A heat pump moves warmth from outside air or ground into your home, rather than burning fuel to create heat

A heat pump is an appliance that works like a refrigerator running in reverse. A refrigerator pulls warmth out of a box and dumps it into your kitchen. A heat pump pulls warmth from outside air (or from the ground) and pushes it into your home. It does this using a refrigerant—a liquid that evaporates and condenses easily—and a compressor that pressurizes it. The whole cycle repeats continuously, moving heat from one place to another without burning anything.

The reason this matters: moving heat uses far less electricity than generating it. An electric resistance heater (like a space heater or old electric baseboard) converts electricity directly into warmth at a 1-to-1 ratio. A heat pump can move three to four units of heat for every unit of electricity it uses, because it is not creating the heat—it is relocating it. That efficiency difference shows up directly on your heating bill.

Key Takeaways

  • A heat pump moves existing warmth from outside into your home using a refrigerant and compressor, rather than generating heat by burning fuel or using resistance.
  • The outdoor unit absorbs heat from air or ground; the indoor unit releases that heat into your home, with a compressor cycling the refrigerant between them.
  • Heat pumps work in cold climates but lose efficiency below freezing, and many models include a backup electric heater for extreme cold or defrost cycles.
  • The four main types are air-source (most common), ground-source (most efficient but expensive), mini-split (ductless), and hybrid (heat pump plus gas furnace).
  • Heat pumps require annual maintenance—cleaning filters and coils—and refrigerant leaks must be repaired by a licensed technician.

The basic cycle: evaporator, compressor, condenser, expansion valve

The heat pump has four main parts, and they work in a loop. The outdoor unit contains the evaporator coil, where cold refrigerant evaporates and absorbs heat from the outside air (or ground). That warm refrigerant gas then flows to the compressor, which squeezes it under high pressure. Compression heats the refrigerant further—this is the key step that makes the whole system work.

The hot, pressurized refrigerant then flows to the indoor unit, where it enters the condenser coil. A fan blows indoor air across this coil, and the refrigerant releases its heat into your home. As the refrigerant cools, it condenses back into a liquid. Finally, the liquid flows through an expansion valve, which reduces its pressure and temperature, and the cycle begins again.

In heating mode, this cycle runs as described. In cooling mode (air conditioning), the system reverses direction using a reversing valve. The outdoor unit becomes the condenser (releasing heat outside), and the indoor unit becomes the evaporator (pulling heat out of your home). The same four parts do the same job—they just swap roles.

Why heat pumps work in cold weather, and where they struggle

Heat pumps can extract warmth from outside air even when it is cold—as long as the air is above absolute zero, it contains some heat energy. An air-source heat pump can heat your home at temperatures well below freezing. However, the colder the outside air, the less heat is available to move, so the compressor has to work harder and longer. Your heating bill rises as outdoor temperature drops.

Below about 25°F to 35°F (depending on the model), most air-source heat pumps become less efficient than a backup heating source. Many systems include a backup electric heater (called auxiliary or emergency heat) that automatically switches on during extreme cold. The heat pump still runs, but the electric heater supplements it. Some newer models stay efficient to much lower temperatures, but they cost more.

Heat pumps also need to defrost themselves. When outdoor humidity is high and temperature is near freezing, frost builds up on the outdoor coil, blocking airflow. The system detects this and reverses to cooling mode briefly, melting the frost. During defrost, your indoor temperature may dip slightly, and the backup heater kicks in to maintain comfort. This cycle lasts a few minutes and repeats as needed.

Air-source heat pumps: the most common type

An air-source heat pump pulls heat from outside air. It is the most affordable type and works in most climates, including cold ones. The outdoor unit looks like an air conditioner condenser—a metal box with a fan and coils. The indoor unit can be a traditional furnace-style unit (if you have ducts), a wall-mounted or ceiling-mounted blower, or a combination of both.

Air-source heat pumps lose efficiency as temperature drops, but they are still cost-effective in climates where winter lows stay above 0°F most of the time. In very cold climates (northern New England, the Upper Midwest, parts of the Mountain West), they work but require more backup heating, which raises your winter bill. Some newer cold-climate models are designed to stay efficient to -15°F or lower.

Ground-source (geothermal) heat pumps: most efficient, highest upfront cost

A ground-source heat pump, also called a geothermal heat pump, pulls heat from the ground or groundwater instead of air. The ground stays at a relatively constant temperature year-round—usually 45°F to 55°F depending on your location. Because the temperature difference between the refrigerant and the ground is smaller than the difference between the refrigerant and winter air, the compressor does not have to work as hard. These systems are the most efficient type.

