Heat pumps move warmth from outside air or ground into your home, rather than burning fuel to create heat

A heat pump works by capturing heat that already exists in the air or ground outside your home and moving it indoors. Even when it feels cold outside, there is still thermal energy available—a heat pump extracts it, concentrates it, and releases it inside. In summer, the system reverses and pulls heat out of your home to cool it down. This is why a heat pump can both heat and cool, and why it uses less energy than a furnace or air conditioner running separately.

The core of the system is a refrigerant—a liquid that circulates through the heat pump and changes between liquid and gas states. As it changes state, it absorbs heat in one location and releases it in another. This cycle repeats continuously, moving thermal energy from where you don't want it to where you do.

Key Takeaways

  • Heat pumps use a refrigerant that cycles between liquid and gas to move heat from outside to inside, rather than generating heat by burning fuel.
  • The outdoor unit contains a coil that absorbs heat from the air or ground, while the indoor unit releases that heat into your home.
  • A compressor pressurizes the refrigerant to raise its temperature so it can warm your home even in cold weather.
  • Heat pumps reverse their cycle in summer to pull heat out of your home and cool it down, making them year-round systems.
  • The efficiency of a heat pump depends on the temperature difference between outside and inside—they work best when outdoor temperatures are above freezing.

The four main parts of a heat pump system

Every heat pump has four essential components that work together: the outdoor unit, the indoor unit, the compressor, and the expansion valve. The outdoor unit contains a coil that sits in the cold air (or ground, for ground-source systems). Refrigerant flows through this coil and absorbs heat from the outside environment. The indoor unit also has a coil, and warm refrigerant flows through it to release heat into your home's air or water system.

The compressor is the engine of the system. It pressurizes the refrigerant gas, which raises its temperature significantly. This is crucial: by compressing the gas, the heat pump can make the refrigerant hot enough to warm your home even when outdoor temperatures are low. The expansion valve does the opposite—it releases pressure on the refrigerant, allowing it to cool down and return to the outdoor coil to absorb more heat. This cycle repeats dozens of times per hour.

How the refrigerant cycle moves heat

The refrigerant begins as a cold liquid in the outdoor coil. As outdoor air passes over the coil, heat transfers from the air into the liquid refrigerant, causing it to evaporate into a gas. This gas is then drawn into the compressor, which squeezes it under high pressure. Compression heats the gas further—this is the key step that makes heating possible in cold weather.

The hot, pressurized gas then flows to the indoor coil. As indoor air passes over this coil, the heat transfers from the refrigerant into your home. The refrigerant cools down and condenses back into a liquid. This liquid then flows through the expansion valve, where pressure drops and temperature falls. The cold liquid returns to the outdoor coil, and the cycle begins again.

This continuous loop is what allows a heat pump to move heat from a cold place to a warm place—something that would not happen naturally without energy input. The compressor provides that energy, powered by electricity.

Air-source versus ground-source heat pumps

An air-source heat pump extracts heat from the outdoor air through the outdoor unit. It is the most common type because installation is simpler and less expensive than other options. Air-source systems work well in moderate climates and can still extract heat when temperatures drop below freezing, though efficiency declines as outdoor temperature falls.

A ground-source heat pump (also called a geothermal heat pump) pulls heat from the ground or groundwater instead of air. Ground temperature remains more stable year-round—typically 45 to 50 degrees Fahrenheit even in winter—so the system operates at higher efficiency. Ground-source systems require digging trenches or drilling wells to install underground loops, which makes them more expensive upfront but often more efficient over time.

Both types use the same refrigerant cycle. The difference is where the outdoor coil sits and what medium surrounds it. Air-source systems are more common in residential homes because they cost less to install and work adequately in most climates.

Why efficiency drops in very cold weather

Heat pump efficiency is measured by how much heat output you get for each unit of electrical energy input. This ratio, called the coefficient of performance (COP), depends on the temperature difference between outside and inside. When outdoor temperature is 50 degrees and you want indoor temperature at 70 degrees, the difference is only 20 degrees, and the compressor does not have to work as hard.

When outdoor temperature drops to 0 degrees, the temperature difference jumps to 70 degrees. The compressor must work much harder to extract heat from very cold air and raise it to room temperature. At some point—usually around 0 to 10 degrees Fahrenheit, depending on the system—a heat pump's efficiency becomes poor enough that a backup heating source kicks in. Many systems include electric resistance heating or a gas furnace as backup for the coldest days.

This is why heat pumps perform best in climates where winter temperatures rarely drop below freezing. In very cold regions, a hybrid system that combines a heat pump with a furnace often makes more sense than a heat pump alone.

The reversing valve and cooling mode

In summer, a component called the reversing valve switches the direction of refrigerant flow. Instead of absorbing heat from outside and releasing it indoors, the cycle reverses: the indoor coil now absorbs heat from your home, and the outdoor coil releases it outside. The compressor still runs, still pressurizes the refrigerant, and the expansion valve still controls flow—only the direction changes.

This reversibility is what makes a heat pump a complete heating and cooling system. You do not need a separate air conditioner. The same equipment that heats your home in winter cools it in summer, which is one reason heat pumps can be more cost-effective than running two separate systems.

Comparing heat pump efficiency to other heating systems

A traditional furnace burns fuel (gas or oil) to create heat, and the best modern furnaces convert about 95 percent of that fuel into usable warmth. A heat pump does not create heat—it moves it—so it can deliver more heat energy than the electrical energy it consumes. A heat pump with a COP of 3, for example, delivers three units of heat for every one unit of electricity used.

In moderate climates, this makes heat pumps significantly more efficient than furnaces over a full heating season. In very cold climates where backup heating runs frequently, the advantage shrinks. The trade-off is upfront cost: heat pumps typically cost more to install than a furnace, though operating costs are often lower.

Heat pumps also provide cooling without a separate air conditioner unit, which can offset some of the higher installation cost if you would otherwise need both a furnace and an AC system.

Frequently Asked Questions

Can a heat pump work when it is freezing outside?

Yes. Heat pumps can extract heat from air even at 0 degrees Fahrenheit or colder, because some thermal energy exists at any temperature above absolute zero. However, efficiency drops significantly in very cold weather, and most systems switch to backup heating (electric resistance or gas) when outdoor temperature falls below 10 to 20 degrees, depending on the model.

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

Frost buildup on the outdoor coil happens in cold, humid conditions. Most heat pumps have a defrost cycle that reverses the system briefly to melt the ice. This is normal and automatic. Loud noises during defrost or startup are common—if noise is constant or unusual, have the system inspected by a technician.

Is a heat pump the same as an air conditioner?

No. An air conditioner only cools and uses a similar refrigerant cycle, but it cannot reverse to provide heating. A heat pump is a reversible system that both heats and cools. All heat pumps can cool, but not all cooling systems can heat.

What size heat pump do I need for my home?

Heat pump sizing depends on your home's square footage, insulation, climate zone, and how cold winters get. A system that is too small will not heat adequately; one that is too large will cycle on and off frequently and waste energy. A technician should perform a load calculation to determine the right size for your specific home.

Do heat pumps work in humid climates?

Yes. Heat pumps work in any climate, but they are most efficient in moderate temperatures. In very hot, humid climates, the cooling load is high and the system runs constantly, which increases operating costs. In very cold climates, heating efficiency drops. Moderate climates—roughly 30 to 80 degrees Fahrenheit for most of the year—are where heat pumps perform best.