Heat pumps work by moving heat from one place to another, not by creating it
A heat pump doesn't generate warmth the way a furnace burns gas or an electric heater glows red. Instead, it captures heat that already exists—even in cold air—and moves it indoors using refrigerant, a liquid that absorbs and releases heat as it cycles through the system. In winter, it pulls heat from outside air (or ground or water) and pushes it into your home. In summer, it reverses direction and removes heat from inside, cooling your house. This is why heat pumps work in both seasons, while a furnace only heats.
The process relies on a straightforward physical principle: refrigerant changes state between liquid and gas depending on pressure and temperature. When pressure drops, the refrigerant evaporates and absorbs heat. When pressure rises, it condenses and releases that heat. A compressor, an expansion valve, and two coils (one inside, one outside) work together to cycle the refrigerant back and forth, moving heat where you need it.
Key Takeaways
- Heat pumps move existing heat rather than creating it, which is why they work even when outdoor air is cold.
- The outdoor coil absorbs heat in winter and releases it in summer; the indoor coil does the opposite.
- A compressor pressurizes the refrigerant to control when it absorbs and releases heat.
- Heat pumps are more efficient than furnaces because moving heat uses less energy than burning fuel to create it.
The refrigerant cycle: how heat actually moves
The refrigerant starts as a cold, low-pressure liquid in the outdoor coil (called the evaporator in heating mode). As outdoor air passes over this coil, heat from that air transfers into the refrigerant, causing it to evaporate into a gas. Even in freezing temperatures, there is still heat energy in the air—the refrigerant straightforward absorbs it.
This warm gas then flows to the compressor, which squeezes it under high pressure. Compression heats the gas further, raising its temperature above the temperature inside your home. The hot, pressurized gas then enters the indoor coil (called the condenser in heating mode). Indoor air is blown across this hot coil, absorbing the heat and warming your house. As the refrigerant releases heat, it cools down and condenses back into a liquid.
The liquid then passes through an expansion valve, which drops its pressure suddenly. This pressure drop cools the refrigerant below outdoor temperature, and the cycle begins again. In summer, the system reverses: the indoor coil becomes the evaporator (absorbing heat from your home), and the outdoor coil becomes the condenser (releasing that heat outside).
Why the outdoor coil works even in winter
Many people assume a heat pump cannot pull heat from air that is already cold. In reality, heat exists in air at any temperature above absolute zero (−459°F). Even at 20°F outside, there is usable heat energy. The refrigerant in the outdoor coil is kept even colder than the outdoor air through the expansion valve, so heat naturally flows from the warmer air into the colder refrigerant.
However, as outdoor temperature drops, the temperature difference between the air and the refrigerant shrinks, so each cycle captures less heat. This is why heat pumps become less efficient in very cold climates. Some systems include a backup electric heater that turns on automatically when outdoor temperature falls below a certain point (often around 35°F to 40°F, depending on the model). This backup heater supplements the heat pump rather than replacing it, so the system still uses the more efficient heat pump whenever possible.
The compressor: the engine that makes it all work
The compressor is the most energy-intensive part of a heat pump. It uses an electric motor to squeeze the refrigerant gas, raising its pressure and temperature. This is the only moving part that requires significant electricity. Everything else—the flow of refrigerant, the transfer of heat across the coils—happens because of the pressure difference the compressor creates.
The compressor runs continuously during heating or cooling, but modern heat pumps often use a variable-speed compressor that adjusts its speed based on how much heating or cooling is needed. A slower compressor uses less electricity and produces less noise. When your home reaches the target temperature, the compressor slows down or stops entirely, rather than cycling on and off like older models. This steady operation is one reason heat pumps are more efficient than furnaces, which must fire up and shut down repeatedly.
Indoor and outdoor coils: where the heat exchange happens
The outdoor coil is a grid of thin metal tubes with fins attached. Air flows across these fins, and heat transfers between the air and the refrigerant inside the tubes. In winter, this coil absorbs heat; in summer, it releases it. The coil is exposed to weather, so it has a fan that runs continuously to keep air moving across it and maximize heat transfer.
The indoor coil sits inside your furnace or air handler (a box that holds the coil and a blower fan). In winter, your home's air is blown across this coil by the blower fan, picking up heat from the warm refrigerant. In summer, the same coil removes heat from your indoor air. The indoor coil is protected from weather and is usually quieter than the outdoor unit, though you may hear the blower fan running.
Why heat pumps are more efficient than furnaces
A furnace burns fuel (gas, oil, or electricity) to create heat. Even the most efficient furnace loses some energy up the chimney or through the walls of the combustion chamber. A heat pump, by contrast, straightforward moves heat that already exists. Moving heat requires far less energy than creating it, which is why heat pumps can deliver two to four units of heat for every unit of electricity they consume. A furnace delivers roughly one unit of heat for every unit of fuel burned.
This efficiency advantage shrinks in very cold climates where the outdoor coil cannot extract much heat, forcing the backup heater to run. In mild climates, heat pumps are dramatically more efficient. Even in cold regions, a heat pump with a backup heater often uses less total energy than a furnace alone, because the heat pump runs for most of the heating season and only the backup kicks in during the coldest weeks.
Reversing valve: how the system switches between heating and cooling
A small component called the reversing valve allows the heat pump to switch between heating and cooling modes. This valve redirects the flow of refrigerant so that the indoor and outdoor coils swap roles. In heating mode, the outdoor coil absorbs heat and the indoor coil releases it. In cooling mode, the indoor coil absorbs heat and the outdoor coil releases it.
The reversing valve is controlled by your thermostat. When you switch from heating to cooling (or vice versa), the thermostat sends a signal that flips the valve. The changeover takes only a few seconds, and you may hear a brief hissing sound as the refrigerant pressure equalizes. Some heat pumps also have a defrost cycle that briefly reverses the system in winter to melt ice off the outdoor coil, then switches back to heating.
Frequently Asked Questions
Can a heat pump work when it is freezing outside?
Yes. Heat pumps pull heat from outdoor air even at freezing temperatures because heat energy exists in all air above absolute zero. However, efficiency drops as outdoor temperature falls, and most systems include a backup electric heater that activates during very cold weather to maintain comfort without straining the heat pump.
What is the loud noise coming from the outdoor unit?
The outdoor fan and the compressor both produce noise. A variable-speed compressor is quieter than an older fixed-speed model. Noise levels vary by brand and model; some units are designed to run at 50 decibels or lower, while others may reach 70 decibels. A professional can assess whether your unit is operating normally or if a repair is needed.
Why does my heat pump sometimes reverse and make a hissing sound?
That is the defrost cycle. In winter, frost can build up on the outdoor coil, blocking airflow. The system briefly reverses to heating mode, warming the outdoor coil to melt the frost, then switches back. This cycle lasts a few minutes and happens automatically. You may feel a brief blast of cool air indoors during defrost, which is normal.
Does a heat pump need refrigerant added regularly?
No. A heat pump is a sealed system, and refrigerant should not leak out under normal operation. If your system is low on refrigerant, it indicates a leak that must be found and repaired by a technician. Adding refrigerant without fixing the leak is temporary and will not solve the problem.
How does a heat pump heat a home if it is colder outside than inside?
The compressor raises the pressure and temperature of the refrigerant gas above indoor temperature, so heat flows from the hot refrigerant into your home's air, even when outdoor air is cold. The refrigerant itself becomes much colder than outdoor air through the expansion valve, allowing it to absorb heat from outside. This temperature difference is what makes the system work.