What an electric heat pump actually does

An electric heat pump doesn't generate heat the way a furnace burns fuel. Instead, it moves heat from one place to another using electricity and a refrigerant—the same basic principle as a refrigerator, but working in reverse. In winter, it pulls heat from the cold air outside (or from the ground) and moves it indoors. In summer, it reverses direction and pulls heat from inside your home and moves it outdoors, cooling your space.

The key difference from traditional heating is efficiency. A furnace converts fuel into heat at roughly 80 to 95 percent efficiency. A heat pump can deliver two to four units of heat for every unit of electricity it uses, because it's moving existing heat rather than creating new heat from scratch. That's why heat pumps cost less to run than electric resistance heating or gas furnaces in most climates.

Key Takeaways

  • Heat pumps move heat from outside air or ground into your home using electricity and a refrigerant, rather than burning fuel to create heat.
  • The refrigerant circulates through an outdoor unit and an indoor unit, changing between liquid and gas states to absorb and release heat.
  • In winter, the outdoor coil absorbs heat from cold air; in summer, the indoor coil absorbs heat from your home and the outdoor coil releases it.
  • Heat pumps work in temperatures down to about 0°F, though efficiency drops as outdoor temperature falls and backup heating may turn on automatically.
  • The compressor, which runs on electricity, does the work of moving refrigerant and is the main energy consumer in the system.

The refrigerant cycle: how heat actually moves

The refrigerant is the working fluid that carries heat through the system. It circulates in a closed loop between the outdoor unit and the indoor unit, constantly changing between liquid and gas states. When refrigerant evaporates (turns to gas), it absorbs heat. When it condenses (turns back to liquid), it releases heat. The heat pump uses this property to move warmth from where you don't want it to where you do.

In heating mode, the outdoor coil acts as an evaporator. Even when the air outside is cold—say 30°F—the refrigerant inside the coil is even colder, so heat flows from the outdoor air into the refrigerant, turning it into a gas. This gas travels indoors through copper tubing to the indoor coil, where a compressor has pressurized it. The pressurized gas releases its heat into your home's air or water, turning back into a liquid. That liquid flows back outside, pressure drops, and the cycle repeats.

In cooling mode, the roles flip. The indoor coil becomes the evaporator, absorbing heat from your home's air. The refrigerant gas travels outside, where the outdoor coil releases that heat to the outside air. The cycle is identical—only the direction changes.

The compressor: the engine that moves heat

The compressor is the electric motor at the heart of the system. It pressurizes the refrigerant gas, which raises its temperature and allows it to release heat more easily indoors (or absorb heat more easily outdoors). Without the compressor, the refrigerant would flow passively and no heat transfer would happen.

The compressor is also the main reason heat pumps use electricity. It runs continuously while the system is heating or cooling, and its energy consumption determines your operating cost. Modern heat pumps use variable-speed compressors that slow down or speed up based on how much heating or cooling you need, rather than running at full power all the time. This reduces energy use on mild days when you need less heating or cooling.

Outdoor and indoor units: where the heat exchange happens

The outdoor unit houses the compressor, one of the two coils, and a fan that blows air across the coil. In winter, this fan pulls cold outside air across the coil so the refrigerant can absorb heat from it. In summer, the fan pulls air across the coil to help the refrigerant release heat to the outdoors. The outdoor unit is where you hear most of the noise from a heat pump—the compressor and fan running together.

The indoor unit contains the other coil and a blower that pushes air from your home across the coil. In winter, warm refrigerant flows through this coil and heats the air, which the blower sends through your ductwork or directly into rooms. In summer, cool refrigerant absorbs heat from your indoor air. The indoor unit is usually quieter than the outdoor unit because the compressor is outside.

The two units are connected by insulated copper tubing that carries refrigerant in both directions. A small amount of electrical wiring also connects them so the indoor thermostat can control the outdoor compressor.

How heat pumps work in cold weather

Heat pumps pull heat from outdoor air even when that air is very cold. At 0°F, there is still heat energy in the air—it's just at a lower concentration. The refrigerant inside the outdoor coil is kept even colder than the outdoor air, so heat still flows into it. However, as outdoor temperature drops, the temperature difference shrinks, and the heat pump must work harder (run the compressor longer) to move the same amount of heat indoors.

