How Heat Pumps Work

The Four Core Components

A heat pump does not create heat — it moves it. Even air at 20°F holds usable thermal energy, and the refrigeration cycle lets us pump that energy uphill into a warmer house. The whole trick rests on one physical fact: when you compress a gas it gets hot, and when you let a liquid boil at low pressure it gets cold. Four components exploit that fact in a closed loop.

• Compressor — the heart of the system. It pulls in low-pressure vapor and squeezes it into high-pressure, high-temperature discharge gas.
• Condenser coil — where the hot discharge gas rejects heat and condenses into a liquid. Whichever coil is receiving discharge gas is acting as the condenser.
• Metering device — a TXV or fixed orifice that drops high-pressure liquid to low pressure, flashing part of it cold.
• Evaporator coil — where the cold, low-pressure refrigerant boils and absorbs heat. The coil fed by the metering device is acting as the evaporator.

In a straight air conditioner those roles are fixed: the indoor coil is always the evaporator, the outdoor coil is always the condenser. A heat pump adds one part — the reversing valve — that swaps which coil does which job. That single valve is why one box can both heat and cool.

Cooling Mode: Moving Heat Out

In cooling, a heat pump behaves exactly like a standard split-system AC. The indoor coil is the evaporator and the outdoor coil is the condenser.

1. Cold, low-pressure refrigerant boils inside the indoor coil, absorbing heat from the return air. Supply air comes off 14–22°F cooler than the return.
2. That vapor returns to the compressor, gets squeezed to high pressure, and leaves as hot discharge gas.
3. Discharge gas flows to the outdoor coil, where the condenser fan blows ambient air across it and the refrigerant dumps its heat outside and condenses to liquid.
4. The liquid passes through the metering device, drops in pressure, and heads back to the indoor coil to do it again.

Heating Mode: Moving Heat In

Flip to heating and the reversing valve reroutes the discharge gas. Now the indoor coil becomes the condenser and the outdoor coil becomes the evaporator. The refrigerant flow through the compressor never changes direction — the compressor still pumps the same way — but the high-side and low-side coils trade places.

1. Hot discharge gas now goes to the indoor coil. The blower pushes house air across it, the refrigerant gives up its heat to that air, and warm supply air comes off the register.
2. The condensed liquid flows out to the metering device serving the outdoor coil and drops to low pressure.
3. In the outdoor coil, that cold refrigerant boils by absorbing heat from the outdoor air — yes, even cold winter air. This is why the outdoor coil runs colder than ambient and frosts up.
4. Low-pressure vapor returns through the reversing valve to the compressor suction, and the loop repeats.

The Reversing Valve

The reversing valve (also called a four-way valve) is a slide valve with four refrigerant connections: one to the compressor discharge, one to the suction line, one to the indoor coil, and one to the outdoor coil. A pilot solenoid, energized by a 24V coil, uses the system’s own pressure differential to shove an internal slide from one end to the other, re-routing discharge gas to whichever coil should be the condenser.

A critical field detail: manufacturers wire the solenoid to be energized in either heating or cooling depending on the brand. Most (Carrier, Trane, Rheem) energize the “O” terminal in cooling. Some legacy Rheem/Ruud equipment uses a “B” terminal energized in heating. Always confirm at the thermostat and unit before you assume a stuck valve.

Trace the Cycle Step by Step

Here is the full loop as you would trace it with a set of gauges and a temp clamp. This example is heating mode on an R-410A system:

Typical Field Pressures & Temps

Exact numbers depend on ambient, load, and refrigerant, but these R-410A ballparks help you know when something is off. Note that R-454B — the A2L replacement now shipping on new equipment — runs within about 3% of these pressures under the same conditions, so your feel for the numbers carries over.

Why a Heat Pump Beats Resistance Heat

Electric resistance heat (strip heat) converts one unit of electricity into one unit of heat — a coefficient of performance (COP) of 1.0. A heat pump does not convert electricity into heat; it uses electricity to move heat that already exists in the outdoor air. Because moving heat costs far less energy than making it, a modern heat pump delivers a COP of roughly 2 to 4 in mild weather. That means for every 1 kW of electricity it draws, it can move 2–4 kW of heat into the house.

That advantage shrinks as it gets colder, because there is less heat in the outdoor air to grab and the coil frosts more. That is why systems add electric strip heat or a gas furnace (dual-fuel) below the balance point — the outdoor temperature where the heat pump’s output can no longer keep up with the building’s heat loss. Understanding that crossover is key to sizing auxiliary heat correctly.

Field Notes & Safety

• EPA 608 applies. Any time you connect gauges and could vent refrigerant, you must hold the correct 608 certification (Type II covers high-pressure heat pumps). Recover — never vent — when opening the system.
• A2L equipment is here. R-454B and R-32 are mildly flammable (A2L). Follow the manufacturer’s handling and leak-detection requirements, use compatible tools and recovery machines, and respect charge limits and ventilation rules from ASHRAE 15/34.
• Don’t misread defrost as a failure. Steam rolling off the outdoor unit and a brief blast of cool indoor air during a heating call is normal defrost operation, not a broken system.
• Check the easy stuff first. A dirty filter or blocked coil throws off pressures and superheat/subcooling in both modes. Verify airflow before you condemn the charge, the metering device, or the reversing valve.
• Auxiliary heat masks problems. A homeowner may not notice a weak or bypassing reversing valve because strip heat keeps them warm — while the electric bill quietly climbs. Watch amp draw and staging.