Heat pumps heat by moving warmth from outside air into your home, not by generating heat like a furnace does
A heat pump is an air-to-air system that pulls warmth from the outdoor air—even when it feels cold outside—and transfers it indoors through refrigerant lines. The outdoor unit contains a fan and a coil that absorbs heat from the air. That heat is then compressed and moved through copper tubing to an indoor unit, where it releases the warmth into your home. In winter, this process runs continuously to maintain your set temperature. In summer, the cycle reverses and the system works as an air conditioner, pulling heat out of your home instead.
The key difference from a furnace is that a heat pump does not burn fuel or use electric resistance to create heat—it harvests heat that already exists in the outdoor air and moves it where you need it. This is why heat pumps are more efficient than electric heating in most climates: moving heat uses far less energy than making it from scratch.
Key Takeaways
- Heat pumps move warmth from outside air into your home using refrigerant that circulates between an outdoor unit and an indoor unit.
- The outdoor coil absorbs heat, a compressor pressurizes the refrigerant to raise its temperature, and the indoor coil releases that heat into your living space.
- Heat pumps work in cold climates, though their efficiency drops as outdoor temperature falls below freezing, and many systems include electric backup heat for extreme cold.
- The system reverses in summer to cool your home by pulling heat out and releasing it outdoors, so one unit handles both heating and cooling.
The refrigerant cycle that moves heat indoors
The heart of a heat pump is a closed loop of refrigerant—a liquid that evaporates and condenses at low temperatures. In heating mode, the outdoor coil acts as an evaporator: refrigerant inside it absorbs heat from the outside air and turns into a gas, even when the air temperature is well below freezing. This warm gas then travels through a copper line to the compressor, which is the engine of the system.
The compressor squeezes the refrigerant gas, raising its pressure and temperature significantly—much hotter than the air it came from. This hot, pressurized gas then flows to the indoor coil, called the condenser. As indoor air passes over this coil, the refrigerant releases its heat into your home and cools back down into a liquid. An expansion valve then lowers the pressure of the liquid, and the cycle begins again. This loop runs continuously during the heating season, moving heat from outside to inside over and over.
The efficiency of this cycle depends on the temperature difference between indoors and outdoors. The larger the gap, the harder the compressor has to work. On a 35-degree day, a heat pump moves heat easily. On a 0-degree day, it still works, but the compressor runs longer and uses more electricity to achieve the same indoor temperature.
Why heat pumps work even in cold weather
Many homeowners assume a heat pump cannot heat when it is freezing outside, but that is not how the system works. The outdoor coil does not need the air to be warm—it only needs the air to contain heat energy, which it does at any temperature above absolute zero. Even at 10 degrees Fahrenheit, the outdoor air holds usable heat that the refrigerant can absorb and move indoors.
However, as outdoor temperature drops, the system becomes less efficient. The refrigerant has to work harder to pull heat from very cold air, so the compressor runs longer and draws more electricity. At some point—usually around 20 to 30 degrees Fahrenheit, depending on the system—many heat pumps switch on electric backup heat (also called auxiliary heat or emergency heat) to supplement the heat pump. This backup is a straightforward electric resistance heater, similar to a space heater, that kicks in automatically when the heat pump alone cannot keep up with demand.
In climates where temperatures rarely drop below freezing, a heat pump alone handles all heating needs. In colder regions, the backup heat runs during the coldest weeks, and the system still uses less total energy than a traditional furnace would over the entire winter.
The indoor and outdoor units and how they connect
A heat pump system has two main components. The outdoor unit houses the compressor, the outdoor coil, a fan, and the expansion valve. It is roughly the size of a central air conditioner unit and sits on a concrete pad outside your home. The indoor unit is typically mounted on a wall or in a closet and contains the indoor coil and a blower fan that pushes warm air through your ductwork or into the room.
The two units are connected by two copper refrigerant lines—one carries warm refrigerant to the indoor unit, and the other returns cooler refrigerant to the outdoor unit. These lines are insulated to prevent heat loss and are usually run through a small hole in your exterior wall. A drain line also runs from the indoor unit to remove condensation that forms on the coil, similar to an air conditioner.
