Heat pumps move warmth from one place to another rather than generating heat by burning fuel
A heat pump operates on a straightforward principle: it captures heat that already exists in the air, ground, or water outside your home and moves it indoors during winter. In summer, it reverses direction and pulls heat out of your home to cool it down. The system uses a refrigerant — a liquid that changes state easily between liquid and gas — to absorb heat at one location and release it at another. This is the same basic process your refrigerator uses, except a heat pump is designed to heat your living space rather than keep food cold.
The reason heat pumps matter is efficiency. Moving existing heat requires far less energy than creating heat from scratch by burning natural gas or using electric resistance coils. A heat pump can deliver two to four units of heating energy for every unit of electricity it consumes, depending on outdoor temperature and the system type. This efficiency advantage shrinks in very cold climates, but even then most heat pumps outperform traditional electric heating.
Key Takeaways
- Heat pumps move heat from outside air, ground, or water into your home using a refrigerant that cycles between liquid and gas states.
- The system reverses in summer to pull heat out of your home and cool it down, so one unit handles both heating and cooling.
- A compressor powered by electricity drives the refrigerant through the cycle, making the process work without burning fuel.
- Heat pumps lose efficiency in very cold climates below freezing, though modern cold-climate models perform better than older designs.
- The outdoor unit looks similar to an air conditioner, while the indoor unit can be a wall-mounted box, a furnace-style cabinet, or coils inside existing ductwork.
The refrigerant cycle: how heat moves through the system
The heart of a heat pump is a closed loop filled with refrigerant. The cycle has four main stages. First, the refrigerant enters the outdoor unit as a cold, low-pressure liquid. There it passes through an evaporator coil that absorbs heat from the outside air (or ground or water, depending on the type). Even when outdoor air feels cold to you, it still contains heat energy that the refrigerant can extract. The refrigerant absorbs this heat and turns into a gas.
Next, a compressor — powered by electricity — sucks in this low-pressure gas and squeezes it. Compression heats the gas further, raising its temperature well above the temperature inside your home. The hot, high-pressure gas then flows to the indoor unit and passes through a condenser coil. As indoor air blows across this coil, the hot refrigerant releases its heat into your home and cools back down into a liquid. Finally, the liquid passes through an expansion valve that reduces its pressure, cooling it further before it returns to the outdoor evaporator coil to start the cycle again.
In cooling mode, the system reverses. The outdoor unit becomes the condenser (releasing heat outside), and the indoor unit becomes the evaporator (pulling heat from your home). A reversing valve — a four-way switch — redirects refrigerant flow to make this reversal happen automatically.
Air-source, ground-source, and water-source heat pumps
The three main types of heat pumps differ in where they pull heat from. Air-source heat pumps are the most common and least expensive. They extract heat from outdoor air using the outdoor unit you see mounted on the side of a house or building. They work in most climates but lose efficiency when outdoor temperatures drop below freezing, because there is less heat available in very cold air. Modern cold-climate air-source models use larger coils and higher-capacity compressors to maintain performance down to around 0°F, though efficiency still declines.
Ground-source heat pumps (also called geothermal) bury pipes in the earth, where soil temperature stays relatively constant year-round — typically 45°F to 60°F depending on location and depth. Because ground temperature does not swing as wildly as air temperature, ground-source systems maintain high efficiency even in harsh winters. The tradeoff is cost: installation requires digging trenches or drilling boreholes, which can run $15,000 to $30,000 or more before incentives. They are most practical for new construction or major renovation.
Water-source heat pumps pull heat from a pond, lake, or well. They perform similarly to ground-source systems but require a suitable water source nearby. Installation is less invasive than ground-source drilling but still requires site-specific engineering.
Indoor and outdoor components you will see
The outdoor unit houses the compressor, the reversing valve, and the evaporator or condenser coil depending on the season. It looks similar to an air conditioner unit — a metal box with a fan that draws air across the coil. The fan runs continuously during operation to maximize heat transfer. In winter, you may see frost or ice building up on the outdoor coil; modern systems include a defrost cycle that temporarily reverses the refrigerant flow to melt ice and restore efficiency.
The indoor unit varies by system type. A ductless mini-split uses a wall-mounted or ceiling-mounted box that blows conditioned air directly into the room. A ducted system uses coils installed inside your existing furnace cabinet or air handler, so heated or cooled air travels through your ductwork just as it would from a traditional furnace. Some homes use a hybrid system that pairs a heat pump with a gas furnace; the heat pump handles most of the heating season, and the furnace kicks in during the coldest days when the heat pump's efficiency drops too far.
