Heat pumps move warmth from outside air or ground into your home in winter, then reverse the flow in summer to cool

A heat pump is not a furnace or air conditioner. It is a machine that captures heat that already exists in the air or soil outside your home and moves it indoors when you need warmth. In summer, it reverses and pulls heat out of your home to cool it. The key difference: instead of burning fuel to create heat, a heat pump relocates heat using refrigerant and a compressor—the same basic parts that run a refrigerator.

This matters because moving heat takes far less energy than making it. On a 35-degree day, there is still heat in the outside air; the heat pump extracts it, concentrates it, and delivers it indoors. That is why heat pumps can heat a home using one-third to one-half the electricity that a traditional electric resistance heater would need for the same warmth.

Key Takeaways

  • Heat pumps work by circulating refrigerant through an outdoor unit and an indoor unit, moving existing heat rather than generating new heat from fuel.
  • In heating mode, the outdoor coil absorbs heat from outside air or ground, and the indoor coil releases that heat into your home.
  • In cooling mode, the process reverses: the indoor coil absorbs heat from your home, and the outdoor coil releases it outside.
  • A reversing valve switches the direction of refrigerant flow between heating and cooling, and a thermostat controls when the system runs.
  • Heat pumps work in cold climates but lose efficiency below about 25 degrees Fahrenheit, which is why some homes use a backup heating system for extreme cold.

The outdoor unit captures heat and the indoor unit delivers it

The outdoor unit of a heat pump looks similar to an air conditioner condenser—a metal box with a fan and coils. Inside, refrigerant flows through the coils and absorbs heat from the outside air (or ground, in a ground-source system). A fan blows outside air across the coils to speed up the heat transfer. Even when the outside temperature is cold, there is still thermal energy in the air that the refrigerant can absorb.

The refrigerant, now warmed, travels through a copper line into your home and enters the indoor unit, which is usually mounted on a wall or in a closet. Inside that unit, the warm refrigerant passes through coils, and a fan blows indoor air across those coils. The heat transfers from the refrigerant to your home's air, and that warm air is pushed through your ductwork (if you have ducts) or directly into the room (if it is a ductless, or mini-split, system). The refrigerant, now cooled, cycles back outside to repeat the process.

The compressor pressurizes refrigerant to make heat transfer work

The compressor is the engine of the heat pump. It squeezes the refrigerant, raising its pressure and temperature. This pressure difference is what allows heat to flow from a cold place (outside air at 30 degrees) to a warm place (your home at 70 degrees). Without the compressor doing work, heat would naturally flow the wrong direction.

Think of it like this: the compressor takes cool, low-pressure refrigerant from the outdoor coil and compresses it into hot, high-pressure refrigerant. That hot refrigerant then enters the indoor coil, where it is hotter than your home's air, so heat flows naturally into your house. The refrigerant cools down and depressurizes as it releases that heat, then cycles back outside to absorb more. The compressor runs continuously while the thermostat calls for heat, and stops when the set temperature is reached.

A reversing valve switches the system between heating and cooling

The reversing valve is a four-way switch inside the outdoor unit that changes the direction refrigerant flows through the system. In winter, it directs refrigerant to absorb heat outdoors and release it indoors. In summer, it reverses the path so refrigerant absorbs heat indoors and releases it outdoors—making the heat pump function as an air conditioner.

Your thermostat signals the reversing valve to switch modes. When you set the thermostat to heating, the valve opens one way. When you switch to cooling, it opens the other way. Some heat pumps have a defrost cycle that briefly reverses the valve in winter to melt frost buildup on the outdoor coil, then switches back to heating. This automatic reversal is one reason heat pumps can handle both heating and cooling without separate equipment.

Expansion devices control refrigerant pressure and flow rate

Between the outdoor and indoor units, refrigerant passes through an expansion device—usually a metering orifice or thermostatic expansion valve. This device acts like a valve that restricts the refrigerant flow, causing a sudden pressure drop. When refrigerant depressurizes, its temperature drops sharply. This cold, low-pressure refrigerant then enters the outdoor coil (in heating mode) where it can absorb heat from the outside air.

