The Basic Loop: How Heat Pumps Circulate Refrigerant
A heat pump works by moving heat from one place to another using a liquid called refrigerant that cycles through four main parts: a compressor, a condenser, an expansion valve, and an evaporator. The refrigerant changes between liquid and gas as it moves through these parts, and those changes allow the system to absorb heat from outside air (even in winter) and release it inside your home.
Think of the refrigerant as a delivery truck. In summer, it picks up heat from inside your home and dumps it outside. In winter, it picks up heat from outside air and brings it in. The same loop runs year-round—the direction of flow just reverses depending on the season and what you need.
The whole process relies on a straightforward fact: when a liquid evaporates into a gas, it absorbs heat from its surroundings. When a gas condenses back into a liquid, it releases that heat. A heat pump forces this cycle to happen over and over, moving heat where you want it to go.
Key Takeaways
- Refrigerant circulates through four parts—compressor, condenser, expansion valve, and evaporator—changing between liquid and gas to move heat in or out.
- In winter, the outdoor unit absorbs heat from outside air and the indoor unit releases it into your home; in summer, the flow reverses.
- The compressor is the engine of the system, pressurizing the refrigerant to make the heat-moving cycle work.
- A reversing valve switches the direction of refrigerant flow so the same equipment heats in winter and cools in summer.
The Compressor: The Engine That Starts the Cycle
The compressor is the most important part of a heat pump. It takes in low-pressure refrigerant gas and squeezes it into high-pressure gas. When you compress a gas, it gets hot—this is the energy that makes the whole system work. The compressor runs on electricity and is usually located in the outdoor unit.
Once the compressor pressurizes the refrigerant, that hot, high-pressure gas moves to the next stage. In winter heating mode, this hot gas goes directly to the indoor unit, where it releases heat into your home. In summer cooling mode, it goes to the outdoor unit first, where it dumps heat outside before cycling back.
The Condenser: Where Heat Gets Released
After leaving the compressor, the hot, pressurized refrigerant gas enters the condenser. This is a coil with a fan that blows air across it. As outside air passes over the coil, the refrigerant inside cools down and condenses from a gas back into a liquid. When it does, it releases the heat it was carrying—and that heat either goes into your home (in heating mode) or out of it (in cooling mode).
In a cooling cycle, the condenser is in the outdoor unit, and the fan pushes the released heat outside. In a heating cycle, the condenser is in the indoor unit, and the released heat warms the air that blows into your rooms. The same coil can work both ways depending on which direction the refrigerant flows.
The Expansion Valve: Pressure Drop That Enables Heat Absorption
After the condenser, the refrigerant is a cool, high-pressure liquid. It then passes through a small opening called the expansion valve (or throttle valve). This valve restricts the flow, causing the pressure to drop suddenly. When pressure drops, the liquid's temperature drops too—it becomes very cold.
This cold, low-pressure liquid is the key to the next step. The expansion valve is a straightforward but essential part: it does nothing but create a pressure difference, but that difference is what allows the refrigerant to absorb heat in the evaporator. Without it, the cycle would not work.
The Evaporator: Where Heat Gets Absorbed
The cold, low-pressure liquid from the expansion valve enters the evaporator, another coil with a fan. In heating mode, this coil is in the outdoor unit and sits in the cold outside air. Even though the air is cold, it is still warmer than the refrigerant inside the coil. Heat flows from the air into the refrigerant, causing the liquid to evaporate into a gas.
As the refrigerant absorbs heat and turns into a gas, it carries that heat back to the compressor, where the cycle begins again. In cooling mode, the evaporator is in the indoor unit, and it absorbs heat from the air inside your home, cooling your rooms while the heat is sent outside.
This is the counterintuitive part that confuses many people: a heat pump can pull heat from cold outside air because the refrigerant is even colder. As long as the refrigerant is colder than its surroundings, heat will flow into it.
The Reversing Valve: Switching Between Heating and Cooling
A reversing valve is a four-way valve that changes the direction of refrigerant flow. When you switch your heat pump from cooling to heating mode (or vice versa), this valve flips, sending the hot refrigerant to a different coil. In heating mode, hot refrigerant goes to the indoor unit to warm your home. In cooling mode, it goes to the outdoor unit to dump heat outside.
Without the reversing valve, you would need two separate systems—one to heat and one to cool. The reversing valve lets one system do both jobs by straightforward changing which coil acts as the condenser and which acts as the evaporator.
How the Outdoor and Indoor Units Work Together
Most heat pumps have two visible parts: an outdoor unit (the condenser/evaporator box) and an indoor unit (usually a wall-mounted or ceiling-mounted handler, or sometimes built into your furnace). Refrigerant lines run between them, carrying the refrigerant back and forth.
In winter, the outdoor unit's fan pulls cold air across the evaporator coil, where refrigerant absorbs heat from that air. The warm refrigerant gas travels through the line into your home, where it enters the indoor unit's condenser coil. A fan in the indoor unit blows your home's air across this coil, and the refrigerant releases its heat into your rooms. The now-cool refrigerant returns to the outdoor unit to repeat the cycle.
In summer, the process reverses. The indoor unit's evaporator coil absorbs heat from your home's air, and the outdoor unit's condenser coil releases that heat outside. The same refrigerant lines carry the heat in both directions—the direction just changes based on the season.
Frequently Asked Questions
Why can a heat pump pull heat from cold outside air in winter?
The refrigerant inside the evaporator coil is colder than the outside air, even in winter. Heat always flows from warmer to colder, so the air outside (say, 30°F) is warmer than the refrigerant (say, 0°F), and heat flows into the refrigerant. The compressor then pressurizes that heat and makes it hot enough to warm your home.
What is the difference between the condenser and evaporator?
Both are coils, but they do opposite jobs. The evaporator absorbs heat (refrigerant inside is colder than the air around it), and the condenser releases heat (refrigerant inside is hotter than the air around it). In a heat pump, the same physical coil can be either one, depending on which direction the refrigerant flows.
Does a heat pump work in very cold weather?
Most air-source heat pumps work down to about 0°F, though efficiency drops as outdoor temperature falls. Below that, a backup heating system (usually electric resistance heat or a gas furnace) kicks in. Some newer cold-climate heat pumps are designed to work efficiently at temperatures well below zero.
Why does my heat pump sometimes make a loud hissing or gurgling sound?
Hissing usually means refrigerant is leaking from a line or connection. Gurgling can be normal refrigerant flow, but if it is loud or new, it may indicate a problem. Either sound warrants a call to a technician—refrigerant leaks reduce efficiency and can damage the compressor.
Can I see the refrigerant moving through the system?
No. Refrigerant is sealed inside copper lines and coils. You cannot see it, and you should never try to open the system yourself—refrigerant requires special handling and recovery equipment. Only licensed technicians should work with refrigerant.