Heat pumps move warmth from outside air or ground into your home in winter, and reverse the flow in summer

A heat pump does not generate heat the way a furnace burns fuel. Instead, it captures warmth that already exists in the air or soil outside your home and moves it indoors. In winter, it pulls heat from cold air (yes, cold air still contains heat energy) and concentrates it inside. In summer, it reverses direction and pulls heat out of your home, cooling it down. This is why heat pumps can both heat and cool—they are not two separate systems, just one system running in opposite directions.

The core of a heat pump is a refrigerant, a liquid that circulates through the system and changes between liquid and gas states as it moves between the outdoor unit and the indoor unit. This cycle of state-change is what allows the pump to move heat. The same basic principle powers your refrigerator, which pulls heat out of the box and releases it into your kitchen.

Key Takeaways

  • Heat pumps move existing warmth from outside into your home rather than creating heat by burning fuel or using electric resistance.
  • A refrigerant liquid circulates between an outdoor unit and an indoor unit, changing states to absorb heat in one location and release it in another.
  • The compressor is the engine of the system—it pressurizes the refrigerant to make the heat-moving cycle work.
  • Heat pumps work in cold climates, though their efficiency drops as outdoor temperature falls, and many homes use a backup heating source below 20°F or so.
  • The indoor unit can be a furnace-style handler, a wall-mounted head, or radiant floor tubing, depending on the type of heat pump you have.

The refrigerant cycle: how heat actually moves

The refrigerant starts as a cold liquid in the outdoor unit during winter. As outside air passes over the outdoor coil, the refrigerant absorbs heat from that air and evaporates into a gas. This is the key moment: the refrigerant is now carrying heat energy, even though the outside air feels cold to you. The gas then travels through a copper line into your home.

Inside, the gas enters the compressor, which is an electric pump that squeezes the gas under high pressure. Pressure makes the gas hotter—this is the same reason a bicycle pump gets warm when you use it. The hot, pressurized gas then flows through the indoor coil, where a fan blows indoor air across it. The heat transfers from the gas to your home's air, and the gas cools back down into a liquid. That warm air is what heats your rooms.

The liquid refrigerant then passes through an expansion valve, which lowers its pressure and temperature, and the cycle repeats. In summer, the system reverses: the outdoor unit becomes the condenser (releasing heat outside) and the indoor unit becomes the evaporator (pulling heat from your home).

Why the compressor is the most important part

The compressor is the engine that makes the whole cycle work. Without it, the refrigerant would not move, and heat would not transfer. The compressor runs on electricity and is the single biggest energy consumer in a heat pump system. When you hear a heat pump running, the sound you hear is usually the compressor.

Modern heat pumps often use a variable-speed compressor, which adjusts its speed based on how much heating or cooling you need. On a mild day, the compressor runs slowly and uses less electricity. On a very cold or very hot day, it runs faster. This is more efficient than an older fixed-speed compressor, which runs at full power or shuts off completely.

How heat pumps work in cold weather

Heat pumps can extract heat from air that is well below freezing because heat is a form of energy, and energy exists in cold air—just in smaller amounts. However, as outdoor temperature drops, the temperature difference between the outside air and the refrigerant shrinks, and the heat transfer becomes slower and less efficient. At around 20°F to 25°F, many heat pumps lose enough efficiency that running them becomes expensive.

For this reason, most heat pump systems in cold climates include a backup heat source—usually an electric resistance heater or a gas furnace that kicks in when the outdoor temperature falls below a set point. The system automatically switches to backup heat without you having to do anything. Some newer cold-climate heat pumps can operate efficiently down to 0°F or below, but they are more expensive and not yet common in all regions.

If you live in a climate where winter temperatures regularly drop below 10°F, ask your installer what the system's rated efficiency is at that temperature and whether backup heat is included in the design.

