Home› Level 1› Unit 3› Section 4› 4.4 Superheat & Subcooling
Unit 3 — Refrigeration System Fundamentals & Maintenance
Section 4 — Vapour Compression Cycle

4.4 — Superheat & Subcooling

Two numbers — superheat and subcooling — tell a technician more about a system’s charge and metering device performance than any single gauge reading. Calculating them correctly and interpreting what they mean is a core field skill.

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4.4.1 — What Is Superheat?

Superheat is the temperature rise of a vapour above its saturation (boiling) temperature at a given pressure. Once all liquid has evaporated, continued heat absorption raises the vapour’s temperature above the saturation point — that excess is superheat.

Superheat = Tactual vapour − Tsaturation (dew point)

Both temperatures in the same unit (°F or °C). Tsat is read from a P–T chart using the measured suction pressure.

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Evaporator Superheat

Measured at the outlet of the evaporator coil (or at the suction service valve). Confirms that all liquid has evaporated before the refrigerant enters the suction line. Typical target: 8–12°F (4–7°C).

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Total (Compressor Inlet) Superheat

Measured at the compressor suction port. Includes heat picked up along the suction line after leaving the evaporator. Always higher than evaporator superheat. Typical target: 20–30°F (11–17°C) at the compressor.

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Discharge Superheat

Temperature of the discharge gas above the condensing saturation temperature. Normally 50–100°F (28–56°C) above condensing temperature. Excessively high discharge superheat indicates overheating of the compressor and often points to high compression ratio or restricted suction.

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Why superheat protects the compressor

A minimum superheat at the compressor suction ensures the refrigerant is 100% vapour. Liquid refrigerant cannot be compressed — it can bend valve reeds, crack pistons, and cause catastrophic compressor failure in a single stroke (liquid slugging).

4.4.2 — How to Calculate Evaporator Superheat

Superheat calculation requires two measurements: a pressure and a temperature. Both must be taken at the same location — the evaporator outlet or suction service valve.

  1. Connect the low-side manifold gauge to the suction service valve (or Schrader port).
  2. Read the suction pressure (psig) once the system is running and stable.
  3. Using the refrigerant’s P–T chart (or a manifold set with a built-in chart), find the saturation temperature that corresponds to that pressure. This is Tsat — the dew-point temperature.
  4. Using a calibrated clamp-on thermometer or thermocouple, measure the suction line surface temperature at the same point (within 6″ of the valve or evaporator outlet). This is Tactual.
  5. Calculate: Superheat = Tactual − Tsat
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Insulate your temperature probe

A bare clamp-on probe on a cold suction line will pick up radiant heat from the surroundings and read higher than actual. Wrap the probe and line with insulation tape for 2–3 minutes before reading for an accurate measurement.

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Worked Example — R-410A System

Suction pressure reads 120 psig. From the R-410A P–T chart, 120 psig corresponds to a saturation temperature of 40°F.

Suction line surface temperature reads 52°F.

Superheat = 52 − 40 = 12°F — within the normal 8–12°F target range for a TXV system.

4.4.3 — Superheat Target Ranges & Diagnosis

Target ranges vary by metering device type. Fixed-orifice/capillary tube systems use a manufacturer-supplied chart that factors in outdoor and indoor conditions. TXV and EEV systems maintain a narrower, more consistent superheat.

Metering Device Normal Evaporator SH Notes
Fixed orifice / capillary tube 10–20°F (5–11°C) Use manufacturer charging chart; SH varies with conditions
TXV (Thermostatic Expansion Valve) 8–12°F (4–7°C) Valve should maintain set-point; check bulb attachment if SH drifts
EEV (Electronic Expansion Valve) 6–10°F (3–6°C) Controlled by ECU; diagnose via manufacturer software/fault codes
High Superheat (> target)
  • Low refrigerant charge
  • Restricted metering device (TXV set too high, blocked orifice)
  • Blocked or dirty filter-drier
  • Restricted suction line or filter
  • Low load on the evaporator (low airflow, dirty coil)
Low Superheat (< target)
  • Overcharge of refrigerant
  • Metering device stuck open or set too low
  • TXV sensing bulb lost charge or improperly mounted
  • High load on evaporator (unusually high heat gain)
  • Risk of liquid floodback to compressor

4.4.4 — What Is Subcooling?

Subcooling is the temperature drop of a liquid below its saturation (condensing) temperature at a given pressure. Once the refrigerant has fully condensed in the condenser, further heat rejection lowers its temperature below the saturation point — that margin is subcooling.

Subcooling = Tsaturation (bubble point) − Tactual liquid line

Tsat is read from a P–T chart using the measured high-side (discharge/liquid line) pressure. Tactual is the liquid line temperature measured at the condenser outlet or liquid service valve.

