Quick answer
The refrigeration cycle is the closed loop in which a refrigerant absorbs heat by boiling at low pressure in an evaporator, is compressed, releases that heat by condensing at high pressure in a condenser, and is expanded back to low pressure to repeat.
It is how every air conditioner, heat pump and refrigerator moves heat from a cold place to a warm one. The refrigerant is not used up; it circulates.
Quick facts
The key facts about Refrigeration Cycle, with sources:
- Also called
- Vapor-compression cycle
- Four main parts
- Evaporator, compressor, condenser, expansion device
- Low side
- Refrigerant boils in the evaporator, absorbing heat
- High side
- Refrigerant condenses in the condenser, releasing heat
- R-32 saturation at 40°F
- 135.65 psia (about 121 psig) 1
- R-32 saturation at 110°F
- 389.58 psia (about 375 psig) 1
- Service checks
- Superheat and subcooling, read from pressure and line temperature 2
Key takeaways
- A refrigerant absorbs heat when it boils and gives it off when it condenses.
- Pressure sets the temperature at which it boils or condenses; that is what the compressor and expansion device control.
- For R-32, 40°F boiling happens near 121 psig and 110°F condensing near 375 psig 1.
- Heat pumps run the same cycle in reverse for heating.
- Opening the refrigerant loop is EPA 608 certified work only 3.
What the refrigeration cycle is
Water boils at 212°F at sea level but at a lower temperature on a mountain, where the pressure is lower. Refrigerants work the same way, only at much lower temperatures.
At the right pressure, a refrigerant boils at 40°F, cold enough to chill house air. At a higher pressure, it condenses at 110°F, hot enough to dump heat into a 95°F day.
The refrigeration cycle uses that trick in a loop. Boiling soaks up a lot of heat; condensing gives it back. Moving the refrigerant between a low-pressure side and a high-pressure side moves heat from indoors to outdoors in an air conditioner, or from outdoors to indoors in a heat pump.
The cycle, labelled
- 1. Evaporator (indoor coil in cooling): cold, low-pressure liquid boils into vapor as room air gives up heat → low-pressure vapor
- 2. Suction line → compressor: vapor is squeezed → hot, high-pressure vapor
- 3. Discharge line → condenser (outdoor coil): vapor gives heat to outdoor air and condenses → warm, high-pressure liquid
- 4. Liquid line → expansion device (TXV, EEV or piston): pressure drops sharply → cold, low-pressure liquid-vapor mix
- → back to the evaporator. Heat pump in heating: a reversing valve swaps the indoor and outdoor coil roles.
How each stage works
- Evaporation. The refrigerant enters the indoor coil as a cold mix of liquid and vapor. Warm room air boils off the liquid. By the coil outlet it is all vapor, a little warmer than its boiling point. That extra is superheat.
- Compression. The compressor raises the vapor's pressure, which also raises its temperature well above outdoor air.
- Condensation. In the outdoor coil the hot vapor cools to its condensing point, turns back to liquid, then cools a little more. That extra cooling is subcooling.
- Expansion. The liquid squeezes through a metering device. Its pressure falls, part of it flashes to vapor, and the rest drops to the low boiling temperature, ready to absorb heat again.
Where the cycle shows up
| Equipment | Evaporator is | Condenser is |
|---|---|---|
| Central AC | Indoor coil | Outdoor coil |
| Heat pump, cooling | Indoor coil | Outdoor coil |
| Heat pump, heating | Outdoor coil | Indoor coil |
| Ductless mini-split | Wall head (cooling) | Outdoor unit (cooling) |
| Geothermal heat pump | Ground loop exchanger (heating) | Indoor coil (heating) |
| Refrigerator | Inside the box | Coil on the back or bottom |
Worked example: an R-32 system on a hot day
Saturation pressures for R-32 (difluoromethane) from NIST's Chemistry WebBook 1. psig = psia − 14.7. The line temperatures are assumed readings. Superheat and subcooling formulas are from a Goodman R-32 installation manual 2.
| Point in the cycle | Pressure | Saturation temperature | Line temperature (assumed) | Result |
|---|---|---|---|---|
| Suction (evaporator outlet) | 135.65 psia ≈ 121 psig | 40°F | 50°F | Superheat 50 − 40 = 10°F |
| Liquid line (condenser outlet) | 389.58 psia ≈ 375 psig | 110°F | 100°F | Subcooling 110 − 100 = 10°F |
| Pressure ratio | 389.58 ÷ 135.65 | — | — | About 2.9 to 1 the compressor must create |
The coil boils refrigerant at 40°F, well below room air, so heat flows in. The condenser holds it at 110°F, well above a 95°F day, so heat flows out.
