The basic principle: moving heat, not generating it
An electric heat pump does not burn fuel or use resistance coils to create warmth the way a furnace or space heater does. Instead, it uses electricity to move heat from one place to another — pulling warmth from outside air, ground, or water and pushing it indoors during winter, then reversing the process in summer to cool your home.
This is the core difference that makes heat pumps efficient. A traditional electric heater converts electricity directly into heat through resistance, which wastes energy. A heat pump uses a small amount of electricity to operate a compressor and pump, allowing it to move much more heat energy than the electricity it consumes. On a 40-degree day, there is still heat in the outside air; the heat pump extracts it.
Key Takeaways
- A heat pump moves existing heat from outside to inside using a refrigerant cycle powered by an electric compressor, rather than generating heat through burning fuel or electrical resistance.
- The refrigerant circulates through four main components — the outdoor coil, compressor, indoor coil, and expansion valve — in a continuous loop that changes the refrigerant between liquid and gas states.
- When the refrigerant evaporates in the outdoor coil, it absorbs heat from outside air; when it condenses in the indoor coil, it releases that heat into your home.
- A reversing valve allows the system to switch direction in summer, pulling heat from inside and releasing it outdoors to cool your home.
- Heat pumps work in cold climates but lose efficiency as outdoor temperature drops, which is why some systems include a backup electric heater for very cold days.
The refrigerant cycle: the engine of the heat pump
The heart of every heat pump is a closed loop of refrigerant — a special liquid that boils and condenses at low temperatures. This refrigerant circulates continuously through four main parts: the outdoor coil, the compressor, the indoor coil, and the expansion valve. As the refrigerant moves through this loop, it changes between liquid and gas states, and those state changes are what allow heat to move.
Think of the refrigerant as a heat shuttle. In winter heating mode, the refrigerant enters the outdoor coil as a cold, low-pressure liquid. The outdoor air — even at 30 or 40 degrees — is warmer than the refrigerant, so heat flows from the air into the liquid. The refrigerant absorbs this heat and boils into a gas. The compressor then sucks in this warm gas and compresses it, raising its temperature and pressure even higher. This hot, high-pressure gas flows to the indoor coil, where it is much hotter than the air inside your home. Heat flows from the gas into your indoor air, and the refrigerant cools down and condenses back into a liquid. The expansion valve then reduces the pressure on this liquid, cooling it further, and the cycle begins again.
The four main components and what each one does
The outdoor coil (also called the evaporator in heating mode) sits in a metal cabinet outside your home. A fan blows outside air across the coil, and the cold refrigerant inside absorbs heat from that air. This is where the heat pump "collects" warmth from the environment.
The compressor is an electric pump driven by a motor. It sucks in the warm gas from the outdoor coil and squeezes it, raising both its temperature and pressure. This is the part that uses electricity, and it is the reason heat pumps are more efficient than resistance heaters — the compressor does not create heat; it straightforward concentrates the heat that was already there.
The indoor coil (called the condenser in heating mode) is usually mounted inside your furnace cabinet or air handler. The hot, compressed refrigerant flows through it, and a fan blows your home's air across the coil. Heat transfers from the refrigerant to your indoor air, warming your home. The refrigerant cools and condenses back into a liquid.
The expansion valve is a small device that restricts the flow of liquid refrigerant, causing a sudden drop in pressure and temperature. This cold liquid then enters the outdoor coil, and the cycle repeats. The expansion valve is what maintains the pressure difference that keeps the refrigerant moving.
How the reversing valve switches between heating and cooling
In summer, you want to cool your home, not heat it. A reversing valve — a solenoid-controlled switch in the refrigerant line — reverses the direction of refrigerant flow. Now the indoor coil becomes the evaporator (where heat is absorbed), and the outdoor coil becomes the condenser (where heat is released). The compressor pulls warm indoor air through the indoor coil, the refrigerant absorbs that heat, and the outdoor unit releases it outside. The cycle is identical in principle; only the direction has flipped.
