What an AC heat pump does, and why it works differently from a furnace

An AC heat pump doesn't generate heat the way a furnace burns fuel. Instead, it moves heat from one place to another using refrigerant — the same chemical that cools a standard air conditioner. In summer, it pulls heat out of your house and dumps it outside. In winter, it reverses direction and pulls heat from the outside air (even when it's cold) and pushes it inside. This is why it's called a heat pump: it pumps heat rather than creating it.

The reason this matters is efficiency. Moving heat takes far less energy than making heat. A heat pump can deliver 2 to 4 units of heating for every 1 unit of electricity it uses. A furnace converts fuel into heat at roughly 80 to 95 percent efficiency, which sounds good until you realize a heat pump can do the same job with half the electricity. In climates where winter temperatures stay above freezing most of the time, a heat pump can cut your heating costs significantly.

Key Takeaways

  • A heat pump moves heat using refrigerant and electricity rather than burning fuel, making it more efficient than a furnace in moderate climates.
  • The outdoor unit contains a compressor that pressurizes refrigerant, and the indoor unit contains an evaporator coil that absorbs or releases heat depending on the season.
  • In winter, a heat pump extracts heat from cold outside air and moves it indoors; in summer, it reverses to cool your house like a standard air conditioner.
  • Heat pumps lose efficiency below 35°F and may need a backup heating source in very cold climates, which is why they work best in mild winters.

The refrigerant cycle: how heat moves through the system

The core of a heat pump is a closed loop of refrigerant — a liquid chemical that boils at very low temperatures. The refrigerant circulates between two coils: one inside your house and one outside. A compressor in the outdoor unit pressurizes the refrigerant, which heats it up. A metering device (usually a small valve) then lets the pressurized refrigerant expand, which cools it down. This pressure-and-expansion cycle repeats continuously.

In heating mode, the outdoor coil acts as an evaporator. Cold refrigerant flows through it and absorbs heat from the outside air — yes, even cold air contains heat energy. The refrigerant warms up and travels indoors to the indoor coil, where it condenses and releases that heat into your house. A fan blows air across the indoor coil, and warm air flows into your rooms. The now-cool refrigerant returns to the compressor, and the cycle starts again.

In cooling mode, the cycle reverses. The indoor coil becomes the evaporator, pulling heat out of your house. The outdoor coil becomes the condenser, releasing that heat outside. This is identical to how a standard air conditioner works, which is why many heat pumps can cool just as well as a dedicated AC unit.

The outdoor and indoor units, and what each one does

The outdoor unit houses the compressor, the outdoor coil, a fan, and the metering device. The compressor is the engine of the system — it pressurizes the refrigerant and consumes most of the electricity the heat pump uses. The outdoor fan pulls air across the outdoor coil to exchange heat with the outside environment. This unit is noisy and sits on a concrete pad outside your house, usually on the ground or mounted on a wall.

The indoor unit contains the indoor coil and a blower fan. In a split system (the most common type), the indoor unit is a wall-mounted box or a cabinet that sits in a closet or basement. The blower pulls air from your house, passes it across the indoor coil, and returns the conditioned air through ducts or vents. Some heat pumps are packaged units, where both the outdoor and indoor components sit in a single cabinet outside, but split systems are more common in homes with existing ductwork.

Refrigerant lines connect the two units and run through your walls or along the exterior of your house. These lines are insulated to prevent heat loss and are typically the only visible connection between indoors and outdoors.

Why heat pumps lose power in very cold weather

A heat pump's efficiency drops as outdoor temperature falls. Below 35°F, the outdoor coil becomes less effective at absorbing heat from the air, and the system has to work harder to extract the same amount of warmth. Below 20°F, many heat pumps lose so much efficiency that they become uneconomical to run — you'd spend more on electricity than you would on a furnace burning natural gas or oil.

To handle this, most heat pumps include a backup heating source: electric resistance coils (essentially a space heater) built into the indoor unit. When outdoor temperature drops below a set threshold — usually 30°F to 40°F depending on the model — the system automatically switches to resistance heating. The heat pump still runs, but the backup coils do most of the work. This keeps your house warm but uses more electricity and defeats the efficiency advantage.

