A heat pump moves warmth from one place to another instead of burning fuel to create heat

A heat pump works by pulling heat energy from the air, ground, or water outside your home and moving it indoors during winter. In summer, it reverses and pulls heat out of your home to cool it down. The system uses a refrigerant—a liquid that changes state easily—and a compressor to do this moving. Think of it like a conveyor belt for heat: it picks up warmth from a cold place and delivers it to a warm place, which seems backwards but works because of how refrigerants behave under pressure.

The reason this matters for your home is efficiency. A furnace burns fuel and converts it to heat, losing some energy in the process. A heat pump doesn't create heat; it relocates it. On a 35-degree day, there is still heat energy in the outside air—the heat pump extracts it and concentrates it indoors. This is why heat pumps can deliver more warmth to your home than the electrical energy they use, something called a coefficient of performance or COP.

Key Takeaways

  • A heat pump moves existing heat from outside to inside (or vice versa) using a refrigerant and compressor, rather than creating heat by burning fuel.
  • The four main parts are the evaporator coil (outside), compressor, condenser coil (inside), and expansion valve, which work together in a continuous cycle.
  • Heat pumps work in cold climates but lose efficiency below about 25 degrees Fahrenheit, which is why many homes in freezing regions use a backup heating system.
  • The reversing valve allows the system to switch between heating mode in winter and cooling mode in summer.
  • Regular filter changes and professional maintenance every one to two years keep the system running at peak efficiency.

The four main parts and what each one does

Every heat pump has four core components that work together in a loop. The evaporator coil sits in the outdoor unit and absorbs heat from the outside air, ground, or water. The refrigerant inside this coil is cold and low-pressure, so it pulls heat energy from the surroundings and boils into a gas.

The compressor is the engine of the system. It takes that refrigerant gas and squeezes it, raising its pressure and temperature. This is the part that uses electricity. The hot, pressurized gas then flows to the condenser coil, which is inside your home. As the hot refrigerant passes through the condenser, it releases its heat into your indoor air (or water, depending on the system type), and the refrigerant cools back into a liquid.

The expansion valve is the final piece. It reduces the pressure of the liquid refrigerant and cools it down before it returns to the evaporator coil to start the cycle again. This continuous loop—evaporate, compress, condense, expand—is what moves heat from outside to inside all winter long.

How the reversing valve lets the system switch between heating and cooling

In summer, you need the opposite: pull heat out of your home and dump it outside. A reversing valve makes this possible. It is a four-way valve that redirects the flow of refrigerant so that the outdoor coil becomes the condenser (where heat is released) and the indoor coil becomes the evaporator (where heat is absorbed). The compressor and expansion valve keep working the same way; only the direction of refrigerant flow changes.

When you switch your thermostat from heating to cooling mode, an electrical signal tells the reversing valve to flip. The changeover is automatic and usually takes a few seconds. Some systems make a hissing sound during the switch—that is normal and is just the refrigerant pressure equalizing.

Why heat pumps lose efficiency in very cold weather

A heat pump can extract heat from cold air, but the colder the outside temperature, the harder the compressor has to work and the less heat it can pull. Below about 25 degrees Fahrenheit, the efficiency drops noticeably. Below 0 degrees, many heat pumps struggle to keep up with heating demand on their own.

For this reason, homes in climates with sustained freezing temperatures often have a backup heating system—usually electric resistance heating or a gas furnace. When outdoor temperature drops below a set point (often 25 to 35 degrees, depending on the system), the backup kicks in automatically. This keeps your home warm but uses more electricity or fuel than the heat pump alone would. Modern heat pumps are more cold-weather capable than older models, but the physics of the system means no heat pump is equally efficient at all temperatures.

The difference between air-source, ground-source, and water-source heat pumps

An air-source heat pump pulls heat from the outside air. It is the most common type because it is cheaper to install—no digging required. It works in most climates but loses efficiency in extreme cold, as described above.

A ground-source heat pump (also called geothermal) pulls heat from the earth, which stays around 50 degrees year-round a few feet below the surface. This makes it much more efficient in cold climates and uses less electricity overall. The tradeoff is cost: you have to dig trenches or bore a well, which can run $15,000 to $30,000 or more depending on your property and soil type. The long-term savings on heating and cooling can offset this, but the upfront investment is steep.

A water-source heat pump pulls heat from a pond, lake, or well. It is rare in residential homes but works well if you have a suitable water source on your property. Installation cost depends on the water source and how far it is from your home.

What happens inside the indoor and outdoor units

The outdoor unit houses the evaporator coil, compressor, reversing valve, and a fan that blows outside air across the evaporator coil to maximize heat transfer. You will see this unit mounted on a concrete pad beside your home. In winter, you may see frost or ice building up on it—this is normal, and most systems have a defrost cycle that periodically reverses the refrigerant flow for a few minutes to melt the ice.

The indoor unit contains the condenser coil and a blower fan that pushes warm (or cool) air through your ductwork and into your rooms. Some systems use a wall-mounted or ceiling-mounted indoor unit instead of ductwork; these are called ductless or mini-split systems. The indoor unit also has a filter that traps dust and debris—this is the part you need to change every one to three months, depending on how much dust your home generates.

How to maintain a heat pump so it works efficiently

The most important maintenance task is changing the indoor filter regularly. A clogged filter forces the blower to work harder, reduces airflow, and can damage the compressor over time. Check the filter monthly and replace it when it looks gray or brown.

Keep the outdoor unit clear of leaves, grass clippings, and snow. The fan needs to pull air freely across the evaporator coil. If the unit is buried in snow, gently clear it away (do not use a pressure washer, which can damage the coil). In fall, trim back any bushes or branches that hang over the unit.

Have a professional service the system every one to two years. They will check refrigerant levels, clean the coils, inspect electrical connections, and test the reversing valve. Low refrigerant is a common problem and reduces efficiency; a technician can detect and fix it. If you notice the system is not heating or cooling as well as it used to, or if you hear unusual noises, call a technician rather than waiting for the annual service.

Frequently Asked Questions

Can a heat pump work when it is freezing outside?

Yes, but with reduced efficiency. Most heat pumps work down to about 0 degrees Fahrenheit, though they lose heating power as temperature drops. Below 25 degrees, a backup heating system usually turns on automatically. In climates where it stays below freezing for weeks, a ground-source heat pump or a hybrid system (heat pump plus furnace) is more practical.

Why is my outdoor unit frosting over in winter?

Frost forms when the cold evaporator coil pulls moisture from the air. This is normal. Most heat pumps have a defrost cycle that runs automatically every 30 to 90 minutes, reversing the refrigerant flow for a few minutes to melt the ice. If frost builds up so thick that the fan cannot spin, call a technician—it may indicate a refrigerant leak or sensor problem.

How much electricity does a heat pump use compared to a furnace?

A heat pump uses less total energy to deliver the same amount of heat because it moves heat rather than creating it. However, it does use electricity continuously, whereas a gas furnace uses gas (which may be cheaper in your area). Your actual cost depends on local electricity and gas prices. A technician can estimate your annual operating cost based on your climate and system size.

What is that hissing sound when my heat pump switches to cooling mode?

That is the reversing valve redirecting refrigerant flow. It is normal and lasts a few seconds. If the hissing is constant or very loud, or if the system does not switch modes properly, contact a technician—it may indicate a valve problem.

Do I need to do anything special to prepare my heat pump for winter?

Clear leaves and debris from the outdoor unit, trim back vegetation, and make sure the area around it drains well so water does not pool. Change the indoor filter before the heating season starts. If you had a professional service in fall, you are set. If not, consider scheduling one before cold weather arrives.