Heat pumps move warmth from outside air into your house, even when it's cold
A heat pump heats by pulling warmth from the air outside—even frigid air contains some heat energy—and moving it indoors through a refrigerant cycle. The system uses a compressor, condenser, and evaporator coil to extract that warmth and release it inside your home. In winter mode, the heat pump reverses the direction of refrigerant flow compared to air conditioning mode, which is why the same unit can both heat and cool.
The process works because heat naturally flows from warm to cold, but a heat pump uses electricity to pump heat against that flow—pulling it from cold outdoor air and pushing it into your warmer indoor space. This is the opposite of how a furnace works, which burns fuel to create heat from scratch. A heat pump's efficiency comes from moving existing heat rather than generating it, which is why it uses less energy than electric resistance heating.
Key Takeaways
- Heat pumps extract warmth from outdoor air using a refrigerant that circulates through coils, then release that warmth indoors through a fan and ductwork.
- The outdoor unit (condenser) absorbs heat from cold air, and the indoor unit (evaporator) releases that heat into your home.
- A reversing valve switches the refrigerant direction between heating and cooling modes, so one system handles both seasons.
- Heat pumps work in temperatures well below freezing, though their heating output drops as outdoor temperature falls.
- Many heat pump systems include electric resistance heating as a backup when outdoor temperatures drop too low for the heat pump alone to meet demand.
The refrigerant cycle that moves heat indoors
Inside a heat pump, a chemical called refrigerant circulates continuously in a closed loop. The refrigerant evaporates in the outdoor coil (called the evaporator in heating mode), absorbing heat from the outside air. Even at 20°F outside, the refrigerant boils at a much lower temperature, so it pulls warmth from the air around it.
The compressor then pressurizes this warm refrigerant gas, raising its temperature further. The hot, pressurized gas flows to the indoor coil (called the condenser in heating mode), where a fan blows indoor air across it. The refrigerant cools and condenses back into liquid, releasing the heat it absorbed outdoors plus the heat added by compression. That warm air blows into your home through ducts or vents.
The liquid refrigerant then flows through an expansion device that lowers its pressure, cooling it down before it returns to the outdoor coil to repeat the cycle. This loop runs continuously as long as your thermostat calls for heat, moving warmth from outside to inside without burning any fuel.
Why the outdoor unit still works when it's freezing
The outdoor air always contains some heat energy, even at temperatures far below freezing. A heat pump's refrigerant boils at temperatures much colder than the air around it—often at minus 20°F or lower—so it can extract heat from 10°F air or colder. The colder the outdoor air, the smaller the temperature difference between the air and the boiling refrigerant, which means the heat pump has to work harder and transfers less heat per cycle.
However, frost can form on the outdoor coil when humidity in the air freezes on contact with the cold metal. A thick frost layer blocks airflow and reduces heat transfer. Most heat pumps include a defrost cycle that temporarily reverses the refrigerant flow, warming the outdoor coil and melting the frost. During defrost, the system may draw heat from indoors or use electric resistance heating to maintain indoor temperature, which is why you might notice a brief dip in warmth during defrost cycles on very cold days.
How the reversing valve switches between heating and cooling
A reversing valve is a four-way solenoid valve that changes the direction of refrigerant flow through the system. In cooling mode, the outdoor coil acts as a condenser (releasing heat outside) and the indoor coil acts as an evaporator (absorbing heat from inside). In heating mode, the valve flips the flow, so the outdoor coil becomes the evaporator (absorbing heat from outside air) and the indoor coil becomes the condenser (releasing heat indoors).
Your thermostat controls when the reversing valve switches. When you set the system to heat and the indoor temperature drops below your setpoint, the thermostat energizes the solenoid in the reversing valve, flipping it to heating mode. The same physical coils and refrigerant lines now work in reverse, pulling heat from outside instead of rejecting it there. This is why a heat pump can serve as both a heater and an air conditioner without separate equipment.
