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

A heat pump is a machine that captures heat that already exists in the air or ground outside your home and moves it indoors. It does not generate heat the way a furnace burns gas or oil. Instead, it uses electricity to run a refrigerant cycle—the same basic process that makes a refrigerator cold—but in reverse. In winter, it pulls warmth from the outdoor air (even when it feels cold) or from the ground below your property, concentrates that warmth, and releases it inside. In summer, it reverses direction and pulls heat out of your home to cool it down.

The reason this matters is efficiency: moving heat uses far less energy than creating it from scratch. A heat pump can deliver three to four units of heat for every unit of electricity it consumes, whereas a furnace converts fuel into heat at roughly 80 to 95 percent efficiency. This is why heat pumps lower heating and cooling costs in most climates, though the savings depend on your local electricity rates, outdoor temperatures, and the type of heat pump you have.

Key Takeaways

  • Heat pumps move existing warmth from outside into your home instead of burning fuel, using a refrigerant cycle powered by electricity.
  • In winter, the system extracts heat from outdoor air or ground; in summer, it reverses to pull heat out of your home and cool it down.
  • The four main parts are the outdoor unit, indoor unit, refrigerant lines, and a reversing valve that switches the direction of heat flow.
  • Heat pumps work in cold climates but lose efficiency below about 25°F, which is why many homes in freezing regions use a backup heating system.
  • The refrigerant cycle involves four stages—evaporation, compression, condensation, and expansion—that repeat continuously to move heat.

The Four Main Parts of a Heat Pump System

Every heat pump has the same core components, whether it pulls heat from the air or the ground. The outdoor unit contains a coil that absorbs heat from the surrounding environment. The indoor unit contains another coil that releases that heat into your home (or absorbs heat from your home in summer). Refrigerant—a liquid chemical that boils and condenses at low temperatures—flows between these two coils through insulated copper lines. A reversing valve is a four-way switch that changes the direction the refrigerant flows, so the system can heat in winter and cool in summer.

The fourth essential part is the compressor, which sits in the outdoor unit and is powered by electricity. The compressor squeezes the refrigerant gas, raising its pressure and temperature. This is the energy input that makes the whole cycle work. When you hear a heat pump running, you are hearing the compressor and the fan that blows air across the outdoor coil. The indoor unit has its own fan that circulates air through the indoor coil and into your home's ductwork or directly into rooms.

How the Refrigerant Cycle Moves Heat

The heat pump's core job happens in four repeating stages. First, evaporation: the refrigerant enters the outdoor coil as a low-pressure liquid. Heat from the outside air (or ground) causes it to boil and turn into a gas, even when outdoor temperatures are well below freezing. This is the stage where heat is absorbed from outside.

Second, compression: the compressor sucks in this low-pressure gas and squeezes it, raising both its pressure and temperature. The refrigerant becomes a hot, high-pressure gas—hotter than the air inside your home. Third, condensation: this hot gas flows to the indoor coil, where it releases its heat to the air inside your home. As it cools, the refrigerant condenses back into a liquid. This is where you feel the warmth coming out of your vents. Fourth, expansion: the liquid refrigerant passes through an expansion device (a small orifice or valve) that lowers its pressure and temperature, turning it back into a low-pressure liquid. It then returns to the outdoor coil to start the cycle again.

In summer, the reversing valve flips the cycle: the outdoor coil becomes the condenser (where heat is released outside), and the indoor coil becomes the evaporator (where heat is absorbed from your home). The same four stages happen, but in the opposite direction.

Air-Source vs. Ground-Source Heat Pumps

An air-source heat pump pulls heat from the outdoor air. It is the most common type because it costs less to install—no digging required—and works in most climates. The outdoor unit looks similar to an air conditioning condenser. Air-source heat pumps lose efficiency as outdoor temperatures drop, especially below 25°F. In very cold climates, many homes pair an air-source heat pump with a backup electric resistance heater or a gas furnace that kicks in when outdoor temperatures fall too low.

A ground-source heat pump (also called a geothermal heat pump) pulls heat from the earth below your property, where temperatures stay relatively constant year-round—usually 45 to 55°F depending on your location. Ground-source systems are more efficient than air-source systems, especially in cold climates, because they do not lose efficiency as outdoor air temperature drops. However, installation costs are much higher because the system requires digging trenches or drilling wells to bury refrigerant lines underground. Ground-source heat pumps make financial sense in cold regions where heating costs are high and you plan to stay in the home for many years.

Why Heat Pumps Work Even in Cold Weather

Many people assume a heat pump cannot work when it is freezing outside, but this misunderstands how the refrigerant cycle works. The refrigerant boils at a much lower temperature than water—typically between minus 40°F and minus 20°F depending on the type of refrigerant. This means the outdoor coil can still absorb heat from 20°F air and convert the refrigerant to a gas. The compressor then concentrates that heat, making it hot enough to warm your home.

