A heat pump moves warmth from outside air or ground into your home, rather than burning fuel to create heat
A heat pump works by capturing heat that already exists in the air or soil outside your home and moving it indoors. Even on a cold day, there is warmth in the outdoor air—a heat pump extracts it, concentrates it, and releases it inside. In summer, the process reverses: the pump pulls heat out of your home and moves it outside, cooling your living space. This is why a heat pump can both heat and cool without a furnace or separate air conditioner.
The machine does this work using a refrigerant—a liquid that easily changes between liquid and gas states. As the refrigerant cycles through the system, it absorbs heat from one place and releases it in another. A compressor, an evaporator coil, a condenser coil, and an expansion valve work together to keep this cycle running. The compressor is the engine that drives the whole process, using electricity to pressurize the refrigerant and move it around the loop.
Key Takeaways
- A heat pump captures existing heat from outside air or ground and moves it indoors using a refrigerant that cycles between liquid and gas.
- The compressor is the electric motor that powers the system by pressurizing the refrigerant and forcing it through the cycle.
- In heating mode, the outdoor coil absorbs heat and the indoor coil releases it; in cooling mode, this process reverses.
- Heat pumps work even in cold weather because air and soil always contain some heat, though efficiency drops as outdoor temperature falls.
- The expansion valve controls how much refrigerant flows through the system, allowing the pump to adjust its heating or cooling output.
The refrigerant cycle: how heat moves through the system
The refrigerant starts as a cool liquid in the outdoor coil (called the evaporator when the pump is heating). As outdoor air passes over this coil, heat from the air transfers into the cold refrigerant, turning it into a gas. This is the first key step: the refrigerant absorbs heat without needing to burn anything.
The gaseous refrigerant then flows to the compressor, which squeezes it under high pressure. Compressing a gas heats it up—this is the same reason a bicycle pump gets warm when you use it. The hot, pressurized gas then moves to the indoor coil (called the condenser when heating). As indoor air is blown across this hot coil, the refrigerant releases its heat into your home and cools back down into a liquid.
The liquid refrigerant then passes through the expansion valve, which acts like a narrow opening. This valve drops the pressure and temperature of the refrigerant before it returns to the outdoor coil to start the cycle again. The whole loop repeats continuously, moving heat indoors as long as the pump is running.
Why heat pumps work even when it is cold outside
Many people assume a heat pump cannot work in freezing weather because there is no heat to extract. This is not true. Even at 0°F, the outdoor air contains heat energy. A heat pump can pull that heat out because the refrigerant in the outdoor coil is kept even colder than the air around it—cold enough to absorb heat from the frigid air.
However, efficiency does drop as outdoor temperature falls. The colder the air, the slower heat transfers into the refrigerant, and the harder the compressor must work to move that heat indoors. In very cold climates, many heat pump systems include a backup electric heating element that turns on when outdoor temperature drops below a certain point—often around 35°F. This backup heater uses more electricity than the heat pump itself, but it keeps your home warm when the pump alone cannot keep up.
Air-source versus ground-source heat pumps
An air-source heat pump pulls heat from the outdoor air. The outdoor unit sits outside your home and looks similar to an air conditioner. Air-source pumps are cheaper to install and work in most climates, but they lose efficiency as outdoor temperature drops. They are the most common type in residential homes.
A ground-source heat pump (also called a geothermal heat pump) pulls heat from the soil or groundwater instead. Underground temperature stays relatively constant year-round—around 50°F in most places—so ground-source pumps maintain steady efficiency even in harsh winters. The trade-off is cost: installing the underground loop system requires digging and is much more expensive than an air-source installation. Ground-source pumps make sense for homeowners planning to stay in their home for many years and have space for the underground piping.
The compressor: the electric motor that powers everything
The compressor is the most important and most energy-hungry part of a heat pump. It is an electric motor that pressurizes the refrigerant gas, forcing it through the system. Without the compressor running, the refrigerant would not move and no heat transfer would happen.
Modern heat pumps often use a variable-speed compressor that adjusts its power based on how much heating or cooling your home needs. On a mild day, the compressor runs slowly, using less electricity. On a very cold day, it runs faster and harder. This variable approach saves energy compared to older compressors that straightforward turn on and off at full power.
Cooling mode: the same cycle in reverse
When you switch a heat pump to cooling mode, the system reverses. The outdoor coil (now the condenser) releases heat, and the indoor coil (now the evaporator) absorbs it. Warm indoor air passes over the cold indoor coil, transferring heat into the refrigerant. That heat is then carried outside and released into the outdoor air.
This reversing happens through a component called the reversing valve, which changes the direction the refrigerant flows. The same pump that heated your home in winter cools it in summer, which is why heat pumps are more efficient than running separate heating and cooling systems.
Common signs a heat pump is not working properly
If your heat pump is running but your home is not warming or cooling, the refrigerant may be leaking. A leak means less refrigerant is available to absorb and move heat, so the system loses power. You will notice the pump running but the temperature inside staying the same. A refrigerant leak requires a technician to find the leak, repair it, and refill the system.
If the outdoor unit is frozen over in winter, the defrost cycle may not be working. Heat pumps have a defrost mode that temporarily reverses the cycle to melt ice buildup on the outdoor coil. If this fails, ice blocks airflow and the pump cannot absorb heat from the air. This also requires professional service.
Strange noises—hissing, bubbling, or grinding—can signal refrigerant problems, compressor wear, or loose parts. Any of these warrant a service call before the problem gets worse.
Frequently Asked Questions
Does a heat pump use a lot of electricity?
A heat pump uses electricity to run the compressor, but it moves heat rather than creating it, so it is more efficient than electric resistance heating. In moderate climates, a heat pump typically uses 25 to 50 percent less electricity than a traditional electric furnace. In very cold climates where the backup heater runs often, the savings shrink but usually remain positive.
Can a heat pump heat a home in winter if it is below freezing outside?
Yes. Even at 0°F, the outdoor air contains heat that a heat pump can extract. The refrigerant in the outdoor coil is kept colder than the air, so heat still flows into it. However, the system works harder and less efficiently in extreme cold, and many systems use a backup electric heater when outdoor temperature drops below 35°F.
What is the difference between a heat pump and an air conditioner?
An air conditioner only cools—it removes heat from your home and releases it outside. A heat pump does both: it can cool in summer and heat in winter by reversing the direction the refrigerant flows. This reversing valve is the main component that makes a heat pump different from a standard air conditioner.
How long does a heat pump last?
Most heat pumps last 15 to 20 years with regular maintenance. The compressor, which does the hardest work, typically lasts the life of the unit. Refrigerant leaks, worn coils, or electrical problems can shorten lifespan, but routine service—cleaning coils, checking refrigerant levels, and inspecting electrical connections—helps a pump reach its full expected life.
Why does my heat pump make noise when it switches between heating and cooling?
The reversing valve clicks or hisses when it changes direction. This is normal and lasts only a few seconds. If the noise is loud, continuous, or accompanied by a loss of heating or cooling, the reversing valve may be stuck and needs service.