Indoor units: how the heat gets into your home

The indoor unit is where the heat (or cooling) actually enters your living space. There are three common types. An air handler looks like a furnace and sits in a basement, attic, or closet; a fan blows air across the coil and pushes it through your existing ductwork. A ductless head is a wall-mounted or ceiling-mounted unit that heats or cools a single room or zone without ducts. A ground-source heat pump uses underground tubing instead of an indoor coil, and the heat transfers through radiant floor tubing or a hydronic system.

The type of indoor unit you have depends on whether your home already has ducts, how many zones you want to heat separately, and your budget. Ductless systems are more flexible for retrofits but cost more per zone. Air handlers work well if you already have ductwork in good condition.

The outdoor unit and how it handles frost

The outdoor unit contains the compressor, the outdoor coil, and a fan. In winter, when the outdoor coil is absorbing heat from cold air, moisture in that air can freeze on the coil's surface. If frost builds up, it blocks airflow and the system stops working efficiently.

To prevent this, heat pumps have a defrost cycle. Periodically, the system reverses itself briefly—the outdoor unit becomes a condenser and releases heat, melting the frost. You may see steam or water dripping from the outdoor unit during defrost. This is normal. The defrost cycle uses energy and temporarily reduces heating, but it only lasts a few minutes and happens only when needed.

Keep the area around your outdoor unit clear of snow, leaves, and debris. If snow piles up against the unit, it can block airflow and force the defrost cycle to run more often than necessary.

Efficiency ratings and what they mean

Heat pump efficiency is measured in HSPF (Heating Seasonal Performance Factor) for heating and SEER2 (Seasonal Energy Efficiency Ratio) for cooling. HSPF is the ratio of heat output to electricity input over a whole heating season. A higher HSPF means more heat per kilowatt-hour of electricity used.

Most modern air-source heat pumps have an HSPF between 8 and 12. Ground-source heat pumps typically have an HSPF between 10 and 15 because the ground temperature is more stable than air temperature. The higher the rating, the lower your heating bills—but higher-rated units also cost more upfront. Your installer can help you understand the trade-off between purchase price and long-term energy savings.

Be aware that HSPF ratings are measured under standard conditions, usually at temperatures above 20°F. If you live in a very cold climate, the real-world efficiency in your home may be lower than the rating suggests, especially if you are relying on backup heat for much of the winter.

Frequently Asked Questions

Can a heat pump heat a home on its own in winter, or do I always need backup heat?

It depends on your climate and the heat pump model. In mild climates where winter temperatures rarely drop below 30°F, a heat pump can handle all heating. In cold climates, backup heat is standard and necessary. Some newer cold-climate heat pumps can operate efficiently to 0°F or below, but they cost more. Ask your installer what temperature your system is rated for and whether backup heat is included.

Why does my heat pump sometimes make a hissing or gurgling sound?

Those sounds are usually the refrigerant flowing through the lines or the expansion valve opening and closing. A loud hissing or gurgling that is new or getting worse may indicate a refrigerant leak, which reduces efficiency and requires a service call. Normal operation sounds like a steady hum from the compressor.

Do heat pumps work in humid climates?

Yes. In fact, heat pumps are very effective at removing humidity in summer because the cooling process naturally dehumidifies the air. In winter, they may slightly dry out indoor air, which is why some people use a humidifier alongside a heat pump in very dry climates.

What happens if the refrigerant leaks out?

The system will lose heating and cooling capacity and will not work properly. Refrigerant is sealed in the system and should never need refilling under normal operation. If you need refrigerant added, it means there is a leak that must be found and repaired. This requires a licensed technician and is not a DIY fix.

Is it normal for water to drip from the outdoor unit?

Yes, during defrost cycles in winter and during cooling in summer. The water is condensation from the air and is harmless. If water is pooling or the dripping seems excessive, check that the condensate drain is not blocked by leaves or ice. A blocked drain can be cleared by your installer or sometimes by running a wet/dry vacuum over the drain opening.