The catch is that as outdoor air temperature drops, the temperature difference between the outdoor coil and the air shrinks, so heat transfer slows down. Below about 25°F, an air-source heat pump's heating output drops noticeably, and the compressor works harder to maintain the same indoor temperature. This is why the system's efficiency rating (called the Heating Seasonal Performance Factor, or HSPF) is always lower in cold climates than in mild ones. In regions where winter temperatures regularly drop below 0°F, a backup heating system is usually necessary to keep your home warm without running the heat pump constantly.

Defrost Cycles and Winter Operation

During winter, moisture in the outdoor air can freeze on the outdoor coil of an air-source heat pump, reducing heat transfer and eventually blocking airflow. To prevent this, the system automatically switches into defrost mode every 30 to 90 minutes (depending on outdoor humidity and temperature). During defrost, the reversing valve flips the cycle temporarily, so the outdoor coil becomes hot and melts the frost. While this is happening, your home is not receiving heat, and in some systems, an electric resistance heater kicks in to maintain indoor temperature.

Defrost cycles typically last 5 to 15 minutes and are a normal part of winter operation. You may notice your indoor temperature drop slightly during defrost, or hear the outdoor unit fan stop and restart. This is not a malfunction. Ground-source heat pumps rarely need defrost cycles because the ground stays above freezing, which is another efficiency advantage in cold climates.

Comparing Heat Pump Efficiency to Other Heating Systems

A heat pump's efficiency is measured by its Coefficient of Performance (COP) or Heating Seasonal Performance Factor (HSPF). A COP of 3 means the system delivers three units of heat for every unit of electricity consumed. A typical air-source heat pump has an HSPF of 8 to 10 in mild climates, meaning it delivers 8 to 10 units of heat per unit of electricity over a full heating season. A gas furnace, by contrast, converts fuel into heat at 80 to 95 percent efficiency—it wastes 5 to 20 percent of the fuel's energy as exhaust.

Because electricity is often more expensive per unit of energy than natural gas, the cost savings from a heat pump depend on your local utility rates. In regions where electricity is cheap and gas is expensive, heat pumps save money year-round. In regions where gas is cheap, heat pumps may save money on cooling but cost more to operate for heating, especially if outdoor temperatures are very cold. Ground-source heat pumps are more efficient than air-source systems but cost more upfront, so the payback period is longer.

Common Issues and How Heat Pumps Handle Them

One frequent concern is noise. Heat pump outdoor units are louder than air conditioning units because the compressor works harder in heating mode. Typical noise levels range from 40 to 50 decibels—similar to a window air conditioner or a quiet refrigerator. If noise is a concern, ask your installer about units with sound-dampening cabinets or consider placing the outdoor unit away from bedrooms.

Another issue is short-cycling: if the indoor temperature rises just one or two degrees above your thermostat setting, the system shuts off, then restarts a few minutes later. This is normal and actually more efficient than running continuously, but frequent cycling can feel uncomfortable. A programmable or smart thermostat with a wider temperature deadband (the range between heating and cooling) can reduce this. A third concern is that some heat pumps produce warm air that feels cooler than air from a furnace, even though the system is working correctly. This happens because heat pump air is typically 90 to 100°F, whereas furnace air is 130 to 140°F. The lower temperature is still warming your home; it just does not feel as hot to your skin.

Frequently Asked Questions

Can a heat pump work if it is 0°F outside?

Yes, a heat pump can extract heat from 0°F air because the refrigerant boils at much lower temperatures. However, its heating output drops significantly, and the compressor works harder, raising your electricity use. Most air-source heat pumps are paired with a backup heater for temperatures below 0°F to keep heating costs reasonable.

Why does my heat pump outdoor unit have ice on it?

Ice buildup is normal in winter and triggers the defrost cycle automatically. The system reverses temporarily to melt the frost, which may cause a brief drop in indoor temperature. If ice is constantly thick or the system is not defrosting, contact a technician—it could indicate a refrigerant leak or sensor problem.

Is a heat pump cheaper to run than a gas furnace?

It depends on your local electricity and gas prices. In regions where electricity is cheap relative to gas, heat pumps usually cost less to operate. In regions where gas is much cheaper, a heat pump may cost more for heating, though it still saves money on cooling. A technician can estimate your costs based on your utility rates.

How long does a heat pump last?

A well-maintained heat pump typically lasts 15 to 20 years. Regular maintenance—cleaning filters, checking refrigerant levels, and inspecting coils—extends the lifespan. Ground-source systems often last longer because they are protected underground and experience less temperature stress.

Can I use a heat pump in a very cold climate?

Yes, but you will need a backup heating system for the coldest days. Many cold-climate homes use an air-source heat pump paired with a gas furnace or electric resistance heater. Ground-source heat pumps are more efficient in cold climates but cost significantly more to install.