A heat pump is an air conditioner running in reverse
A heat pump works by moving heat from one place to another instead of generating heat from scratch. In winter, it pulls warmth from the outside air (even when it feels cold) and pushes it indoors. In summer, it reverses direction and pulls heat out of your house and releases it outside — exactly like an air conditioner. The same refrigerant loop does both jobs by changing direction.
The reason this matters: moving heat uses far less electricity than creating it. An electric furnace heats by running current through a wire until it glows hot, like a toaster. A heat pump uses that same electricity to run a compressor and fan, but gets three to four times as much warmth into your house because it is relocating existing heat rather than making new heat from electrical resistance.
Key Takeaways
- A heat pump moves heat between your house and the outside air using a refrigerant loop, switching direction seasonally to heat in winter and cool in summer.
- The outdoor unit pulls heat from cold air and compresses it to raise the temperature; the indoor unit releases that heat into your home.
- Heat pumps use 50 to 75 percent less electricity than electric furnaces or baseboard heaters because they move heat rather than generate it.
- The system includes an outdoor unit, an indoor unit, refrigerant lines connecting them, and a thermostat that controls when the compressor runs.
- Most heat pumps lose efficiency below 32°F, so homes in very cold climates often keep a backup heating source for the coldest days.
The refrigerant loop: how heat actually moves
Inside the heat pump is a closed loop of refrigerant — a chemical that boils at very low temperatures. The loop has four main parts: a compressor, a condenser coil, an expansion valve, and an evaporator coil. In heating mode, the outdoor unit contains the evaporator coil, and the indoor unit contains the condenser coil.
The outdoor evaporator coil sits in the cold air and absorbs whatever heat exists there. Even at 20°F, air contains heat energy; the refrigerant boils and turns into a gas at a temperature lower than the air around it, so heat flows into the refrigerant. That gas travels through a line to the indoor unit, where the compressor squeezes it. Compression raises the temperature of the gas to perhaps 120°F — hot enough to warm your house. The hot gas passes through the indoor condenser coil, and a fan blows your home's air across it. Heat transfers from the refrigerant to your air, and the refrigerant cools back into a liquid. That liquid returns to the outdoor unit through another line, passes through an expansion valve that lowers its pressure, and the cycle repeats.
In summer, a reversing valve flips the direction. Now the indoor coil becomes the evaporator (pulling heat out of your house) and the outdoor coil becomes the condenser (releasing that heat outside). The refrigerant loop is identical; only its direction changes.
The outdoor and indoor units working together
The outdoor unit is a metal box containing the compressor, one of the coils, a fan, and the reversing valve. The fan pulls outside air across the coil to exchange heat between the refrigerant and the air. You hear this fan running whenever the heat pump is heating or cooling. The compressor is the loudest component — it sounds like a car engine idling and uses most of the system's electricity.
The indoor unit is usually mounted on a wall or in a closet and contains the other coil and a blower fan. This fan runs whenever you want heating or cooling and pushes conditioned air into your rooms. Some systems use a traditional furnace or air handler as the indoor unit instead of a wall-mounted unit; the heat pump outdoor unit connects to it with refrigerant lines, and the furnace's blower distributes the air.
Refrigerant lines — typically two copper tubes of different sizes — connect the outdoor and indoor units. These lines are insulated to prevent heat loss and are run through a wall or along the exterior of your house. A drain line also runs from the indoor unit to remove condensation that forms when the coil cools air in summer.
Why efficiency drops in very cold weather
Heat pumps work best when the temperature difference between indoors and outdoors is small. On a 50°F day, the outdoor coil can easily absorb heat and the compressor does not have to work hard. On a 0°F day, the outdoor air holds much less heat energy, so the refrigerant has to boil at an even lower temperature to pull any heat out. The compressor must run longer and work harder to compress the gas to a useful temperature. Electricity use climbs, and the amount of heat delivered per unit of electricity falls.
Most air-source heat pumps lose significant efficiency below 32°F and become impractical below 0°F. In those conditions, many systems automatically switch to a backup heater — either electric resistance coils inside the indoor unit or a gas furnace if the system is a hybrid. The backup heater is less efficient than the heat pump but keeps your house warm when the heat pump alone cannot do the job. Homes in climates with frequent sub-zero temperatures often choose a hybrid system (heat pump plus gas furnace) or a ground-source heat pump, which pulls heat from the earth instead of the air and stays efficient even in cold weather.
