A heat pump moves warmth instead of making it
A heat pump doesn't burn fuel or use electric resistance to create heat the way a furnace or space heater does. Instead, it moves heat from one place to another—from outside air into your home in winter, or from inside your home to the outside air in summer. It works like a refrigerator running in reverse: a refrigerator pulls heat out of a box and dumps it into your kitchen; a heat pump pulls heat out of cold outdoor air and pushes it into your living space.
This is why heat pumps are more efficient than electric resistance heating. You're not generating heat; you're relocating it. Even on a 30-degree day, there is still heat energy in the outdoor air—the heat pump extracts it and concentrates it indoors.
Key Takeaways
- A heat pump moves heat from outside to inside (or vice versa) using refrigerant and a compressor, rather than burning fuel or using electric coils.
- In heating mode, the outdoor unit absorbs heat from cold air, and the indoor unit releases that heat into your home.
- In cooling mode, the process reverses: heat is pulled from inside your home and released outdoors.
- A reversing valve switches the direction of refrigerant flow, allowing the same system to heat and cool without separate equipment.
- Most heat pumps include a backup electric heating element that turns on automatically when outdoor temperatures drop below the system's effective range.
The refrigerant loop: the core of how it works
At the heart of every heat pump is a closed loop filled with refrigerant—a liquid that evaporates and condenses easily. The refrigerant circulates between two metal coils: one outside your home and one inside. A compressor (powered by electricity) pressurizes the refrigerant, which raises its temperature. A reversing valve is a four-way switch that changes which direction the refrigerant flows, letting the same equipment heat or cool.
In heating mode, here's what happens: the outdoor coil (called the evaporator) absorbs heat from the outside air. The refrigerant inside that coil evaporates—it turns from liquid to gas. The compressor then sucks up that warm gas, compresses it, and makes it even hotter. That hot gas flows to the indoor coil (called the condenser), where it releases its heat into your home's air. The refrigerant cools back down to a liquid and the cycle repeats.
In cooling mode, the reversing valve flips the flow. Now the indoor coil absorbs heat from your home's air, and the outdoor coil releases that heat outside. It's the same equipment doing the opposite job.
Why outdoor temperature matters for heating
A heat pump can extract heat from outdoor air even when it's cold, but the colder it gets, the less heat is available to move. Most air-source heat pumps (the most common type) work efficiently down to about 25 to 35 degrees Fahrenheit. Below that, the system has to work much harder, and the amount of heat it can deliver drops sharply.
This is why most heat pumps include a backup electric heating element inside the indoor unit. When outdoor temperatures fall below a certain threshold—often around 20 degrees—the system automatically switches on this electric resistance heater to supplement the heat pump. You'll see this labeled as "auxiliary heat" or "emergency heat" on your thermostat. It uses more electricity than the heat pump alone, but it keeps your home warm when the heat pump can't do the job by itself.
In very cold climates, some homeowners keep a gas furnace or other backup heater alongside the heat pump. The heat pump runs when it's efficient, and the furnace takes over during the coldest days.
The indoor and outdoor units work as a team
A typical air-source heat pump has two visible parts: an outdoor unit (which looks similar to an air conditioner) and an indoor unit (usually mounted on a wall or in a closet). The outdoor unit houses the compressor, the reversing valve, and one of the coils. The indoor unit holds the other coil, a fan, and usually the backup electric heater.
Refrigerant lines (small copper tubes) connect the two units and run through your wall. A larger line carries the cold refrigerant from indoors to outdoors; a smaller line carries the warm refrigerant back. Your home's ductwork or a mini-split system's air handlers distribute the heated or cooled air throughout your rooms.
The two units communicate through electrical wiring. Your thermostat sends a signal to the outdoor unit telling it what mode to run in and how hard to work. Sensors in both units monitor temperature and pressure to keep the system running safely.
How a heat pump differs from a furnace or air conditioner
A furnace burns fuel (gas, oil, or electricity) to create heat. An air conditioner only cools—it has no heating function. A heat pump does both heating and cooling with one system, using the same refrigerant loop and just reversing its direction. This is why a heat pump is sometimes called a "reverse-cycle" system.
Because a heat pump moves heat rather than making it, it uses less electricity than an electric furnace or electric resistance heater. However, it is not as fast at heating as a furnace can be. When you turn up the thermostat on a cold day, a furnace responds almost when ready; a heat pump takes longer to warm the house because it's moving heat gradually from cold outdoor air. This is one reason many heat pump systems include that backup electric element.
A heat pump also requires outdoor space for the outdoor unit and clear airflow around it. If your outdoor unit is blocked by snow, ice, or debris, its heating performance will drop. Regular maintenance—clearing leaves and checking refrigerant levels—keeps it working well.
Ductless (mini-split) heat pumps work the same way
A ductless heat pump, or mini-split, operates on the same refrigerant-loop principle as a ducted system. The main difference is that instead of one large indoor unit connected to ductwork, you have one or more small wall-mounted or ceiling-mounted air handlers in individual rooms. Each handler has its own coil and fan.
Mini-splits are popular in homes without existing ductwork, in room additions, or when you want to heat and cool different zones independently. The outdoor unit is the same—a compressor and coil that moves refrigerant to and from the indoor handlers. The refrigerant lines are thinner and easier to run through walls than ductwork, and installation is usually less invasive than adding ducts.
Maintenance keeps the system efficient
Heat pumps have fewer moving parts than furnaces and no combustion, so they generally need less maintenance. However, they do require regular attention. The outdoor unit should be cleared of leaves, dirt, and snow so air can flow freely through the coil. In spring and fall, a technician should inspect the refrigerant charge, check electrical connections, and clean or replace the indoor filter.
If your heat pump is not heating or cooling as well as it used to, the most common cause is a low refrigerant charge—usually from a small leak. This is not something to fix yourself; call a licensed HVAC technician. Refrigerant handling requires certification and special equipment.
Frequently Asked Questions
Can a heat pump work in very cold climates?
Yes, but with limits. Most air-source heat pumps work down to about 25 degrees. Below that, the backup electric heater turns on. In climates where temperatures regularly drop below zero, a ground-source heat pump (which pulls heat from the earth instead of the air) is more efficient, but it costs more to install. Many cold-climate homeowners use a heat pump with a gas furnace backup.
Why does my heat pump make noise when switching between heating and cooling?
The reversing valve clicks or hisses when it switches direction. This is normal. You may also hear the outdoor fan speed up or down as the system adjusts. If you hear grinding, squealing, or banging, call a technician—those sounds indicate a problem.
Does a heat pump need defrosting in winter?
Yes. When outdoor air is cold and humid, frost can build up on the outdoor coil, blocking airflow. Most heat pumps have an automatic defrost cycle that reverses the refrigerant flow briefly, warming the outdoor coil and melting the frost. You may see warm air or water dripping from the outdoor unit during defrost—this is normal.
How long does a heat pump last?
A well-maintained heat pump typically lasts 15 to 20 years. Lifespan depends on climate, how often it runs, and whether it receives regular maintenance. In very cold climates where the backup heater runs frequently, the system works harder and may wear out sooner.
What's the difference between a heat pump and a hybrid system?
A hybrid system pairs a heat pump with a gas furnace. The heat pump runs when it's efficient (usually above 30 to 40 degrees), and the furnace automatically takes over during cold snaps. This saves energy compared to a furnace alone, while ensuring fast, reliable heating in deep winter. It costs more upfront but can lower heating bills in moderate climates.