This reversing happens automatically when you switch your thermostat from heating to cooling mode. Some heat pumps can also operate in a third mode called defrost, which temporarily reverses the cycle to melt ice buildup on the outdoor coil during cold, humid weather.
Why efficiency matters: coefficient of performance
Heat pump efficiency is measured by a number called COP, or coefficient of performance. A COP of 3 means the heat pump moves three units of heat for every one unit of electricity it uses. The remaining two units come from the environment — the outside air, ground, or water.
COP varies with outdoor temperature. On a mild 50-degree day, a typical air-source heat pump might have a COP of 3 to 4. On a very cold 0-degree day, the COP might drop to 1.5 or lower, because there is less heat available outside and the compressor has to work harder to extract it. This is why many heat pump systems include a backup electric heater (called auxiliary or emergency heat) that turns on automatically when outdoor temperatures fall below a certain threshold — usually around 35 degrees, depending on the system.
Air-source versus ground-source heat pumps
The most common type is an air-source heat pump, which pulls heat from outside air. It is cheaper to install than other types and works in most climates, though efficiency drops in very cold weather. The outdoor unit looks similar to an air conditioner condenser.
A ground-source heat pump (also called a geothermal heat pump) pulls heat from the ground or groundwater, which stays at a more constant temperature year-round — usually 45 to 55 degrees. This means the compressor does not have to work as hard, and the COP stays higher even in winter. Ground-source systems are much more expensive to install because they require digging trenches or drilling wells, but they are more efficient and can work in very cold climates without much auxiliary heat.
Common misconceptions about how heat pumps work
Many people think a heat pump "stops working" when it is cold outside. In reality, the system continues to extract heat from the air even at 0 degrees — there is still molecular motion and thermal energy present. What changes is the rate of heat transfer and the amount of work the compressor must do. The system becomes less efficient, not non-functional.
Another misconception is that the outdoor unit is "heating" the outside air. It is actually the opposite: the outdoor coil is absorbing heat from the outside air (or ground) and removing it. This is why the outdoor unit of a heat pump in heating mode feels warm or even hot to the touch — it is releasing the heat it extracted from indoors during the cooling season, or it is in defrost mode.
Some people also assume that a heat pump requires special wiring or a larger electrical service than a traditional furnace. While heat pumps do use electricity continuously (rather than burning gas), most homes can accommodate a standard air-source heat pump on their existing electrical service. Ground-source systems may require upgrades depending on the compressor size.
Frequently Asked Questions
Can a heat pump work when it is freezing outside?
Yes. Heat pumps extract heat from air even at 0 degrees or below, though efficiency drops significantly. Most systems include a backup electric heater that activates automatically when outdoor temperature falls below a set point, usually 35 degrees. This ensures your home stays warm without relying solely on the less-efficient compressor.
Why does my heat pump outdoor unit feel hot in winter?
The outdoor unit feels hot because it is releasing heat — either heat it extracted from your home during cooling season, or heat it is actively pulling from the outside air and concentrating. If the system is in defrost mode, it temporarily reverses to melt ice on the coil, which also makes the outdoor unit feel warm.
What is the difference between a heat pump and an air conditioner?
An air conditioner only cools; it removes heat from inside and releases it outdoors. A heat pump does both: it can cool in summer and heat in winter by reversing the direction of refrigerant flow. The indoor and outdoor coils swap roles depending on the season.
How much electricity does a heat pump use compared to a furnace?
A heat pump uses electricity continuously, while a gas furnace uses electricity only for the blower fan and controls. However, because a heat pump moves heat rather than generating it, it typically uses less total energy to heat a home than electric resistance heating. Compared to a gas furnace, energy use depends on local electricity and gas prices, but heat pumps are often competitive or cheaper over time.
Can a heat pump work with my existing ductwork?
Yes. Most air-source heat pumps connect to your existing furnace and ductwork through an air handler or by replacing the furnace with a heat pump unit. The ductwork itself does not need to change. A professional installer will assess your system to confirm compatibility.