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Why Subcooling Matters

Subcooled liquid arriving at the metering device is 100% liquid with no vapour bubbles. This ensures the metering device can meter refrigerant accurately and maximises the amount of liquid available for evaporation, improving efficiency and capacity.

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Zero or Negative Subcooling

Liquid that has not been adequately subcooled (or that has flashed to a two-phase mixture) contains vapour bubbles. These bubbles cause erratic metering device operation, reduced capacity, and a bubbly sight glass — even when the system is not undercharged.

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Subcooling & Flash Gas

Every extra degree of subcooling reduces the amount of flash gas formed when the liquid is throttled through the metering device. Less flash gas means more refrigerant enters the evaporator as usable liquid, increasing system capacity.

4.4.5 — How to Calculate Subcooling

Like superheat, subcooling requires a pressure reading and a temperature reading — both taken on the high-pressure (liquid) side of the system.

  1. Connect the high-side manifold gauge to the liquid line service valve (or high-side Schrader port).
  2. Read the liquid line pressure (psig) with the system running and stable.
  3. Using the refrigerant’s P–T chart, find the saturation temperature (bubble-point) corresponding to that pressure. This is Tsat.
  4. Using a calibrated clamp-on thermometer or thermocouple, measure the liquid line surface temperature at the condenser outlet or as close to it as possible. This is Tactual.
  5. Calculate: Subcooling = Tsat − Tactual
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Worked Example — R-410A System

Liquid line pressure reads 400 psig. From the R-410A P–T chart, 400 psig corresponds to a saturation temperature of 105°F.

Liquid line surface temperature reads 90°F.

Subcooling = 105 − 90 = 15°F — within the normal 10–20°F target range for a TXV system.

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Measure close to the condenser outlet

The liquid line gains heat as it runs toward the metering device. Measuring the liquid line temperature at the indoor unit or metering device inlet will give a higher subcooling reading than the actual condenser outlet value. For the most meaningful diagnostic data, measure as close to the condenser outlet as possible.

4.4.6 — Subcooling Target Ranges & Diagnosis

Subcooling is primarily used to diagnose charge level and condenser performance. Unlike superheat, which is controlled by the metering device, subcooling is controlled by the amount of refrigerant charge in the system and condenser efficiency.

System Type Normal Subcooling Notes
TXV or EEV system 10–20°F (5–11°C) Most common residential and commercial equipment; check nameplate
Fixed orifice / cap tube Varies; use charging chart Subcooling method less reliable; use manufacturer superheat chart
System with receiver 5–10°F (3–5°C) Receiver stores excess liquid; subcooling reflects condenser performance
High Subcooling (> target)
  • Overcharge of refrigerant
  • Liquid backed up in condenser (restricted liquid line)
  • Blocked receiver outlet valve
  • High discharge pressure combined with high subcooling = overcharge
Low Subcooling (< target)
  • Low refrigerant charge (most common cause)
  • Bubbles visible in sight glass
  • Dirty or undersized condenser coil
  • Insufficient condenser airflow
  • High ambient temperature reducing condenser effectiveness

4.4.7 — Using Superheat & Subcooling Together for Diagnosis

The real diagnostic power comes from reading both values at the same time. The combination of superheat and subcooling readings can distinguish between problems that produce similar gauge pressures but very different root causes.

Superheat Subcooling Most Likely Cause Action
Normal (8–12°F) Normal (10–20°F) System operating correctly No action needed; verify airflow and pressures
High (>15°F) Low (<8°F) Low refrigerant charge Check for leaks; weigh in refrigerant to spec after repair
Low (<5°F) High (>20°F) Overcharge of refrigerant Recover refrigerant to manufacturer specification
High (>15°F) Normal or high Restricted metering device or low evaporator load Check TXV bulb, replace filter-drier, verify airflow
Low (<5°F) Normal Metering device stuck open or TXV sensing bulb lost charge Replace TXV or sensing bulb; check for flooding at compressor
High (>15°F) Low (<5°F) + bubbles in sight glass Saturated filter-drier creating pressure drop Measure pressure drop across drier; replace if >2 psi drop
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Before Adjusting Charge — Verify These First

Superheat and subcooling readings are only valid when the system is operating under the conditions they were designed for. Before acting on your readings:

  • Confirm return air temperature and airflow are within design range
  • Verify the condenser is clean and has adequate airflow clearance
  • Check that all supply and return registers are open and unobstructed
  • Allow the system to run for at least 15 minutes to stabilise before reading
  • Confirm which refrigerant is in the system before consulting P–T charts
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Never add refrigerant to a system that has a leak

Adding refrigerant to a leaking system is a temporary fix that only moves the problem forward. All refrigerant additions must follow leak detection, repair, and pressure test procedures as required by provincial and federal regulations. Venting refrigerant is illegal.

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