The role of the compressor and fan in moving heat
The compressor is the engine of the heat pump. It pressurizes the refrigerant gas, which raises its temperature and prepares it to release heat indoors. The compressor runs on electricity and is the single largest energy consumer in the system. Most residential heat pumps use a scroll compressor or rotary compressor that runs continuously when heating is needed, though some newer models use variable-speed compressors that adjust their output based on demand.
The indoor and outdoor fans move air across the coils to transfer heat between the refrigerant and the air. The outdoor fan pulls air through the outdoor coil so the cold refrigerant can absorb heat from it. The indoor fan (often part of your existing furnace or air handler) blows indoor air across the warm indoor coil, picking up heat and distributing it through your ductwork. Without these fans, heat transfer would be too slow to warm your home effectively.
Backup electric heating when outdoor temperatures drop very low
As outdoor temperature falls, a heat pump's heating output decreases because there is less heat available in the cold air to extract. Most systems reach a point called the balance point—typically around 30°F to 40°F depending on the model and your home's insulation—where the heat pump alone cannot meet your heating demand. Below that temperature, the system switches on electric resistance heating (also called auxiliary heat or emergency heat) to supplement the heat pump.
Electric resistance heating works like a toaster: electricity flows through a wire coil that glows hot, and a fan blows air across it. This heating method is less efficient than the heat pump because it converts electricity directly to heat rather than moving existing heat, but it ensures your home stays warm even during extreme cold. You may hear your system switch to resistance heating as a brief click or feel a change in air temperature as the resistance coils set up. Some systems use resistance heating automatically; others require you to manually switch to emergency heat mode if the heat pump fails.
Common misconceptions about heat pump heating
Many people believe heat pumps cannot heat below freezing, but this is false. Heat pumps work effectively at temperatures well below zero, though their output drops and they may need backup heating to keep up with demand. The outdoor coil does not need to be warmer than the air around it—the refrigerant inside is much colder, so heat flows into the coil regardless of outdoor temperature.
Another misconception is that heat pump air feels cold compared to furnace heat. Heat pump air is typically 90°F to 100°F, which is cooler than furnace air (120°F to 130°F) but still warm enough to heat your home. Some people interpret the lower temperature as the system not working, when in fact the heat pump straightforward delivers more air at a lower temperature to distribute the same amount of heat. If you prefer warmer air, electric resistance heating can raise the temperature, though at higher energy cost.
Frequently Asked Questions
Does a heat pump work when it's below freezing outside?
Yes. Heat pumps extract heat from outdoor air even at temperatures well below freezing. However, their heating output decreases as temperature drops, and they may need electric resistance backup heating to meet your home's full heating demand on very cold days. Frost buildup on the outdoor coil is managed by automatic defrost cycles.
Why does my heat pump air feel cooler than my old furnace?
Heat pump air is typically 90°F to 100°F, while furnace air is 120°F to 130°F. The heat pump delivers more air volume at lower temperature to distribute the same total heat. Both methods warm your home; the furnace just does it with hotter, drier air. If you prefer warmer air, you can set up electric resistance heating, though this increases energy use.
What is the defrost cycle and why does my heat pump do it?
The defrost cycle temporarily reverses refrigerant flow to warm the outdoor coil and melt frost buildup. Frost blocks airflow and reduces heating efficiency. The cycle lasts a few minutes and may briefly reduce indoor warmth. It runs automatically when the system detects frost, typically several times per day during humid, cold weather.
Can a heat pump heat a whole house by itself in winter?
A heat pump can heat most homes alone down to its balance point temperature, usually 30°F to 40°F. Below that, electric resistance heating kicks in to supplement. In very cold climates, some homes may need resistance heating for much of the winter, which increases operating cost but ensures consistent warmth.
How much electricity does a heat pump use for heating?
A heat pump typically uses 30 to 50 percent less electricity than electric resistance heating to produce the same warmth, because it moves heat rather than generating it. Exact consumption depends on outdoor temperature, your home's insulation, thermostat settings, and how often backup heating runs. Colder climates and homes with poor insulation use more electricity.