A heat pump moves warmth from outside air into your home, rather than burning fuel to create heat

A heat pump is an appliance that pulls heat energy from the air outside—even when it's cold—and moves it indoors to warm your home. It does not generate heat the way a furnace does by burning gas or oil. Instead, it uses electricity to run a cycle that captures existing warmth from the outdoor air and transfers it inside. In winter, it heats; in summer, it reverses and cools like an air conditioner.

The reason this matters is efficiency. Moving heat takes far less energy than creating it from scratch. A heat pump can deliver three to four units of heat for every unit of electricity it uses. A traditional electric heater or furnace cannot match that ratio, which is why heat pumps lower heating bills in most climates.

Key Takeaways

  • A heat pump uses refrigerant and electricity to move heat from outside air into your home, not to generate heat by burning fuel.
  • The outdoor unit pulls heat from cold air, the refrigerant carries it indoors, and the indoor unit releases that warmth into your living space.
  • A reversing valve lets the same system cool your home in summer by moving heat in the opposite direction.
  • Heat pumps work in cold climates, but their efficiency drops as outdoor temperature falls, and some homes need a backup heating source for the coldest days.
  • Regular filter changes and annual professional inspection keep a heat pump running at peak efficiency.

The basic cycle: refrigerant, pressure, and phase change

At the heart of every heat pump is a closed loop filled with refrigerant—a fluid that boils and condenses at low temperatures. The cycle has four main parts: the outdoor unit (evaporator coil), a compressor, the indoor unit (condenser coil), and an expansion device. Refrigerant circulates through all four continuously.

In winter heating mode, the outdoor coil absorbs heat from the cold air outside. The refrigerant inside that coil boils—it turns from liquid to gas—and carries that heat energy. The compressor then pressurizes the refrigerant gas, which raises its temperature even higher. That hot gas flows to the indoor coil, where it condenses back into liquid and releases its warmth into your home. The expansion device drops the pressure again, and the cycle repeats.

The key insight is that refrigerant boils at a much lower temperature than water does. So even when outdoor air is 30°F, the refrigerant can still absorb heat from it and turn into a gas. That heat gets concentrated and moved indoors.

How the outdoor unit captures heat from cold air

The outdoor unit looks like an air conditioner box mounted on your wall or ground. Inside is a coil of thin metal tubes carrying cold refrigerant, surrounded by aluminum fins. A fan pulls outdoor air across those fins and the coil.

Even at 20°F or lower, outdoor air contains heat energy. The cold refrigerant in the coil is colder still, so heat naturally flows from the air into the refrigerant. The refrigerant absorbs that warmth and boils into a gas. The fan keeps pulling fresh air across the coil so the process continues. This is why heat pumps work in cold climates—they do not need the air to be warm, only warmer than the refrigerant inside the coil.

As outdoor temperature drops, the temperature difference between the air and the refrigerant shrinks, so less heat transfers per cycle. This is why heat pump efficiency declines in very cold weather. Below about 0°F, many heat pumps lose enough efficiency that a backup heating system (electric resistance heat or a gas furnace) kicks in automatically.

The indoor unit releases heat into your home

The indoor unit is usually mounted on a wall or in a closet and contains another coil. Hot refrigerant gas from the compressor enters this coil and condenses back into liquid, releasing its heat. A fan blows indoor air across the coil, and that warm air flows into your home through ducts or vents.

The temperature of the indoor coil is much higher than the outdoor coil because the compressor pressurized the refrigerant. This temperature difference is what allows heat to flow into your living space even though the outdoor air is cold. The warmer the refrigerant, the more heat your home receives per cycle.

After the refrigerant condenses in the indoor coil, it flows back to the expansion device, where its pressure drops sharply. This cooling prepares it to return to the outdoor coil and absorb more heat. The cycle then repeats continuously.

Summer cooling: the reversing valve flips the direction

In summer, a component called the reversing valve changes the direction of refrigerant flow. The outdoor coil now acts as the condenser (releasing heat), and the indoor coil acts as the evaporator (absorbing heat). Heat is pulled from inside your home and moved outdoors, cooling your living space.

From the homeowner's perspective, you straightforward switch the thermostat from heat to cool mode. The reversing valve shifts automatically, and the same equipment that warmed you in winter now cools you in summer. This is why a heat pump is often called a two-in-one system.

Why efficiency matters and how it varies

A heat pump's efficiency is measured by its Seasonal Energy Efficiency Ratio (SEER) for cooling and Heating Seasonal Performance Factor (HSPF) for heating. Higher numbers mean more heat moved per unit of electricity used. A modern heat pump might have an SEER of 16 and an HSPF of 9 or higher.

Real-world efficiency depends on outdoor temperature, indoor temperature setting, and how well your home is insulated. On a 50°F day, a heat pump is highly efficient. On a 0°F day, it works harder and uses more electricity. In very cold climates, a heat pump paired with a gas furnace or electric backup heat often costs less to operate than a heat pump alone, because the backup system takes over when the heat pump's efficiency drops too far.

Regular maintenance also affects efficiency. A dirty outdoor coil or clogged indoor filter forces the system to work harder. Changing the filter every 1 to 3 months and having a professional inspect the system once a year keeps it running at peak performance.

When a heat pump needs backup heat

In climates where winter temperatures regularly drop below 0°F, a heat pump alone may not meet your home's heating demand. Most heat pump systems include a backup heating source—usually electric resistance coils in the indoor unit or a gas furnace. The thermostat automatically switches to backup heat when outdoor temperature falls below a set point, often around 20°F to 35°F depending on the system.

Backup heat is less efficient than the heat pump itself, so it costs more to run. But it ensures your home stays warm on the coldest days. Some homeowners in moderate climates never use backup heat; others in cold climates use it for 20 to 30 percent of the heating season. The exact amount depends on your location and how cold the winter is.

Frequently Asked Questions

Can a heat pump work when it's freezing outside?

Yes. Heat pumps work in freezing temperatures because they pull heat from cold air, not warm air. However, their efficiency drops as outdoor temperature falls. Most heat pumps work well down to about 0°F, but below that, backup heat usually activates to maintain comfort and avoid excessive electricity use.

Why does my heat pump sometimes make a loud noise or hiss?

A hissing sound often means refrigerant is leaking from the coils or connections. A loud grinding or squealing noise may indicate a failing compressor or fan motor. Both require professional repair. A brief whooshing sound during the heating cycle is normal—it is the reversing valve switching direction.

What is the difference between a heat pump and an air conditioner?

An air conditioner only cools by moving heat outdoors. A heat pump does both: it cools in summer and heats in winter using the same equipment. The reversing valve is what makes this possible. If you have only an air conditioner, you need a separate heating system for winter.

How often should I have my heat pump serviced?

A professional inspection once per year is standard. The technician checks refrigerant charge, cleans coils, inspects electrical connections, and tests the reversing valve. You should also change the indoor filter every 1 to 3 months depending on dust and pet hair in your home. Regular maintenance extends the system's life and keeps it efficient.

Will a heat pump work if my home is poorly insulated?

A heat pump will heat a poorly insulated home, but it will run constantly and cost more to operate. Heat escapes through gaps, thin walls, and old windows. Before installing a heat pump, consider sealing air leaks and adding insulation. A well-insulated home lets the heat pump work less and saves money on heating bills.