In climates where winter temperatures regularly drop below 20°F, a heat pump alone is usually not the best choice. In mild climates where winter lows stay above 35°F most of the time, a heat pump can run efficiently all season. Some newer cold-climate heat pumps can operate down to 0°F or lower, but they cost more and are still less efficient than a furnace in extreme cold.

Heating mode versus cooling mode: how the system switches

A reversing valve inside the outdoor unit controls which direction the refrigerant flows. In heating mode, the valve directs refrigerant to the outdoor coil first, where it absorbs heat. In cooling mode, the valve reverses, sending refrigerant to the indoor coil first. The switch happens automatically based on the thermostat setting — if you set the thermostat to heat and the house temperature drops below your target, the system switches to heating mode. If you set it to cool and the house temperature rises above your target, it switches to cooling mode.

Some heat pumps have a defrost cycle that runs automatically in winter. When frost builds up on the outdoor coil (which reduces heat absorption), the system temporarily reverses to cooling mode, warming the outdoor coil and melting the frost. This cycle lasts a few minutes and repeats as needed. During defrost, the backup heating coils usually turn on to keep your house warm while the heat pump is running in reverse.

Comparing heat pump efficiency to furnaces and air conditioners

A heat pump's efficiency is measured in HSPF (Heating Seasonal Performance Factor) for heating and SEER (Seasonal Energy Efficiency Ratio) for cooling. Higher numbers mean more efficient. A typical heat pump has an HSPF of 8 to 10 and a SEER of 14 to 20. A modern furnace has an AFUE (Annual Fuel Utilization Efficiency) of 90 to 98 percent, which sounds better until you account for the energy cost of fuel versus electricity in your area.

In a region where electricity costs $0.14 per kilowatt-hour and natural gas costs $1.20 per therm, a heat pump with an HSPF of 9 can heat your house for less money than a 95 percent efficient furnace — but only if winter temperatures stay above 35°F most of the time. If you live where winter lows regularly hit 10°F, the furnace wins because the heat pump's backup coils kick in and consume expensive electricity.

For cooling, a heat pump with a SEER of 16 is roughly equivalent to a standalone air conditioner with the same SEER rating. The advantage of a heat pump is that you get both heating and cooling in one system, which can save money on installation and maintenance compared to owning a furnace plus a separate AC unit.

Noise, maintenance, and how long a heat pump lasts

The outdoor unit of a heat pump is louder than a furnace because the compressor and fan run continuously during heating and cooling. Typical noise levels range from 70 to 80 decibels — roughly as loud as a vacuum cleaner or heavy traffic. If the outdoor unit sits near a bedroom window, noise can be a problem. Some newer models are quieter, but expect sound as a trade-off for efficiency.

Heat pumps require annual maintenance: cleaning or replacing the indoor air filter, checking refrigerant levels, and inspecting the outdoor coil for debris and frost buildup. The compressor and refrigerant lines are sealed and rarely need service unless there's a leak. A well-maintained heat pump typically lasts 15 to 20 years, similar to a furnace or air conditioner. Compressor failure is the most expensive repair, costing $1,500 to $3,000 or more, which is why some homeowners buy extended warranties.

Frequently Asked Questions

Can a heat pump work in a house with no ductwork?

Yes. Ductless (or mini-split) heat pumps mount directly on interior walls and don't require ducts. Each indoor unit heats or cools one room or zone. Ductless systems are more expensive to install than ducted systems but work well in homes with radiant heating, baseboard heat, or room-by-room temperature control needs.

What happens to a heat pump if the power goes out?

The heat pump stops running. Unlike a furnace, which can often restart automatically when power returns, a heat pump may need manual reset. In a prolonged outage during winter, your house will cool down. This is why some people keep a backup space heater or generator in cold climates.

Do heat pumps work in humid climates?

Yes. Heat pumps cool and dehumidify the same way air conditioners do — by pulling moisture out of the air as it passes across the cold indoor coil. In very humid climates, you may need to run the system longer to reach your target humidity level, which uses more electricity.

Is a heat pump right for my climate?

Heat pumps work best where winter temperatures stay above 35°F most of the time and you have moderate heating needs. If your area has very cold winters or you heat primarily with natural gas (which is cheap), a furnace may cost less to operate. A contractor can compare your local fuel and electricity prices to your home's heating and cooling needs and tell you which system makes financial sense.