Raise outdoor temperature and the condensing pressure climbs; the compressor works harder, which is why efficiency falls on hot afternoons. Never use a chart for one refrigerant on another; each has its own pressure-temperature curve.
R-32 pressure-temperature reference (NIST)
| Saturation temperature | Pressure (psia) | Approx. gauge (psig) |
|---|---|---|
| 30°F | 113.79 | 99 |
| 40°F | 135.65 | 121 |
| 50°F | 160.54 | 146 |
| 90°F | 296.02 | 281 |
| 100°F | 340.37 | 326 |
| 110°F | 389.58 | 375 |
| 120°F | 444.04 | 429 |
Where you meet the cycle as a homeowner
- The large insulated copper line is the suction line (cool vapor). The small one is the liquid line (warm liquid).
- Water at the drain: moisture condensed on the cold evaporator.
- Warm air from the outdoor fan in summer: heat rejected by the condenser.
- Steam from a heat pump in winter: defrost melting frost off the outdoor coil.
- A tech's gauge readings and superheat or subcooling numbers on a service ticket.
Strengths and limits of vapor compression
What it does well
- Moves several units of heat per unit of electricity.
- Works in both directions in a heat pump.
- Dehumidifies as it cools.
- Scales from window units to large buildings.
What limits the cycle
The bigger the gap between the evaporating and condensing temperatures, the harder the compressor works. That is why AC output and efficiency fall on very hot days, and heat pump output falls on very cold ones.
Air side problems show up as refrigerant side symptoms. A dirty filter or weak blower lowers evaporator pressure and can freeze the coil. A dirty outdoor coil raises condensing pressure. A technician reads superheat and subcooling to tell the cases apart 2.
What the cycle means for cost
The cycle itself has no price, but its health drives running cost and repair bills. A wrong refrigerant charge, a dirty coil or low airflow all push pressures away from design and raise electricity use. Leaks cost twice: lost refrigerant and lost efficiency.
HyreHVAC does not print repair prices without enough local quotes; the repair cost calculator gives ranges, and refrigerant leak repair explains what a leak job involves.
How a technician checks the cycle
- Confirm airflow first: filter, blower speed, static pressure.
- Attach gauges or digital probes to the service ports (EPA 608 certified work) 3.
- Read suction and liquid pressures and convert to saturation temperatures with the refrigerant's own data 1.
- Measure suction and liquid line temperatures.
- Calculate superheat and subcooling and compare with the maker's targets 2.
- Diagnose: charge, metering device, airflow or coil condition.
Owner tasks that protect the cycle
- Change filters to keep evaporator airflow up.
- Keep the outdoor coil clear of leaves, grass and dryer lint.
- Keep the condensate drain clear.
- Book a yearly check of charge and coils.
- Never open refrigerant ports or add refrigerant yourself; it requires EPA 608 certification 3.
Signs the cycle is in trouble
- Ice on the suction line or indoor coil.
- Outdoor unit runs but the house does not cool.
- Hissing at the coil or lines, or oily spots at joints: possible leak.
- Very hot outdoor fan air with weak cooling inside: possible dirty condenser or overcharge.
- Turn the system off and call a certified technician.
Rules for the refrigerant in the cycle
Handling. EPA rules under Clean Air Act Section 608 require technicians who maintain, service, repair or dispose of equipment that could release refrigerants to be certified 3.
Which refrigerants. Under EPA's Technology Transitions rule, new residential and light commercial AC and heat pump systems use refrigerants with a GWP below 700, with a 1 January 2025 compliance date and no installation date for systems whose parts were all made or imported before then 4.
EPA's reference table lists GWPs of 2,088 for R-410A, 675 for HFC-32 and 465 for R-454B 5.
For the full regulatory picture, see refrigerant and our refrigerant transition research.
