Heat pump refrigerant cycle

For homeowners

A heat pump moves heat from one place to another using a circulating refrigerant. The compressor pressurizes the refrigerant, which heats it up. The hot refrigerant gives up its heat to whichever side you want hot. Then the refrigerant expands through a small valve, which cools it dramatically. The cold refrigerant absorbs heat from whichever side you want cold.

In cooling mode: the indoor coil is cold (absorbing heat from your home’s air) and the outdoor coil is hot (dumping that heat outside).

In heating mode: the reversing valve flips, and now the outdoor coil is cold (absorbing heat from outside air — even cold outside air still has heat to give) and the indoor coil is hot (releasing heat into your home).

Same compressor, same coils, opposite direction of heat flow. The reversing valve is the component that makes the flip possible. That’s the only physical difference between a straight-cool AC and a heat pump.

For technicians

A few things this diagram puts cleanly:

The compressor never reverses. Same component, same direction of rotation, same discharge port, same suction port. What changes is everything downstream — the reversing valve sends the hot discharge gas to whichever coil is supposed to be hot, and the cold suction return draws from whichever coil is supposed to be cold. The compressor is the only motor in the system, and it just runs.

The refrigerant cycles through the same four states regardless of mode — hot high-pressure gas, warm high-pressure liquid, cold low-pressure mixture, cool low-pressure gas. What changes is where each state is located physically. In cooling, the outdoor coil holds the warm states (rejecting heat to outside air); in heating, the outdoor coil holds the cool states (absorbing heat from outside air). The fact that you can pull useful heat out of 35°F outside air is the whole magic of the heat pump — the refrigerant after the expansion valve is colder than that, so heat still flows from the air to the refrigerant. Thermodynamics doesn’t care which direction is “natural”; the compressor’s work input drives heat from cold to hot.

The TXV is bi-flow on a true heat pump. Either a single bi-directional valve, or more commonly two TXVs with check valves around them — one TXV is active in cooling mode (metering into the indoor coil) and bypassed via its check valve in heating mode; the other TXV is the inverse. Same idea, different plumbing. A fixed-orifice (piston) system uses a single piston that works marginally well in both directions and is the cheapest way to build a heat pump, which is why you see it on entry-level equipment.

The defrost cycle is literally this. The unit is happily running in heating mode, frost builds on the outdoor coil because it’s below freezing while pulling moisture from the air, the defrost board sees the coil temperature drop too low for too long, and it sends the reversing valve back to cooling position for a few minutes. Compressor keeps running. Hot discharge gas now flows out to the (frosty) outdoor coil and melts the ice. Indoor blower is shut off during this so the people inside don’t get a blast of cold air. Once the coil hits about 60°F at the sensor, defrost ends, valve flips back to heating, blower starts again, and you smell that distinctive “wet refrigerant smell” briefly as everything stabilizes. Customers call this “the unit blew cold air on me” — it didn’t, it just stopped blowing hot air for ninety seconds.

Practical diagnostics this diagram informs. A reversing valve stuck mid-stroke gives you a unit that can’t really cool or heat — gauges show pressures that don’t make sense for either mode because the valve is partially routing flow both ways. A reversing valve solenoid that doesn’t energize means the valve never flips at all — unit acts like a one-mode AC. A reversing valve that DOES flip but stays loud and shuddery means refrigerant velocity is wrong (often low charge), or the slide is worn. All three present differently on gauges and amp readings, and the difference matters for which part you replace.

Coefficient of performance (COP) — why heat pumps are efficient. A resistance heater turns 1 unit of electricity into 1 unit of heat. A heat pump turns 1 unit of electricity into 2-4 units of heat in cooling mode (the EER/SEER rating) and 2-3.5 units of heat in heating mode (the HSPF rating), because most of the heat is being MOVED rather than CREATED. The compressor’s electrical work is the only “new” energy added; the rest is heat pulled from one location to another. As outdoor temperature drops in heating mode, the available heat in the source air decreases and COP drops with it. At very low outdoor temperatures (below freezing for older units, below 0°F for cold-climate models), the system either needs auxiliary heat strips to keep up or runs less efficiently than pure resistance heat would. Modern variable-speed heat pumps maintain useful COP down to -15°F or lower; older fixed-speed units lose effectiveness around 25°F and switch over to electric backup.