The tradeoff is installation cost. A ground-source system requires drilling wells or burying long loops of pipe underground, which can cost $15,000 to $30,000 or more depending on your soil, lot size, and local drilling costs. The system itself lasts 25 to 50 years, so the cost spreads over decades, but the upfront expense is substantial. Ground-source heat pumps make the most sense if you are replacing an old heating system and plan to stay in your home for at least 10 years.

Mini-split and ductless heat pumps for homes without ducts

A mini-split heat pump (or ductless heat pump) has a small outdoor unit connected by refrigerant lines to one or more indoor wall-mounted or ceiling-mounted units. Each indoor unit can be controlled independently, so you can heat one room while leaving another cool. Mini-splits are ideal for homes without ductwork, additions, or rooms that are hard to heat with a central system.

Installation is simpler than a central system because you do not need to run ducts through walls and ceilings. The refrigerant lines are small and can fit through a 3-inch hole in the wall. Mini-splits cost less to install than a full central system or ground-source heat pump, but more than a window air conditioner. They are increasingly popular in older homes and renovations where ductwork is impractical.

Hybrid heat pumps: heat pump plus gas furnace

A hybrid heat pump system pairs a heat pump with a gas furnace. The heat pump runs when conditions favor it (mild weather, when it is most efficient), and the gas furnace takes over during extreme cold or when the heat pump cannot keep up. The system automatically switches between them based on outdoor temperature and heating demand.

Hybrid systems are common in cold climates where a heat pump alone would need frequent backup heating. By using the gas furnace instead of electric resistance heat, you save money on winter bills. However, you still need a gas line and a chimney or vent, and you pay for both the heat pump and furnace equipment. A hybrid system makes sense if you already have gas heating and want to reduce heating costs without replacing the entire system.

Maintenance: filters, coils, and when to call a technician

Heat pumps need less maintenance than furnaces because they have no flame or combustion, but they still require regular care. Change or clean the indoor air filter every 1 to 3 months, depending on dust and pets. A clogged filter reduces airflow and forces the compressor to work harder, raising your bill and shortening the system's life.

Once a year, have a technician inspect the system. They will clean the outdoor coil (which collects dust and debris), check refrigerant pressure, inspect electrical connections, and test the reversing valve and backup heater. This annual visit costs $100 to $200 and catches problems early. If you notice ice buildup on the outdoor unit that does not melt during a defrost cycle, or if the system is not heating or cooling properly, call a technician—refrigerant leaks and compressor problems require professional repair.

Do not attempt to add refrigerant yourself. Refrigerant is pressurized, toxic, and regulated by the EPA. Only a licensed HVAC technician can handle it. If your system is low on refrigerant, there is a leak, and the technician must find and repair it before refilling.

Frequently Asked Questions

Can a heat pump heat a home in winter if it is below freezing outside?

Yes. Heat pumps extract warmth from outside air even in freezing temperatures because air always contains some heat energy. However, efficiency drops as temperature falls, and most systems include a backup electric heater for extreme cold. Below about 25°F, the backup heater may run alongside the heat pump to maintain comfort and reduce strain on the compressor.

How long does a heat pump last?

A well-maintained air-source heat pump typically lasts 15 to 20 years. Ground-source heat pumps last longer—often 25 to 50 years—because the underground pipes are protected from weather and temperature swings. Regular maintenance (annual inspections, filter changes, coil cleaning) extends the life of any system.

What is the difference between a heat pump and an air conditioner?

An air conditioner only cools; it removes heat from your home and dumps it outside. A heat pump does both: it can cool in summer and heat in winter by reversing the direction of refrigerant flow. A heat pump costs more upfront but provides both heating and cooling, whereas an air conditioner only provides cooling.

Do heat pumps work in very cold climates?

Air-source heat pumps work in cold climates but lose efficiency below freezing and rely more on backup heating. Ground-source heat pumps are more efficient in cold climates because the ground stays warmer than winter air. Hybrid systems (heat pump plus gas furnace) are popular in very cold regions because they use the most efficient heating source for each temperature range.

Why is my heat pump making noise during heating?

During defrost cycles, the system reverses and may make hissing or gurgling sounds as refrigerant changes direction. A brief dip in indoor temperature is normal. If you hear grinding, squealing, or loud rattling, call a technician—these sounds suggest a compressor problem, loose parts, or a refrigerant leak that needs professional repair.