Superheat (Fixed Orifice)

How to calculate superheat

Superheat is how many degrees the refrigerant vapor leaving the evaporator sits above its boiling point at the pressure it is under. It is the difference between two readings you take at the suction service valve:

1. Connect gauges and read suction pressure at the service valve with the system running and settled (10–15 min).
2. Convert that pressure to saturation temperature using the refrigerant's P/T chart — use the dew point column for blends with glide.
3. Clamp a thermometer on a clean section of suction line within 6 in of the valve and insulate it from ambient air.
4. Subtract saturation temperature from line temperature. That difference is your measured superheat.

Worked example (R-410A)

• Suction pressure: 118 psig → saturation temperature ≈ 40 °F
• Suction line temperature: 52 °F
• Superheat = 52 − 40 = 12 °F

On a TXV system 12 °F is right in the normal band. On a fixed-orifice system, compare it against the target the calculator above returns for your indoor wet bulb and outdoor dry bulb — on a hot, dry day the target can drop to single digits.

Target superheat ranges

What high or low superheat means

High superheat — the coil is starved

Undercharge or a leak, a restriction (plugged filter drier, kinked liquid line), an underfeeding TXV or a plugged orifice. Check subcooling too: high superheat with low subcooling points at undercharge, while high superheat with high subcooling points at a restriction between the condenser and the coil.

Low superheat — the coil is flooding

Overcharge, an overfeeding metering device, or low evaporator airflow (dirty filter, dirty coil, slipping blower). Superheat near 0 °F means liquid is reaching the compressor — shut down before it washes the bearings.

Superheat vs subcooling

Superheat is a suction-side reading and tells you how well the evaporator is being fed. Subcooling is a liquid-side reading and tells you how much liquid is stacked in the condenser. Reading both is what separates a guess from a diagnosis: the pair together identifies undercharge, overcharge, restriction and airflow faults that either number alone cannot.

Common mistakes

• Using the bubble point instead of the dew point on a glide blend (R-407C, R-448A) — the reading can be off by several degrees.
• Measuring line temperature without insulating the sensor, so ambient air pulls the reading high.
• Reading before the system has run 10–15 minutes and settled.
• Charging a TXV system by superheat — a TXV holds superheat nearly constant, so the number barely moves as you add refrigerant.
• Confusing evaporator superheat with total superheat at the compressor; a long uninsulated suction line adds several degrees.

How to use this calculator

Find measured superheat from suction pressure and line temperature, and the target for a fixed-orifice system.

1. Select the refrigerant and enter suction pressure with the correct psig/psia basis.
2. Enter suction line temperature measured near the service valve.
3. Read measured superheat as line temperature minus saturation (dew point) temperature.
4. Enter indoor wet bulb and outdoor dry bulb to get the target superheat for a fixed-orifice system.

Frequently asked questions

How do you calculate superheat?

Superheat = suction line temperature − saturation (dew point) temperature for that refrigerant at the measured suction pressure. Example: R-410A at 118 psig saturates at about 40 °F; a 52 °F suction line gives 52 − 40 = 12 °F of superheat.

What should superheat be?

On a fixed-orifice system the target comes from the indoor wet bulb and outdoor dry bulb chart and can range from about 5 °F to 30 °F. A TXV system is normally 8–12 °F of evaporator superheat.

What does high superheat mean?

High superheat usually means the evaporator is starved: undercharge, a restriction, a plugged filter drier, or an underfeeding metering device.

What does low superheat mean?

Low superheat means the coil is flooding — overcharge, an overfeeding metering device, or low airflow across the evaporator. Near-zero superheat risks liquid floodback to the compressor.

What is the difference between superheat and subcooling?

Superheat is measured on the suction line and tells you how much refrigerant vapor is leaving the evaporator above its boiling point. Subcooling is measured on the liquid line and tells you how much liquid is stacked in the condenser. Charge fixed-orifice systems by superheat and TXV systems by subcooling.

Where do I measure suction line temperature?

On a clean section of the suction line within about 6 in of the service valve, with the sensor insulated from ambient air.

Can I calculate superheat without a pressure gauge?

No. Superheat needs saturation temperature, which only comes from suction pressure and the refrigerant's P/T relationship. A clamp thermometer alone cannot give it.