The thermostat and controls
Your thermostat tells the heat pump when to run and in which direction. In winter, you set it to heating mode and a target temperature. When your house drops below that temperature, the thermostat signals the outdoor unit to start the compressor and the reversing valve to enter heating mode. The indoor blower fan turns on and pushes warm air into your rooms. Once the house reaches the target temperature, the compressor shuts off. The blower may continue running briefly to push out any remaining warmth from the coil.
If the house temperature drops again before the compressor can restart (usually within 30 minutes), the backup heater kicks in instead. This prevents short-cycling — the compressor turning on and off constantly — which wastes energy and wears out the compressor faster. In summer, you switch the thermostat to cooling mode, and the same logic applies in reverse.
Modern thermostats can be programmable or smart, allowing you to set different temperatures for different times of day or to control the system remotely. Some systems also have a "defrost mode" that reverses the refrigerant flow briefly in winter to melt ice that builds up on the outdoor coil when humidity is high.
Single-zone versus multi-zone systems
A single-zone heat pump has one outdoor unit and one indoor unit, heating or cooling your entire house as a single space. You set one thermostat, and the system treats all rooms the same. This is the most common and least expensive setup.
A multi-zone (or ductless) system has one outdoor unit connected to two, three, or more indoor wall units. Each indoor unit has its own thermostat, so you can heat the bedroom to 68°F while keeping the living room at 72°F. Each indoor unit has its own blower and can run independently. This flexibility costs more upfront but lets you avoid heating or cooling unused rooms and can reduce overall energy use in larger homes. Installation is also simpler than running ductwork, making ductless systems popular for additions or retrofits.
Maintenance and how long they last
Heat pumps need less maintenance than furnaces because they have no combustion, no gas lines, and no chimney. The main tasks are cleaning or replacing the air filter on the indoor unit every one to three months (depending on dust and pets), and having a technician clean the outdoor coil and check refrigerant levels once a year. Leaves, dirt, and ice can block the outdoor unit, so clear debris around it regularly.
Most heat pumps last 15 to 20 years with routine care. The compressor is the most expensive component to replace, costing $1,500 to $3,000 if it fails outside the warranty period. Refrigerant leaks are the second most common repair; a small leak can be patched, but a major leak requires the system to be evacuated, repaired, and recharged, which costs $500 to $1,500 depending on the size and type of refrigerant.
Frequently Asked Questions
Can a heat pump heat a house on a freezing day?
Yes, but less efficiently. On days below 32°F, the heat pump still works but uses more electricity to extract heat from cold air. Most systems automatically switch to a backup heater (electric resistance or gas) when outdoor temperature drops below a certain point, usually around 20°F to 35°F depending on the model. The backup heater takes over to maintain comfort without overworking the compressor.
Why does my heat pump sometimes blow cool air in winter?
During defrost mode, the system reverses to melt ice off the outdoor coil, and the indoor unit may blow cool air for a few minutes. This is normal and happens automatically. If cool air blows for more than 10 minutes or happens constantly, the reversing valve may be stuck and needs service.
How much does a heat pump cost to run compared to electric heat?
A heat pump typically costs 50 to 75 percent less to operate than electric resistance heating (baseboard heaters or electric furnaces) because it moves heat instead of generating it. Exact savings depend on your local electricity rates, the efficiency rating of your unit, and how cold your winters are. Hybrid systems (heat pump plus gas furnace) are usually cheaper to run than all-electric heat pumps in very cold climates.
Do heat pumps work in humid climates?
Yes. In fact, heat pumps remove humidity from the air in summer as they cool, similar to air conditioners. The indoor coil condenses moisture from the air, and a drain line carries it away. In winter, heat pumps do not remove humidity, but they also do not add it, so indoor humidity stays at whatever level your home naturally maintains.
What size heat pump do I need?
Size is measured in tons (12,000 BTU per ton). A technician calculates the right size by measuring your home's square footage, insulation, window area, and local climate. An undersized unit will not heat or cool adequately; an oversized unit will cycle on and off too often and waste energy. Most homes need 2 to 5 tons. Never guess — an incorrect size costs more to run and wears out faster.