Most heat pumps operate efficiently down to about 0°F. Below that, many systems automatically switch on a backup heater—usually electric resistance heating or a gas furnace—to supplement the heat pump. This backup heater is less efficient than the heat pump but ensures your home stays warm. Some newer cold-climate heat pumps can operate down to -13°F or lower, though they are more expensive.

On very cold days, you may notice the outdoor unit has frost or ice on it. This happens because the outdoor coil is colder than the dew point of the outside air. Most heat pumps have a defrost cycle that reverses the refrigerant flow for a few minutes to melt the ice, then returns to heating mode. During defrost, your indoor temperature may drop slightly, and you may hear the system switch modes.

Expansion devices and airflow: completing the cycle

Between the two coils, the refrigerant passes through an expansion device—usually a metering valve or capillary tube—that reduces the pressure and temperature of the liquid refrigerant. This pressure drop is what allows the refrigerant to evaporate in the next coil and absorb heat. Without the expansion device, the refrigerant would remain a high-pressure liquid and couldn't absorb or release heat efficiently.

Airflow across the coils is also critical. If the outdoor coil is blocked by leaves, snow, or ice, the refrigerant can't absorb heat from the outdoor air, and the system's heating capacity drops sharply. If the indoor coil is blocked by dust or a clogged filter, the system can't transfer heat to your home. Most heat pump systems include a filter that you should check monthly and replace every three months, just as you would with a furnace.

Efficiency ratings and what they mean

Heat pump efficiency is measured by the Coefficient of Performance (COP) or Heating Seasonal Performance Factor (HSPF) for heating, and Seasonal Energy Efficiency Ratio (SEER) for cooling. These numbers tell you how many units of heat the system delivers for each unit of electricity it uses. A COP of 3 means the system delivers three units of heat for every unit of electricity—much better than a 95 percent efficient furnace, which delivers only 0.95 units of heat per unit of fuel energy.

HSPF and SEER ratings are seasonal averages that account for varying outdoor temperatures throughout the year. A heat pump with an HSPF of 10 and SEER of 20 is considered high-efficiency. Actual performance depends on outdoor temperature, how well your home is insulated, and how you operate the system. On very cold days, efficiency drops, which is why backup heating exists.

Frequently Asked Questions

Why does my heat pump seem to run all the time in winter?

Heat pumps move heat slowly compared to furnaces, so they run longer to deliver the same warmth. On very cold days, the system may run continuously because the outdoor temperature is so low that the heat pump can only move a small amount of heat per hour. This is normal and does not mean the system is broken. If your home isn't reaching the set temperature, the backup heater should turn on automatically.

Can a heat pump heat a home that's already very cold?

Yes, but it takes longer than a furnace. If you've been away and your home has dropped to 50°F, a heat pump will gradually warm it back up, though the backup heater may run to speed the process. Heat pumps are designed for continuous operation in cold climates, not for rapid recovery from very low temperatures. If you plan to be away in winter, it's better to keep the thermostat set to at least 55°F.

What's the difference between an air-source and ground-source heat pump?

An air-source heat pump pulls heat from outdoor air; a ground-source (or geothermal) heat pump pulls heat from the ground or groundwater, which stays at a more stable temperature year-round. Ground-source systems are more efficient because the ground is warmer than winter air, but they cost more to install because they require digging or drilling. Air-source heat pumps are the most common type for homes.

Do heat pumps work in humid climates?

Yes. In fact, heat pumps are very common in humid regions because they cool and dehumidify at the same time—the indoor coil removes moisture from the air as it cools it. In very humid climates, the system may run longer in cooling mode to remove moisture, but this is normal and actually improves indoor air quality.

Why does my heat pump make noise when it switches to defrost mode?

Defrost mode reverses the refrigerant flow, which causes the system to switch direction and can produce a hissing or whooshing sound as the valves shift. You may also hear the outdoor fan stop briefly. This is normal and lasts only a few minutes. If the noise is very loud or the system doesn't return to heating mode afterward, contact a technician.