Some heat pump systems are ductless (also called mini-splits), meaning the indoor unit blows warm air directly into the room without connecting to central ductwork. Others are ducted and tie into your existing furnace ducts, so warm air travels through your home the same way it would from a traditional heating system. The heating principle is identical in both cases—only the delivery method differs.
How the system switches between heating and cooling
In summer, a heat pump reverses its operation by changing the direction of refrigerant flow. A component called a reversing valve switches the roles of the indoor and outdoor coils. Now the indoor coil becomes the evaporator—it absorbs heat from your home's air, and the outdoor coil becomes the condenser—it releases that heat outside. The result is air conditioning: your home cools down, and hot air is expelled outdoors.
This reversing happens automatically when you switch your thermostat from heating to cooling mode, or on some modern systems, the heat pump detects the indoor temperature and switches on its own. Because one unit handles both heating and cooling, you do not need a separate air conditioner, which saves space and installation cost. The same refrigerant lines, compressor, and fans do both jobs depending on the season.
Thermostat control and how the system maintains temperature
Your heat pump is controlled by a thermostat that tells the system when to run and when to stop. When you set the thermostat to 70 degrees in winter, the system runs continuously until your home reaches 70 degrees, then cycles off. As soon as the temperature drops below your set point, the compressor and outdoor fan restart automatically. This on-and-off cycling continues throughout the day and night to hold your home at the temperature you chose.
On very cold days when backup heat is active, your thermostat may show a second heating stage. The first stage is the heat pump itself; if it cannot keep up, the second stage (electric resistance heat) turns on automatically. You may also see a "heat pump" indicator and a separate "auxiliary heat" indicator on your thermostat display, showing which stage is running.
Modern smart thermostats can learn your schedule and adjust temperatures automatically, which reduces energy use. Some also display how much time the backup heat ran, helping you understand how hard the system is working during cold snaps.
Maintenance that keeps the heating cycle working
Heat pumps require less maintenance than furnaces because there is no combustion, no filter changes as frequent, and no annual inspection by a technician is strictly required—though it is recommended. The main maintenance tasks are cleaning or replacing the air filter every one to three months (more often if you have pets or dust), keeping the outdoor unit clear of leaves and snow, and having the refrigerant checked by a professional if the system is not heating as well as it used to.
If your heat pump is not warming your home adequately, the problem is usually low refrigerant (a leak), a frozen outdoor coil (which the system should defrost automatically, but sometimes does not), or a failed compressor. These are not homeowner repairs—you will need to call a heat pump technician. Refrigerant is a controlled substance and requires certification to handle, and compressor replacement is expensive and specialized work.
Keeping the outdoor unit free of obstructions helps the system run efficiently. Remove leaves, snow, and debris from around the outdoor coil, and do not stack items against the unit. In winter, some snow on the outdoor coil is normal and the system will defrost it automatically, but heavy snow buildup should be cleared away gently.
Frequently Asked Questions
Can a heat pump heat a home in freezing weather?
Yes. Heat pumps extract warmth from outdoor air even when it is below freezing. However, as temperature drops, the system becomes less efficient and may switch on electric backup heat to maintain your set temperature. The system still works, but uses more electricity on the coldest days.
Why does my heat pump make noise when it switches to heating?
When a heat pump switches from cooling to heating mode, or when it defrosts the outdoor coil on a cold day, you may hear hissing, gurgling, or a brief loud sound. This is the reversing valve changing direction or refrigerant moving through the lines. It is normal and not a sign of damage.
What is the defrost cycle and why does my heat pump seem to stop heating sometimes?
On cold, humid days, frost can build up on the outdoor coil and block airflow. The system automatically reverses to cooling mode briefly to melt the frost, which may make your home feel slightly cooler for a few minutes. This is normal operation and the system resumes heating once the coil is clear.
How long does a heat pump last?
Most heat pumps last 15 to 20 years with regular maintenance. The compressor, which is the most expensive component, typically lasts the life of the unit. Proper filter changes and keeping the outdoor unit clean extend the system's lifespan.
Is a heat pump more efficient than my old furnace?
In most climates, yes. Heat pumps move existing heat rather than creating it, so they use less energy per unit of warmth delivered. However, efficiency varies by outdoor temperature and your local climate. A technician can compare your furnace's fuel cost to a heat pump's electricity cost for your specific situation.