What it is

Every air conditioner, heat pump, and refrigerator runs on the same loop. We call it the vapor-compression cycle. The whole point is to move heat from a place you want cool to a place you don't care about. Refrigerant is just the bus that carries the heat. It picks up heat indoors, hauls it outside, and dumps it. Then it comes back empty and does it again.

There are four jobs that have to happen in order, and there are four major components that do them. Once you can picture this loop in your head, every gauge reading and every diagnosis starts to make sense.

How it works

Refrigerant doesn't create cold. There's no such thing as "cold." There's only heat and the absence of heat. The cycle moves heat in one direction by changing the refrigerant's pressure and letting it boil and condense at the temperatures we want.

The trick is that a liquid boils at a temperature that depends on its pressure. Raise the pressure, the boiling point goes up. Drop the pressure, the boiling point drops. We use that on purpose. We give the refrigerant a low pressure indoors so it boils cold and soaks up heat, and a high pressure outdoors so it condenses hot and rejects that heat to outdoor air.

In the field

Walk the loop with me, starting at the compressor:

  1. Compression (the compressor). The compressor pulls in cool, low-pressure refrigerant vapor from the suction line and squeezes it. Squeezing a gas raises both its pressure and its temperature. It leaves the compressor as a hot, high-pressure vapor. The compressor is the pump that makes the whole cycle go — it's the line between the low side and the high side.
  1. Condensing (the condenser). That hot vapor flows into the outdoor coil (in cooling). The condenser fan blows outdoor air across it. Because the vapor is now hotter than the outdoor air, heat flows out of the refrigerant into the air. As it loses heat, the vapor condenses into a liquid — same as steam fogging up a cold window. It leaves the condenser as a warm, high-pressure liquid.
  1. Metering (the metering device). Next the liquid hits the metering device — a TXV or a fixed orifice (piston). This is the dividing line on the other end. It's a deliberate restriction that drops the pressure way down. The instant the pressure drops, the boiling point drops with it, and the refrigerant becomes a cold, low-pressure mix of liquid and a little flash gas.
  1. Evaporating (the evaporator). That cold liquid enters the indoor coil. The blower pushes warm room air across it. Now the refrigerant is colder than the room air, so heat flows into the refrigerant and boils it back into a vapor. Boiling absorbs a lot of heat — that's where the cooling actually happens. It leaves as a cool, low-pressure vapor and heads back to the compressor to start over.

That's the loop. Compress, condense, meter, evaporate. Forever, as long as the unit runs.

Normal values & targets

You don't memorize pressures — they ride on the refrigerant and the conditions. But here's the shape of it on a healthy R-410A cooling system on a mild day:

  • Low side (suction): roughly 115–135 psig, which saturates around 38–45°F.
  • High side (liquid/discharge): roughly 350–425 psig, depending on outdoor temp; condensing temp usually lands about 15–25°F above outdoor ambient.
  • Superheat at the compressor: generally 10–20°F so the compressor only swallows vapor.
  • Subcooling at the condenser outlet: generally 8–12°F on a TXV system so the metering device gets solid liquid.

The exact numbers shift with refrigerant, weather, and load. The relationships don't.

Common faults & what they mean

  • Warm air, low suction, low head: often low charge or a restriction starving the evaporator.
  • High head, high subcooling: often overcharge or a dirty/blocked condenser that can't reject heat.
  • High superheat everywhere: the evaporator is starved — low charge, undersized metering, or a restriction upstream.
  • Low superheat, liquid coming back: the evaporator is flooded — overcharge or a metering device feeding too much.

Every one of those is just one of the four processes not doing its job. Find which process is off and you've found the problem.

Tech tips & gotchas

  • The compressor and the metering device are the two "walls" between the high side and the low side. Everything between the compressor discharge and the metering device is high side. Everything from the metering device outlet back to the compressor suction is low side. Know which side you're touching.
  • A condenser rejects more heat than the evaporator absorbs. It has to dump both the heat picked up indoors AND the heat the compressor added by squeezing. That's why the outdoor coil runs hot.
  • "Subcooling" and "superheat" aren't extra concepts bolted on — they're just how far the liquid is below its boiling point (subcool) or how far the vapor is above it (superheat). They tell you the state of the refrigerant at that point in the loop.

Safety / code notes

  • Refrigerant under pressure can flash-freeze skin and eyes — wear safety glasses and gloves when connecting and disconnecting hoses.
  • Recover refrigerant per EPA 608; venting most refrigerants is a federal violation.
  • Service and handling of newer A2L refrigerants carry added requirements per the equipment listing and applicable code — confirm before working on those systems.