Why efficiency drops in very cold weather
Heat pumps become less efficient as outdoor temperature falls because there is less heat available to extract from cold air. When outdoor air is 0°F, the refrigerant must work harder to pull heat out, and the compressor must run longer to reach the temperature you want indoors. The system also spends energy on defrost cycles to clear ice from the outdoor coil, which temporarily stops heating your home.
Below a certain temperature — typically around 20°F to 35°F depending on the model — many air-source heat pumps switch to backup heating. This is usually electric resistance coils or a gas furnace that takes over when the heat pump cannot keep up. Backup heating is less efficient than the heat pump itself, so your energy costs rise. Cold-climate air-source models push this switchover point lower, sometimes to 0°F or below, by using larger compressors and coils. Ground-source systems avoid this problem because ground temperature stays stable.
The compressor and refrigerant: what makes the cycle work
The compressor is the most energy-intensive part of the system. It is essentially a pump that pressurizes the refrigerant gas, which heats it up. Older compressors ran at a fixed speed — either fully on or fully off — which meant the system cycled on and off throughout the day. Modern heat pumps use variable-speed compressors that ramp up or down based on heating or cooling demand. A variable-speed compressor runs at lower capacity on mild days, using less electricity, and only speeds up when the outdoor temperature is extreme.
The refrigerant itself is a chemical engineered to change state at the temperatures and pressures the system creates. Common refrigerants include R-410A and R-32. Refrigerant does not get used up — it circulates in a closed loop indefinitely. If the system develops a leak, a technician must locate and repair it, then refill the refrigerant. Older systems used R-22, which is being phased out due to environmental concerns; if your heat pump uses R-22 and develops a leak, repairs become expensive because R-22 is no longer manufactured in large quantities.
Heating, cooling, and defrost modes
During heating season, the reversing valve directs refrigerant to the outdoor evaporator coil, where it absorbs heat from outside air. The compressor pressurizes this heat and sends it to the indoor condenser coil, where it warms your home. The system continues this cycle until indoor temperature reaches your thermostat setting, then the compressor shuts off or reduces speed.
In cooling mode, the reversing valve flips, and the indoor coil becomes the evaporator. Refrigerant absorbs heat from your home's indoor air and carries it outside, where the outdoor coil releases it. This is identical to how an air conditioner works.
When frost or ice builds up on the outdoor coil in cold weather, the system enters defrost mode. The reversing valve temporarily switches the system to cooling mode, which warms the outdoor coil and melts the ice. During defrost, your home's heating stops for a few minutes — typically 5 to 15 minutes depending on how much ice has accumulated. Modern systems minimize this disruption by defrosting only when necessary and by using electric resistance coils to maintain indoor temperature during the defrost cycle.
Frequently Asked Questions
Can a heat pump heat a home in winter if it is freezing outside?
Yes, but with reduced efficiency. Even at 0°F, outdoor air contains heat that a heat pump can extract. The system will keep your home warm, but the compressor works harder and may use backup heating on the coldest days. Ground-source heat pumps maintain full efficiency in freezing weather because ground temperature stays constant.
What is the white vapor coming from the outdoor unit?
That is moisture from indoor air that condenses and freezes on the outdoor coil during heating mode. It is normal. In defrost mode, the system melts this ice, and you may see water dripping from the unit. Make sure the drain line is clear so water can flow away from your foundation.
How often does a heat pump need maintenance?
Most systems need a professional inspection once per year, ideally before the heating season starts. You should clean or replace the indoor air filter every one to three months depending on dust levels. Outdoor coils can be rinsed with a garden hose to remove debris, though a technician can do this during annual service.
Why does my heat pump make noise when it switches to defrost mode?
The reversing valve produces a loud click or hiss when it switches direction. You may also hear the compressor working harder as it pressurizes refrigerant. These sounds are normal. If the noise is a grinding or squealing that continues during normal operation, contact a technician.
Can I use a heat pump if I have an old furnace?
Yes. A ducted heat pump can use your existing ductwork and air handler, or it can be paired with your furnace in a hybrid setup where the heat pump handles most heating and the furnace provides backup on very cold days. A ductless mini-split does not require ducts and can be installed in homes with radiators or no existing heating system.