A heat pump moves warmth instead of creating it
A heat pump doesn't burn fuel or use electric resistance to make heat the way a furnace or space heater does. Instead, it moves heat from one place to another—pulling warmth from outside air (or ground) and pushing it indoors in winter, then reversing the process in summer to pull heat out and cool your home. This is why a heat pump works even on cold days: there is still heat energy in outdoor air, even when the temperature is below freezing.
The machine that does this moving is called a refrigerant—a liquid that circulates through copper tubing inside and outside your home. As the refrigerant changes between liquid and gas states, it absorbs heat in one location and releases it in another. The whole cycle repeats continuously, powered by an electric compressor that pressurizes the refrigerant and keeps it moving.
Key Takeaways
- A heat pump moves existing heat from outside to inside (or vice versa) rather than generating heat by burning fuel or using electric resistance.
- The outdoor unit pulls heat from air or ground; the indoor unit releases that heat into your home, or removes it during cooling season.
- A refrigerant liquid circulates through both units, changing state to absorb heat in one place and release it in another.
- An electric compressor pressurizes the refrigerant and keeps the cycle running, making heat pumps more efficient than electric resistance heating.
- When outdoor temperatures drop very low, most heat pumps switch to electric resistance backup heat or a secondary system to maintain comfort.
The outdoor unit captures heat from the air or ground
In an air-source heat pump—the most common type—the outdoor unit looks similar to an air conditioning condenser. It contains a fan, a coil filled with cold refrigerant, and a compressor. In heating mode, the cold refrigerant inside the coil absorbs heat from the outdoor air and evaporates into a gas. This happens even when the outdoor temperature is well below freezing, because the refrigerant is colder than the air around it.
A ground-source heat pump (also called a geothermal heat pump) works the same way, but the outdoor coil is buried in the ground or submerged in a pond instead of exposed to air. Ground temperature stays more stable year-round—typically 45 to 55 degrees Fahrenheit depending on your location—so ground-source systems are more efficient in very cold climates. They cost more to install because digging or drilling is required, but they lose less efficiency as outdoor temperature drops.
The compressor pressurizes the refrigerant and raises its temperature
Once the refrigerant absorbs heat from outside and becomes a gas, it flows to the compressor—an electric pump powered by your home's electrical service. The compressor squeezes the refrigerant gas, which raises its temperature and pressure. This high-pressure, hot gas then travels indoors through insulated copper lines.
The compressor is the part that uses the most electricity in a heat pump system. It runs continuously while the system is heating or cooling, but because the heat pump is moving heat rather than creating it, the compressor uses less energy than an electric furnace or baseboard heater would to produce the same amount of warmth. This efficiency advantage is why heat pumps can lower heating costs compared to electric resistance heating, though the savings depend on your climate and local electricity rates.
The indoor unit releases heat into your home
The hot, pressurized refrigerant gas enters the indoor coil, which is usually mounted in or near your furnace or air handler. As indoor air is blown across this coil by a fan, the refrigerant releases its heat to the air and cools down, turning back into a liquid. That warm air is then distributed through your ductwork (or through a ductless system of wall-mounted units) into your rooms.
The cooled liquid refrigerant then flows back outdoors through a thin copper line, where it enters an expansion device—a small valve that drops the pressure and temperature of the refrigerant so it can absorb more heat from the outdoor air. The cycle then repeats.
In cooling mode, the cycle reverses
During summer, a reversing valve inside the heat pump switches the direction of refrigerant flow. Now the indoor coil becomes the cold side that absorbs heat from your home's air, and the outdoor unit becomes the hot side that releases that heat outside. This is why a heat pump can both heat and cool—it is the same machine running in opposite directions.
The reversing valve is a four-way solenoid valve controlled by your thermostat. When you switch from heating to cooling mode, the valve shifts, and refrigerant that was flowing one direction now flows the other. No new equipment is needed; the system straightforward operates in reverse.
Backup heat kicks in when outdoor temperatures drop very low
As outdoor temperature falls, a heat pump becomes less efficient because there is less heat available to extract from the air. Most systems include electric resistance heating—essentially heating elements similar to those in a toaster—that turn on automatically when the outdoor temperature drops below a certain point (often around 35 to 40 degrees Fahrenheit, depending on the system). Some homes also have a gas furnace or oil boiler that serves as backup.
When backup heat is running, your system is no longer as efficient as a heat pump should be, because it is generating heat rather than moving it. However, backup heat only runs when the heat pump alone cannot keep up with demand, so it is used sparingly in most climates. In very cold regions, a heat pump may run backup heat frequently during winter, which reduces the overall efficiency advantage compared to a traditional furnace.
Efficiency depends on outdoor temperature and system design
A heat pump's efficiency is measured by its Coefficient of Performance (COP) or Heating Seasonal Performance Factor (HSPF). These numbers compare the amount of heat delivered to the amount of electricity used. A COP of 3, for example, means the system delivers three units of heat for every unit of electricity consumed—far better than electric resistance heating, which has a COP of 1.
However, COP drops as outdoor temperature falls. At 47 degrees Fahrenheit, a typical air-source heat pump might have a COP of 3 to 4. At 17 degrees Fahrenheit, the same system might have a COP of 2 to 2.5. This is why heat pump efficiency varies significantly by climate. In mild climates where winter temperatures rarely drop below 30 degrees, a heat pump can maintain high efficiency all season. In cold climates, efficiency drops more often, and backup heat runs more frequently.
Frequently Asked Questions
How does a heat pump work on very cold days?
Even when outdoor air is below freezing, it still contains heat energy. The refrigerant inside the outdoor coil is even colder than the air, so heat flows from the air into the refrigerant. The compressor then pressurizes that refrigerant to raise its temperature high enough to warm your home. Below a certain threshold (usually 35 to 40 degrees), backup electric heating turns on to help maintain comfort.
Why is a heat pump more efficient than electric baseboard heating?
Electric baseboard heating converts electricity directly into heat with no waste—a COP of 1. A heat pump moves existing heat from outside, so it delivers more heat energy than the electricity it consumes. Even at low outdoor temperatures, a heat pump typically delivers 2 to 2.5 units of heat per unit of electricity, making it significantly more efficient than resistance heating.
What is the reversing valve and why does it matter?
The reversing valve is a solenoid-controlled switch that changes the direction refrigerant flows through the system. In heating mode, it sends hot refrigerant to the indoor coil. In cooling mode, it reverses the flow so the indoor coil absorbs heat instead. Without it, a heat pump could only heat or only cool, not both.
Does a heat pump need a furnace or can it work alone?
A heat pump can work alone in mild climates where winter temperatures rarely drop below freezing. In colder regions, most systems include backup electric resistance heating or a secondary furnace to maintain comfort when outdoor temperatures fall too low. Some homes use a heat pump for primary heating and keep an existing gas furnace as backup.
Why does my heat pump run more in winter than summer?
In heating mode, the heat pump must work harder to extract warmth from cold outdoor air, so the compressor runs longer and more frequently. In cooling mode, indoor air is warmer than outdoor air, so heat flows more easily from inside to outside, and the system reaches the thermostat setpoint faster. This is normal operation.