The refrigeration cycle is how an air conditioner moves heat from inside a home to the air outside: refrigerant circulates through four components — an evaporator, a compressor, a condenser and a metering (expansion) device — picking up heat from the indoor air at the evaporator and releasing it outdoors at the condenser.
DOE's Home Cooling 101 guide compares the process to how a refrigerator works.
Below: the four components first, then the pressure–temperature readings the cycle hands you on service calls.
What are the four main components?
Three of the four parts come straight from DOE's consumer Home Cooling 101 guide, which names them in a labelled diagram: the evaporator — the cooling coils that remove heat and humidity from indoor air using refrigerant; the condenser — the hot coils that release the collected heat outdoors; and the compressor, which DOE calls the pump that moves refrigerant between the evaporator and the condenser.
The fourth component — the metering device — is named in a building-science textbook chapter hosted on DOE's Building Science Education site, which lists the basic air-conditioner parts as the outdoor coil, the indoor coil, a compressor and an expansion valve.
The same chapter settles which coil is which: in an air conditioner moving heat out, the outdoor coil is always the condenser and the indoor coil the evaporator.
| Component | Where it sits | What it does |
|---|---|---|
| Evaporator | Indoor coil (in an air conditioner) | Removes heat and humidity from the indoor air, using refrigerant |
| Compressor | Between the two coils | Pump that moves refrigerant between the evaporator and the condenser |
| Condenser | Outdoor coil (in an air conditioner) | Releases the collected heat into the outside air |
| Metering device (expansion valve) | Between the condenser and the evaporator | Lets the refrigerant expand before the evaporator |
Those four parts are the whole basic refrigeration cycle, and every refrigerant-side skill in the trade builds on them — the HVAC technician skills guide shows where this one sits in the full technical set.
What happens to refrigerant at each stage?
The refrigerant is what makes the loop work.
The DOE-hosted chapter describes it as a state-change material: it shifts from a liquid to a gas and back again, and heat energy is absorbed and released as it changes state.
Follow one charge of refrigerant around the loop and each component does one job to it:
- Evaporation (indoor coil). Liquid refrigerant enters the evaporator and changes to a gas — and as it changes state it absorbs heat, removing heat and humidity from the indoor air.
- Compression. The compressor pumps the refrigerant out of the evaporator and toward the condenser, keeping refrigerant moving between the two coils.
- Condensation (outdoor coil). At the condenser the refrigerant releases the collected heat into the outside air and shifts back into a liquid.
- Expansion. The expansion valve lets the refrigerant expand — which the chapter says causes the refrigerant to become very cold — before it re-enters the evaporating coil and starts over.
In heating mode a heat pump runs these same components with the coils' roles swapped: the chapter explains that the outdoor coil acts as the evaporator and the indoor coil as the condenser.
The state changes are the same; the chapter describes the coils swapping roles.
That is the whole HVAC refrigeration cycle: four components, two state changes, one loop.
How pressure and temperature relate (the PT chart)
The readings that matter on the refrigerant side are comparisons against the saturation temperature, which you read from the refrigerant's pressure–temperature chart rather than from a thermometer on the line — the temperature the refrigerant holds while it is evaporating or condensing at the pressure you measure.
The refrigerant's pressure–temperature (PT) chart is the lookup that gets you there — match your gauge pressure for the refrigerant in the system, and the chart returns the saturated temperature that goes with it.
Goodman's subcooling procedure for its GVXC20 condensing unit runs exactly on that lookup: read the liquid pressure, convert it to temperature with a pressure/temperature chart, measure the liquid line temperature at the liquid service valve, and subtract to get subcooling.
Goodman's instructions define the two readings as:
- Superheat is suction line temperature minus saturated suction temperature — how far the vapor in the suction line has warmed past its saturation temperature.
- Subcooling is saturated liquid temperature minus liquid line temperature — how far the liquid leaving the condenser has cooled below its saturation temperature.
What the chart does not give you is a target.
Targets belong to the equipment: Goodman's GVXC20 instructions, for example, give its systems a target subcooling of 8°F ± 1°F (10°F ± 1°F for 5-ton models) when the indoor coil has a thermal expansion valve, and below 65°F outdoor ambient they say to weigh in the charge rather than set it by subcooling.
ACCA's verification protocol, ANSI/ACCA 9 QIvp, judges charge the same relative way: charge passes when measured superheat is within ±5°F of the OEM-specified superheat, or measured subcooling is within ±3°F of the OEM-specified subcooling.
There is no universal PT number
How the cycle shows up in diagnostics
Once the cycle stops being theory and becomes a map, the metering device type decides which reading leads.
ACCA's installation-verification protocol, 9 QIvp, requires the subcooling test on systems with thermal expansion valves and the superheat test on systems with fixed metering devices — a fixed orifice, capillary tube or piston — leaving the other test optional.
Goodman's older installation instructions teach the same split from the manufacturer's side: fixed-orifice systems are charged by superheat using the manufacturer's superheat table (adding charge lowers superheat; removing it raises superheat), while expansion-valve systems are set by liquid-line temperature below saturation — subcooling.
The conditions around the reading matter as much as the reading.
The ACCA 9 QIvp charge-verification form says not to run the cooling-mode subcooling test below 60°F outdoor air or the superheat test below 55°F, and to run the system at least 10 minutes — or until readings stabilize — before measuring.
ACCA's residential maintenance standard, ANSI/ACCA 4 QM, calls measuring superheat or subcooling a good diagnostic field practice for confirming refrigerant charge, and tells techs to follow OEM instructions on inverter-driven equipment.
Goodman's GVXC20 manual shows why that last part matters: its subcooling readings are only valid while the unit runs at 100% capacity, in its charge mode.
This is the reasoning loop a working HVAC technician runs on a no-cool call: state what the cycle intends for each component, take the readings, and compare them against the manufacturer's numbers to confirm whether the refrigerant charge is where the OEM wants it.
Where this is tested: EPA 608 Core
The refrigerant this loop moves is regulated, and EPA's published topics for the Core section of the EPA 608 test include recovery and evacuation techniques, the "three Rs" (recover, recycle, reclaim), and refrigerant safety — alongside ozone depletion, the Clean Air Act and Montreal Protocol, the venting prohibition, the refrigerant sales restriction and shipping labels.
Those are the rules that govern the refrigerant this loop moves.
Apprentices get one built-in accommodation: the apprentice exemption lets you work on a refrigerant circuit without your own 608 certification, but only while you are closely and continually supervised by a certified technician — the rule sets no time limit on the exemption, and the supervising technician and apprentice share responsibility for compliance.
Our guide for HVAC apprentices covers the supervised route into the trade.
The credential itself is its own topic — the exam structure, who must hold it and how to get certified are covered in our EPA 608 certification guide.
Career information, not legal advice: confirm the refrigerant-handling rules with the EPA, and exam details with the certifying organization you test with.
Refrigerant work is federally regulated