Refrigeration cycle vs related terms
| Term | Role in the cycle |
|---|---|
| Evaporation temperature | Boiling point on the low side |
| Condensation temperature | Condensing point on the high side |
| Superheat | Vapor warmed above boiling at the evaporator outlet |
| Subcooling | Liquid cooled below condensing at the condenser outlet |
| COP | Heat moved per unit of energy in |
Common misconceptions
- Myth An AC makes cold.
- Reality It removes heat; cold is the absence of the heat it moved outside.
- Myth Refrigerant gets used up.
- Reality It circulates in a sealed loop; a low charge means a leak.
- Myth Any pressure chart works.
- Reality Each refrigerant has its own pressure-temperature curve 1.
- Myth Higher pressure means more cooling.
- Reality Pressures must match design; too high often means a problem.
Climate and the cycle
Hot climates raise condensing pressure: on a 110°F afternoon in Arizona, the condenser must run hotter than in a 90°F Ohio summer, so the compressor works harder and efficiency falls.
That is one reason DOE adds an EER2 test at 95°F for split ACs installed in the Southwest. Cold climates push the other end, lowering evaporating temperature for heat pumps in winter.
When this matters to you
The refrigeration cycle matters most when a technician says your system is low on refrigerant: refrigerant circulates in a sealed loop and is not used up, so a low charge means a leak.
Understanding the cycle helps you read a service ticket and spot a bad diagnosis. A tech who adds refrigerant without checking airflow, superheat and subcooling, or without looking for a leak, is guessing.
Next step: if your system is low on refrigerant, ask for leak repair first; see who can legally handle refrigerant.
Questions about Refrigeration Cycle
What are the four stages of the refrigeration cycle?
Evaporation, compression, condensation and expansion. The refrigerant boils in the evaporator, is compressed, condenses in the condenser and drops in pressure through an expansion device before returning to the evaporator.
How does the refrigeration cycle cool a house?
Refrigerant in the indoor coil boils at a low temperature, taking heat from the air blown across it. The compressor carries that heat to the outdoor coil, where the refrigerant condenses and releases it outside.
Why does pressure matter in the refrigeration cycle?
A refrigerant's boiling and condensing temperature depends on pressure. Low pressure lets it boil cold indoors; high pressure lets it condense hot outdoors. For R-32, NIST data show about 121 psig at 40°F and 375 psig at 110°F.
Is a heat pump the same cycle?
Yes. A heat pump runs the same cycle and adds a reversing valve so the outdoor coil can act as the evaporator in winter, pulling heat from outdoor air and releasing it indoors.
What is superheat and subcooling?
Superheat is how far the vapor leaving the evaporator is above its boiling temperature. Subcooling is how far the liquid leaving the condenser is below its condensing temperature. Technicians use both to check charge and metering.
Does refrigerant need to be replaced?
No. It circulates in a sealed loop and is not consumed. If the charge is low, there is a leak, which should be found and repaired by an EPA 608 certified technician before recharging.
Why is my outdoor unit blowing hot air?
That is normal in cooling. The condenser fan blows outdoor air across the hot coil to carry away the heat removed from your house. Very hot air with poor indoor cooling can point to a problem.
What does the expansion valve do?
It drops the pressure of the liquid refrigerant before it enters the evaporator, which drops its temperature. A thermostatic or electronic valve also meters the flow to keep superheat in range.
Sources
- NIST Chemistry WebBook, Thermophysical Properties of Fluid Systems: difluoromethane (R-32) saturation properties, 30°F–120°F, retrieved .
- Goodman / ACiQ, A/GPHM3 13.4 SEER2 M-Series packaged heat pump with R-32, installation instructions (IOG-3035B, 2025), retrieved .
- U.S. EPA, Section 608 Technician Certification, retrieved .
- U.S. EPA, Technology Transitions HFC Restrictions by Sector, retrieved .
- U.S. EPA, Technology Transitions GWP Reference Table, retrieved .
Expert review
Written by the HyreHVAC Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: R-32 saturation pressures copied from the NIST Chemistry WebBook (psig computed as psia − 14.7); superheat and subcooling formulas from the Goodman R-32 manual; GWPs from EPA's reference table; certification and transition rules from EPA pages, all read 10 October 2026. Line temperatures in the example are assumed.
Suggest a correction. We fix errors and say what changed.