The basic cycle: moving heat instead of making it

An air-to-air heat pump doesn't generate heat the way a furnace does. Instead, it captures heat that already exists in outdoor air — even when that air feels cold — and moves it indoors. In summer, it reverses the process and pulls heat out of your home to cool it down. The system uses a refrigerant (a chemical that changes between liquid and gas easily) and four main parts working in a loop to make this transfer happen.

The reason this matters for your heating bill is efficiency. Moving heat uses far less electricity than creating it. A heat pump can deliver three to four units of heat for every unit of electricity it consumes, whereas a traditional electric heater converts electricity directly into heat at a one-to-one ratio.

Key Takeaways

  • An air-to-air heat pump moves existing heat from outside air into your home in winter, and from inside air outdoors in summer, rather than generating heat from scratch.
  • The system cycles refrigerant through four stages: compression, condensation, expansion, and evaporation, with the outdoor and indoor coils switching roles depending on the season.
  • The compressor is the engine of the system — it pressurizes the refrigerant to raise its temperature so heat can flow into your home.
  • Heat pumps work in cold climates, but their efficiency drops as outdoor temperature falls, and most models include electric resistance heating as backup below 25 to 35 degrees Fahrenheit.

The four-part cycle that moves the heat

The refrigerant loop has four main components, and understanding what each one does makes the whole system clearer. The outdoor coil (called the evaporator in heating mode) sits in the outdoor unit and absorbs heat from the outside air. Even at 20 degrees Fahrenheit, there is still heat energy in the air — the refrigerant inside the coil boils at a much lower temperature, so it pulls that heat in and turns from liquid to gas.

The compressor is the heart of the system. It sucks in the refrigerant gas and squeezes it under high pressure. This compression heats the gas significantly — the same way a bicycle pump gets warm when you use it. The hot, pressurized gas then flows to the indoor coil (the condenser in heating mode), where it releases its heat into your home's air. As the refrigerant cools, it turns back into a liquid.

The expansion valve is the final piece. It's a small opening that lets the high-pressure liquid refrigerant drop suddenly to low pressure. This pressure drop cools the refrigerant back down to a temperature lower than the outdoor air, so the cycle can start again. The whole process repeats continuously while the system runs.

Why the system reverses in summer

In cooling mode, the heat pump does the exact same cycle but in the opposite direction. The indoor coil becomes the evaporator — it absorbs heat from your home's air, cooling the space. The outdoor coil becomes the condenser — it releases that heat outside. A reversing valve (a switch inside the outdoor unit) redirects the refrigerant flow so the coils swap roles.

This reversibility is what makes a heat pump more efficient than running separate heating and cooling systems. You get both functions from one unit, and the same refrigerant loop that heats your home in January cools it in July.

How outdoor temperature affects performance

Heat pumps work in cold weather, but not all cold is equal. At 47 degrees Fahrenheit, a typical air-to-air heat pump operates at full efficiency. As the outdoor temperature drops, the temperature difference between the outside air and the refrigerant shrinks, so the heat transfer slows down. The system has to run longer to deliver the same amount of heat.

Most air-to-air heat pumps include electric resistance heating (essentially a backup electric heater) that kicks in automatically when outdoor temperature falls below 25 to 35 degrees Fahrenheit, depending on the model. Below that threshold, the heat pump alone cannot keep up with the heating demand, and the resistance element takes over. This is why heat pumps are most cost-effective in climates where winter temperatures rarely drop below 20 degrees for extended periods.

Modern cold-climate heat pumps are engineered to work efficiently down to 5 to 10 degrees Fahrenheit, but they cost more than standard models. If you live in a region with very harsh winters, comparing the upfront cost of a cold-climate unit against the long-term savings is worth doing.

The role of the outdoor and indoor units

The outdoor unit houses the compressor, the reversing valve, the outdoor coil, and the fan that blows air across the coil. The indoor unit contains the indoor coil and a blower that circulates your home's air across it. In a split system (the most common type), these two units are connected by refrigerant lines running through your wall.

The outdoor fan runs whenever the system is heating or cooling. In winter, you may notice the outdoor unit frosting over — this happens because the outdoor coil is colder than the dew point of the outside air. Most systems include a defrost cycle that reverses the refrigerant flow temporarily to melt the frost, then resumes heating. This cycle uses energy and reduces heating output for a few minutes, but it's necessary to keep the coil clear.

Efficiency ratings and what they mean

Heat pump efficiency is measured by HSPF (Heating Seasonal Performance Factor) for heating and SEER (Seasonal Energy Efficiency Ratio) for cooling. HSPF is the ratio of total heat delivered over a season to total electricity consumed. A heat pump with an HSPF of 8 delivers eight units of heat for every unit of electricity used over a typical heating season.

Minimum federal standards require an HSPF of 7.7 for new heat pumps. High-efficiency models reach 9 to 10 HSPF. The higher the rating, the lower your heating costs, but the unit also costs more upfront. Whether the energy savings justify the extra cost depends on your local electricity rates and how many heating hours your climate demands each year.

Common issues that affect how well the system works

A heat pump's performance depends on proper installation and maintenance. If the refrigerant charge is low (a leak somewhere in the loop), the system cannot move enough heat and will run constantly without reaching your set temperature. If the outdoor coil is dirty or blocked, airflow is restricted and heat transfer drops. If the indoor filter is clogged, the indoor blower has to work harder and the system loses efficiency.

The expansion valve can also fail, either by sticking partially closed (reducing flow) or by leaking (allowing refrigerant to escape). A failed reversing valve may leave the system stuck in heating or cooling mode. These are not maintenance issues — they require a technician to repair or replace the part. Regular filter changes and annual professional inspection catch most problems before they become expensive.

Frequently Asked Questions

Can a heat pump heat a home when it's below freezing outside?

Yes, but with reduced efficiency. At 20 degrees Fahrenheit, a heat pump still extracts heat from the air, but the system runs longer to deliver the same warmth. Most models automatically switch to electric resistance backup heating below 25 to 35 degrees, which is less efficient but ensures your home stays warm.

Why does my heat pump seem to run constantly in winter?

In cold weather, the outdoor temperature is far from your indoor set point, so the system runs nearly all the time to keep up. This is normal. If it runs constantly even at mild temperatures (above 40 degrees), the refrigerant charge may be low or the outdoor coil may be dirty — have a technician check it.

What's the difference between a heat pump and a furnace?

A furnace burns fuel (gas, oil, or electricity) to create heat. A heat pump moves existing heat from one place to another using electricity. Heat pumps are more efficient in moderate climates but may need backup heating in very cold regions. Furnaces work well in harsh winters but cost more to operate because they generate rather than move heat.

Does a heat pump work if the outdoor unit is covered with snow?

Snow buildup blocks airflow across the outdoor coil, reducing heat transfer and forcing the system to work harder. Most systems have a defrost cycle that melts frost, but heavy snow should be cleared by hand. Don't use a pressure washer — it can damage the coil fins.

How often should a heat pump be serviced?

Professional inspection once per year is standard. A technician checks refrigerant charge, tests electrical connections, cleans coils, and listens for unusual noises. Homeowners should replace the indoor filter every one to three months depending on use and air quality in your home.