The basic layout: where the parts sit
A heat pump has two metal boxes connected by copper pipes carrying refrigerant. One box sits outside your home (the outdoor unit), and one sits inside near your furnace or in a closet (the indoor unit). The outdoor unit looks like an air conditioner — a rectangular metal cabinet with a fan visible through a grille on top. The indoor unit is usually a metal box mounted on a wall or sitting on the floor, with vents where warm or cool air comes out.
The two units are connected by a bundle of insulated copper tubes that run through your wall or foundation. One tube carries cold refrigerant to the indoor unit, and the other carries warm refrigerant back to the outdoor unit. A thin electrical cable and a drain line also run between them. Everything is sealed and pressurized — you should never open these tubes yourself.
Key Takeaways
- A heat pump has an outdoor unit that looks like an air conditioner and an indoor unit that mounts on a wall or floor, connected by insulated copper pipes.
- The compressor, which sits in the outdoor unit, is the engine of the whole system — it pressurizes refrigerant to move heat in or out of your home.
- The expansion valve in the indoor unit controls how much cold refrigerant flows through the coils, which is what actually heats or cools the air.
- A reversing valve lets the system run backward in winter, pulling heat from outside air and pushing it indoors instead of cooling.
- The fan in each unit moves air across the coils so the refrigerant can exchange heat with the air in your home.
The compressor: the engine that moves heat
The compressor is a pump about the size of a small car engine, bolted to the bottom of the outdoor unit. It is the most important part — it pressurizes the refrigerant so that heat can move from one place to another. When you turn on your heat pump, the compressor starts running and sucks in low-pressure refrigerant gas from the indoor unit through one of the copper pipes. It squeezes that gas, raising its pressure and temperature, then pushes it back toward the indoor unit through the other pipe.
The compressor runs continuously while the heat pump is heating or cooling. You will hear it humming or buzzing when it is on. In winter, the compressor works the same way, but a valve inside the outdoor unit reverses the flow so that the hot refrigerant goes to the indoor coils instead of the outdoor ones. The compressor is sealed and does not have user-serviceable parts — if it fails, a technician must replace it.
The coils: where heat actually transfers
Both the outdoor and indoor units contain coils — long loops of thin copper tubing with aluminum fins attached. These fins look like the radiator in a car. The refrigerant flows through the tubing, and air blows across the fins. As the air passes over the cold coils, heat moves from the air into the refrigerant (cooling mode), or as air passes over warm coils, heat moves from the refrigerant into the air (heating mode).
In the outdoor unit, the coils sit behind the fan grille. In the indoor unit, the coils are usually hidden inside the metal cabinet, but you may see them if you remove the front panel. The coils are delicate — dirt and ice can block them and reduce efficiency. A technician should clean them during yearly maintenance.
The expansion valve: controlling the flow
The expansion valve (also called a metering device) is a small brass fitting, usually about the size of a pen, located where the copper pipes enter the indoor unit. It acts like a gate, controlling how much cold refrigerant flows into the indoor coils. When the valve opens wide, more refrigerant flows through and the coils get colder. When it closes, less refrigerant flows and the coils warm up.
The valve responds to the temperature of the refrigerant leaving the indoor coils — if the coils are too cold, the valve closes a bit to warm them up. This prevents the coils from freezing and damaging the system. You cannot adjust this valve yourself; it is factory-set and sealed. If it sticks or fails, a technician must replace it.
The reversing valve: switching between heating and cooling
The reversing valve is a solenoid-operated switch inside the outdoor unit that reverses the direction of refrigerant flow. In cooling mode, it directs hot refrigerant to the outdoor coils so heat can escape outside. In heating mode, it flips and directs hot refrigerant to the indoor coils so heat can enter your home. You will hear a brief hissing or clicking sound when the system switches between heating and cooling — that is the reversing valve opening or closing.
The reversing valve is what makes a heat pump different from a regular air conditioner. Without it, the system could only cool. The valve is sealed inside the outdoor unit and does not require maintenance, but if it fails to switch, the system will either only heat or only cool.
The fans and motors
Both the outdoor and indoor units have electric fans and motors. The outdoor fan pulls air through the outdoor coils so the refrigerant can exchange heat with the outside air. The indoor fan (sometimes called a blower) pushes air across the indoor coils and out through the vents into your home. Both fans run whenever the system is heating or cooling.
The outdoor fan is visible through the grille on top of the outdoor unit — it looks like a propeller. The indoor fan is usually hidden inside the cabinet, but you can hear it running. If either fan stops working, the system will not heat or cool properly. A technician can replace a failed fan motor without replacing the entire unit.
The refrigerant lines and insulation
The two copper pipes connecting the outdoor and indoor units are the refrigerant lines. One carries cold liquid refrigerant to the indoor unit (the smaller line), and the other carries warm gas back to the outdoor unit (the larger line). The larger line is wrapped in thick foam insulation to prevent the warm refrigerant from losing heat as it travels outside.
These lines are pressurized and sealed — they should never leak. If you see frost or ice on the outdoor unit or the lines, or if you notice a hissing sound, the system may have a refrigerant leak. Do not try to repair this yourself; call a technician. The system cannot work without the correct amount of refrigerant.
The thermostat and controls
Your thermostat is the command center. It sits on your wall inside the home and tells the heat pump when to turn on, what temperature to maintain, and whether to heat or cool. Modern thermostats are digital and may have a screen showing the current temperature and settings. Some can be programmed to change temperature at different times of day.
The thermostat sends electrical signals through a thin wire to the outdoor unit and indoor unit. When you set the thermostat to heat mode and a certain temperature, the thermostat signals the compressor to start and the reversing valve to switch to heating. When the indoor temperature reaches your set point, the thermostat signals the compressor to stop. If the temperature drops again, the cycle repeats.
Frequently Asked Questions
Why is the outdoor unit so loud?
The compressor and fan in the outdoor unit both run continuously while the system is heating or cooling, and they create noise. A humming sound is normal. If you hear grinding, squealing, or rattling, something may be loose or failing — have a technician inspect it.
What is the liquid dripping from the outdoor unit?
That is condensation from the outdoor coils. In heating mode, moisture condenses on the cold outdoor coils and drains out. This is normal and should drain away from your foundation. If water pools around the unit, the drain line may be clogged.
Can I clean the outdoor unit myself?
You can gently rinse the outdoor coils with a garden hose to remove dirt and leaves, but do not use a pressure washer — it can damage the fins. Turn off the system first. For a thorough cleaning, hire a technician during yearly maintenance.
What happens if the refrigerant leaks?
The system will gradually lose cooling or heating power. You may notice frost on the lines or outdoor unit, or hissing sounds. A technician can find the leak, repair it, and refill the refrigerant. Do not ignore a leak — running the system low on refrigerant can damage the compressor.
Do I need to replace any parts inside the heat pump?
The compressor, coils, and valves are sealed and designed to last 15 to 20 years. The fan motor may need replacement after 10 to 15 years. The expansion valve rarely fails. A technician can tell you if any part needs replacement during maintenance.