The expansion device is essential because it maintains the pressure difference that makes the whole cycle work. Without it, refrigerant would flow freely and the system could not transfer heat efficiently. Different heat pump designs use different types of expansion devices, but they all serve the same purpose: control the refrigerant state so heat moves in the right direction.

Heat pump efficiency drops in very cold weather

Heat pumps are most efficient when the temperature difference between indoors and outdoors is small. On a 50-degree day, the outdoor coil can absorb heat easily. On a 10-degree day, there is still heat to absorb, but the refrigerant must work harder to extract it, and the compressor uses more electricity. Below about 25 degrees Fahrenheit, many heat pumps lose enough efficiency that a backup heating system becomes cost-effective.

Some homes use a heat pump paired with electric resistance heating or a gas furnace. When outdoor temperature drops below a set point (often 25 to 35 degrees, depending on the system), the backup heater turns on automatically. This hybrid approach keeps the heat pump running when it is efficient and switches to the backup when it is not. Cold-climate heat pumps are designed to work down to 0 degrees or lower, but they cost more and are not necessary in all regions.

Ductless systems deliver heat directly to rooms without ductwork

A ductless heat pump, also called a mini-split, has an outdoor unit connected by refrigerant lines to one or more indoor wall-mounted units. Instead of pushing warm air through ducts, each indoor unit has its own fan and blows heat directly into the room. This design is useful for homes without ductwork, for additions, or for heating and cooling individual zones separately.

Ductless systems use the same heat-moving principle as ducted heat pumps, but they avoid the energy loss that happens when warm air travels through ducts. They also allow you to set different temperatures in different rooms—one unit might heat the bedroom while another cools the living room. Installation requires running refrigerant lines through walls, which is more involved than installing a ducted system, but the efficiency gain and zoning flexibility appeal to many homeowners.

Frequently Asked Questions

How does a heat pump work when it is freezing outside?

Even at freezing temperatures, there is thermal energy in the air that the heat pump can extract. The refrigerant absorbs that heat, the compressor pressurizes it, and the indoor coil releases it into your home. Below about 25 degrees, the process becomes less efficient because the temperature difference is larger, so the compressor works harder and uses more electricity. Many systems include a backup heater for extreme cold.

Why is a heat pump more efficient than electric resistance heating?

Electric resistance heating (like a space heater) converts electricity directly into heat at roughly a one-to-one ratio. A heat pump moves existing heat using electricity to power the compressor, so it can deliver two to three units of heat for every unit of electricity it uses. This is why heat pumps are often called having a COP (coefficient of performance) of 2 to 3, while resistance heating has a COP of 1.

Do heat pumps work in humid climates?

Yes. Heat pumps cool and dehumidify at the same time because the indoor coil is cold, and moisture from indoor air condenses on it and drains away. In heating mode, the system does not dehumidify, but that is not usually a problem because heating naturally dries air. In very humid climates, you may want to run a separate dehumidifier in summer if the heat pump alone does not reach your comfort level.

What happens to the refrigerant when the heat pump is off?

The refrigerant stays inside the sealed system and does not escape. When the thermostat is satisfied and the compressor stops, the refrigerant remains pressurized in the lines and coils, ready to circulate again when the thermostat calls for heating or cooling. A properly installed heat pump is a closed loop that holds the same refrigerant for years.

Can a heat pump replace my furnace completely?

In mild climates, yes. In cold climates, most homeowners pair a heat pump with a backup heater (electric or gas) for days when outdoor temperature drops below the heat pump's efficient range. Some newer cold-climate heat pumps can heat efficiently down to 0 degrees or below, which may eliminate the need for backup heat in some regions. Your installer can advise based on your local winter temperatures and your home's insulation.