speed can reduce capacity and efficiency. If you have a heat pump, you may need a special programmable thermostat to make the most of your savings all year.

Capacity and upkeep play a part too. If the heat pump's capacity is too small for the coldest days, backup heat makes up the difference at a much lower efficiency. A dirty filter strains a heat pump and leads to higher energy bills. A technician can look at the settings, the backup heat and the outdoor unit together.

Source: U.S. Department of Energy · U.S. EPA

A heat pump works all year, so it needs steady care. Make sure your heat pump gets professional maintenance each year. Keep the indoor and outdoor units clean and clear of dust, snow, leaves and other debris. Keep the outdoor unit's airflow open by clearing obstructions throughout the year. Work with dealers whose technicians are EPA-certified to handle refrigerants.

Ratings help you compare models. A heat pump's cooling efficiency is given as SEER and its heating efficiency as HSPF or COP, depending on the type. To earn the ENERGY STAR, a ducted split-system heat pump needs at least 8.1 HSPF2 and 15.2 SEER2. Many utilities offer incentives for installing ENERGY STAR certified air-source heat pumps. Program terms change, so check what is offered before you buy.

Source: U.S. Department of Energy · ENERGY STAR · U.S. EPA