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Refrigeration Cycle Theory for Field Techs

Reference Equipment Updated Aug 24, 2026 · HVAC

Why this matters

Every diagnostic decision in HVAC comes back to the refrigeration cycle. Wrong understanding = wrong diagnosis = wrong part = callback. The cycle isn't complicated, but most techs learn it once + then "feel" rather than "know" it. This reference is the working framework - pressure-temperature relationship, P-H diagram, the four key components, what each one tells you when readings are off.

The four components (the cycle)

Every vapor-compression refrigeration system has:

  1. Compressor - raises pressure + temperature of refrigerant vapor
  2. Condenser - rejects heat to outdoor air; refrigerant condenses (vapor → liquid)
  3. Metering device - drops pressure (expansion valve OR cap tube); refrigerant flashes partially
  4. Evaporator - absorbs heat from indoor air; refrigerant boils (liquid → vapor)

Then back to compressor. Closed loop. Same refrigerant cycles forever (no consumption unless leaking).

Pressure-temperature relationship (the foundation)

For each refrigerant, saturation temperature changes with pressure. At any given pressure, the refrigerant boils OR condenses at one specific temperature.

R-410A example (gauge pressure, dew-point saturation; R-410A glide is under 0.5 F so bubble and dew are interchangeable in the field):

Pressure (psig) Saturation temp (°F)
50 1
100 31
150 53
200 70
250 84
300 96
350 107
400 117

R-32 (current) (gauge pressure, saturation temperature):

Pressure (psig) Saturation temp (°F)
50 1
100 30
150 52
200 68
250 82
300 94
350 105

The relationship is fixed. Knowing pressure tells you the saturation temperature.

Use in diagnostics: measure low-side pressure → look up the saturation temperature → compare to measured suction line temp → calculate superheat.

Superheat (the suction-side measurement)

Superheat = actual suction line temperature - saturation temperature at suction pressure.

Why it matters: superheat tells you whether the evaporator is properly fed.

  • Superheat HIGH (15+ °F): evaporator starved (low refrigerant OR restriction in metering device OR clogged filter-drier)
  • Superheat LOW (under 5 °F): too much refrigerant fed (TXV not closing properly, OR overcharge); liquid floods back to compressor - dangerous
  • Superheat NORMAL (8 - 12 °F typical residential AC): system properly charged + metering

For TXV systems: target superheat 8 - 12 °F at design conditions.

For cap-tube / fixed-orifice systems: superheat varies with load + ambient. Use charging chart instead.

Subcooling (the liquid-side measurement)

Subcooling = saturation temperature at high-side pressure - actual liquid line temperature.

Why it matters: subcooling tells you about the condenser + the refrigerant charge.

  • Subcooling LOW (under 5 °F): undercharged OR condenser not rejecting heat
  • Subcooling HIGH (15+ °F): overcharged OR liquid line restriction OR condenser fan failing
  • Subcooling NORMAL (10 - 15 °F at design conditions): system properly charged

For TXV systems: subcooling is the primary charging criterion.

Pressure-enthalpy (P-H) diagram

Two axes: pressure runs up the page, enthalpy (heat content per pound) runs to the right. Get the axes straight and the cycle draws itself, because heat added moves you right and heat rejected moves you left.

  P (pressure)
  ^
  |   3 <-------------- condense --------------- 2
  |   |    (high side, top of the diagram)       ^
  | metering                                 compression
  |   |                                          |
  |   4 --------------- evaporate -------------> 1
  |        (low side, bottom of the diagram)
  +--------------------------------------------------> H (enthalpy)

The cycle, in order:

  1. Bottom right (point 1): low-pressure saturated-to-slightly-superheated vapor leaving the evaporator and entering the compressor. This is the highest-enthalpy point on the low side, which is why it sits at the right end of the bottom leg.
  2. 1 to 2, bottom right up to top right: compression. Pressure up, temperature up, and enthalpy up as well, because the compressor is doing work on the refrigerant. Point 2 is the far right of the diagram.
  3. 2 to 3, across the top right to left: condensation. Heat is rejected to outdoor air, so enthalpy falls. The refrigerant desuperheats, condenses, and then subcools. Point 3 is high-pressure liquid at the top left.
  4. 3 to 4, straight down the left side: expansion through the metering device. Pressure drops, some liquid flashes to vapor, and the line is vertical because no heat is added or removed. Same enthalpy, lower pressure.
  5. 4 to 1, along the bottom left to right: evaporation. Heat is absorbed from indoor air, enthalpy climbs, and you land back at point 1.

The horizontal width of the bottom leg is the refrigerating effect per pound. The horizontal width of the compression leg is the work you paid for. Each component does ONE thing. Failure in any one breaks the cycle.

Diagnostic by symptom

Both high-side + low-side HIGH:

  • Overcharge (too much refrigerant)
  • Condenser problem (dirty coils, fan failure)
  • Recovery system needed; check + correct

Both high-side + low-side LOW:

  • Undercharge (refrigerant loss; find + repair leak)
  • Compressor weak (less common)

High-side HIGH + low-side LOW:

  • Restriction (filter-drier clog, kinked line, TXV stuck closed)
  • Diagnostic: pressure-test through system

High-side LOW + low-side HIGH:

  • Compressor weakness (worn rings, valves leaking)
  • Reversing valve internal leak (heat pumps)

Low-side high but compressor cycles on overload:

  • Possibly overcharge + compressor overheating
  • Possibly contactor problem
  • Check refrigerant + amperage simultaneously

Refrigerants in service today

R-410A (phase-out): standard for residential AC + heat pump 2010 - 2025. Higher pressure than R-22. EPA SNAP-listed; still in service but new equipment manufacture stopped.

R-32: A2L mildly flammable. Becoming dominant for residential 2025+. Lower GWP than R-410A. Requires A2L-rated tools + leak detection.

R-454B: A2L blend; competing with R-32. Similar properties + handling.

R-22: production and import banned in the US as of January 1, 2020. Still in service in legacy units, supplied only from reclaimed and stockpiled stock, which is why per-pound cost on the retrofit market runs far above anything current.

R-134a: residential refrigerators + some commercial. Different system + lower pressures.

Critical diagnostic skill: see vs feel

Many techs "feel" the system + guess. Pros measure:

  • Always pull gauges (don't skip; even on PM)
  • Always measure superheat OR subcooling (one of them, depending on metering type)
  • Always check compressor amperage
  • Always check thermostat differential vs return air

Document all readings. Pattern over time reveals system health.

The single most-impactful HVAC diagnostic discipline is TAKING THE CHARGE READING ON EVERY VISIT - even PM visits. On a TXV system that is subcooling, and outside the 10 - 15 °F range is the FIRST sign of trouble. On a fixed-orifice system it is superheat against the manufacturer's charging chart for that day's outdoor dry bulb and indoor wet bulb. Catch the drift on PM before it becomes a no-cool emergency 6 months later. Document the reading; track trends. Tech who measures + tracks identifies failures BEFORE customers do.

References

  • ASHRAE Handbook, Refrigeration volume, and ASHRAE Handbook, Fundamentals volume (refrigerant thermophysical property tables and P-H charts)
  • AHRI Standards
  • "Refrigeration + Air Conditioning Technology" by Whitman, Johnson, Tomczyk
  • Manufacturer service manuals
  • Manuall internal: Refrigerant Transition A2L, Annual HVAC System Maintenance, AC Won't Cool - Diagnostic Tree

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