Air heat pumps pull warmth from outside air and move it indoors, even when it is cold

An air heat pump does not generate heat the way a furnace does. Instead, it captures warmth that already exists in the outside air — even at temperatures well below freezing — and transfers that heat indoors using a refrigerant cycle. In summer, the same system reverses to pull heat out of your home and dump it outside, making it work as both heating and cooling in one unit.

The core principle is straightforward: heat exists everywhere air exists, and a heat pump's job is to move it from one place to another. A compressor, condenser, evaporator, and expansion valve work together in a closed loop to accomplish this. The refrigerant inside absorbs heat on one side of the system and releases it on the other, powered by electricity rather than by burning fuel.

Key Takeaways

  • Air heat pumps move existing heat from outside to inside rather than creating heat, making them more efficient than electric resistance heating or gas furnaces in most climates.
  • The refrigerant cycle — evaporation, compression, condensation, and expansion — repeats continuously to transfer heat between outdoor and indoor units.
  • A reversing valve allows the same system to heat in winter and cool in summer by changing the direction refrigerant flows.
  • Air-source heat pumps work in cold climates but lose efficiency as outdoor temperature drops, so many homes in harsh winters use a backup heating source.
  • Installation requires both an outdoor unit and an indoor unit connected by refrigerant lines, and the system needs electrical power to run the compressor.

The refrigerant cycle that moves heat

The evaporator coil sits in the outdoor unit and contains cold refrigerant. Outside air passes over this coil, and because the refrigerant is colder than the air around it, heat from the outdoor air transfers into the refrigerant, causing it to evaporate into a gas. This happens even when outdoor temperatures are below freezing — the refrigerant is kept even colder than the air, so heat still flows into it.

The gaseous refrigerant then flows to the compressor, which is powered by electricity. The compressor squeezes the gas, raising its pressure and temperature. This hot, high-pressure gas then moves to the condenser coil inside your home. Indoor air passes over this hot coil, and heat transfers from the refrigerant into your living space. As the refrigerant cools, it condenses back into a liquid.

The liquid refrigerant then passes through an expansion valve, which reduces its pressure and temperature dramatically. This cold liquid returns to the outdoor evaporator coil, and the cycle repeats. The compressor is the only moving part that requires energy input — everything else happens because of the pressure and temperature differences the compressor creates.

How the reversing valve switches between heating and cooling

A reversing valve is a four-way switch inside the heat pump that changes which coil acts as the evaporator and which acts as the condenser. In heating mode, the outdoor coil absorbs heat and the indoor coil releases it. When you switch to cooling mode, the valve reverses the refrigerant flow so the indoor coil absorbs heat and the outdoor coil releases it.

This reversal happens automatically when you change your thermostat setting from heat to cool. The valve itself is controlled by a small solenoid — an electromagnet — that shifts when the system receives the signal to switch modes. Because the same hardware does both jobs, a heat pump is more compact and often less expensive to install than separate heating and cooling systems.

Why air heat pumps are more efficient than resistance heating

A heat pump moves heat rather than creating it, which means it can deliver more warmth to your home than the electrical energy it consumes. A traditional electric resistance heater (like a space heater or electric furnace) converts electricity directly into heat at a one-to-one ratio: one unit of electricity produces one unit of heat. A heat pump can deliver two to four units of heat for every unit of electricity it uses, depending on outdoor temperature and system design.

This efficiency advantage shrinks as outdoor temperature drops. When it is very cold outside, the temperature difference between the outdoor air and the refrigerant becomes smaller, so less heat transfers into the refrigerant per cycle. The compressor must work harder to move that smaller amount of heat, using more electricity. Below a certain temperature — often around 20 to 30 degrees Fahrenheit, depending on the system — many heat pumps become less efficient than a backup heating source like a gas furnace or electric resistance heater.

Split systems versus packaged units

Most residential air heat pumps are split systems, meaning the outdoor unit and indoor unit are separate and connected by refrigerant lines. The outdoor unit houses the compressor, outdoor coil, and reversing valve. The indoor unit (called an air handler) contains the indoor coil and a blower that pushes heated or cooled air through your ductwork or directly into rooms.

Some installations use a packaged unit, where all components sit in a single cabinet, usually mounted on the roof or outside the home. Packaged units are less common in residential settings but are sometimes used in homes without basements or crawl spaces. Split systems are more flexible because you can place the outdoor unit anywhere with adequate clearance and run refrigerant lines through walls or along the exterior.

What happens when outdoor temperature drops

As outdoor air temperature falls, the temperature difference between the outside air and the cold refrigerant in the evaporator coil shrinks. Heat transfer slows down, and the compressor must run longer and work harder to move the same amount of heat indoors. This is why heat pump efficiency ratings are given at specific outdoor temperatures — typically 47 degrees Fahrenheit for the standard rating, and sometimes 17 degrees for cold-climate performance.

Many heat pumps include a defrost cycle that automatically reverses the system periodically during cold, humid weather. When frost builds up on the outdoor coil, it blocks airflow and reduces heat transfer. The defrost cycle temporarily switches the system to cooling mode, warming the outdoor coil and melting the frost. During defrost, your home may receive no heat for a few minutes, and some systems use electric resistance heating as backup during this time.

In climates where winter temperatures regularly drop below 20 degrees, homeowners often install a backup heating source — a gas furnace, electric resistance heater, or boiler — that takes over when the heat pump becomes inefficient. The system automatically switches to the backup when outdoor temperature drops below a set point, usually around 30 to 35 degrees.

Ductless mini-split systems

A ductless mini-split is an air heat pump without central ductwork. The outdoor unit connects to one or more small indoor units mounted on walls or ceilings in individual rooms. Each indoor unit has its own thermostat, so you can heat or cool different rooms independently. Refrigerant lines run through small holes in the wall between outdoor and indoor units.

Ductless systems are popular in homes without existing ductwork, in room additions, or in spaces where central ducting is impractical. They avoid the energy losses that occur when heated or cooled air travels through ducts, so they can be more efficient than ducted systems in some situations. Installation is simpler than running ductwork, but the indoor units are visible on walls and require regular filter cleaning.

Frequently Asked Questions

Can a heat pump work when it is freezing outside?

Yes. Heat pumps extract heat from outdoor air even at temperatures well below freezing because the refrigerant is kept colder than the outside air. However, efficiency drops significantly as temperature falls, and below about 20 to 30 degrees Fahrenheit, a backup heating source often becomes more cost-effective to operate.

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

Hissing usually comes from refrigerant flowing through the expansion valve or lines, which is normal. Gurgling can indicate refrigerant movement in the lines or a defrost cycle in progress. Loud grinding, squealing, or banging sounds are not normal and suggest a compressor or mechanical problem that needs professional inspection.

How often does a heat pump need maintenance?

Most manufacturers recommend professional service once per year, ideally before the heating season begins. Homeowners should clean or replace air filters monthly during heavy use and keep the outdoor unit clear of leaves, snow, and debris. Refrigerant leaks and compressor wear require professional repair and cannot be serviced at home.

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

An air conditioner cools only and uses the same refrigerant cycle as a heat pump, but without a reversing valve. A heat pump reverses the cycle to provide both heating and cooling. In cooling mode, they work identically; in heating mode, a heat pump moves heat indoors while an air conditioner cannot.

Do heat pumps work in very humid climates?

Yes, but humidity affects performance. High humidity can cause frost to build up on the outdoor coil more quickly in cold weather, triggering more frequent defrost cycles. In hot, humid climates, heat pumps cool effectively, though they may run continuously on the hottest days and use more electricity than in drier regions.