How superheat is calculated
Superheat = suction line temperature - saturation temperature at suction pressure
ASHRAE defines superheat as the difference between a vapor temperature and the dew-point temperature at the same pressure. A single compound such as R-22 or R-32 has one saturation temperature per pressure. For a blend such as R-454B or R-407C, the calculator uses the dew point, which ASHRAE calls the saturated vapor temperature. Lennox's R-454B instructions say the same: saturated vapor temperature is used to calculate suction superheat. Using the bubble column instead would overstate superheat by the glide; see bubble point, dew point and glide.
Where to measure
Take pressure and temperature at the same place: the suction line at the outdoor unit. Carrier's installation instructions for piston systems say to read suction pressure at the suction valve service port and suction temperature on the suction line at the service valve. California's Title 24 charge test puts its pipe sensor on the suction line near the low-side service valve. The DOE Building America diagnostics guide adds that on a long line set running through a hot attic, you may want to measure suction line temperature at the indoor coil instead.
- Run the system in cooling for at least 15 minutes before reading. Carrier and the Title 24 procedure both use 15 minutes; Trane asks for 20 minutes between charge adjustments.
- Clean the line where the sensor goes so it makes good thermal contact (DOE guide). Daikin also says to insulate the suction line thermometer for the best reading.
- Look up saturation for the refrigerant on the outdoor unit's rating plate, which Payne says to check before charging.
- On a fixed-orifice system, also measure the wet bulb of the air entering the indoor coil and the dry bulb of the air entering the condenser, with the sensor shaded from direct sun. Carrier, Title 24 and the DOE guide all read the target superheat from those two.
More detail on probes and readings that mislead: how to measure superheat and subcooling.
What superheat tells you
Superheat shows whether the refrigerant finished boiling before it left the evaporator: ASHRAE describes a dry expansion evaporator as one the refrigerant leaves fully vaporized, usually with superheat. The DOE guide warns that in an overcharged system liquid refrigerant may not completely evaporate and can slug the compressor, and Carrier says not to charge by superheat where its chart shows no target, because refrigerant slugging may occur. With too little charge, the DOE guide says, low suction pressure and saturation temperature can ice the evaporator.
The guide reads superheat together with subcooling: low superheat with high subcooling typically indicates an overcharge, high superheat with low subcooling an undercharge, and high with high or low with low points to other problems. Its diagnostic procedure looks for liquid line restrictions, defective expansion devices and non-condensable gases. Conditions move superheat too: lower outdoor temperatures raise superheat on a fixed-orifice system, and the guide says airflow below 350 cfm per ton makes standard charge tests invalid, so it measures airflow first.
Fixed orifice vs TXV and EEV systems
Fixed metering devices (piston or capillary tube). The device cannot adjust, so superheat moves with the charge and with indoor and outdoor conditions. That makes superheat the charging measurement. Carrier's instructions say units with indoor pistons "require charging by the superheat method", compared against the target superheat on the manufacturer's chart for the measured indoor wet bulb and outdoor dry bulb. If superheat is above target, add refrigerant; if below, remove it. Carrier allows a tolerance of plus or minus 3°F; Title 24 gives installers plus or minus 5°F of the target.
TXV and EEV systems. ASHRAE describes a thermostatic expansion valve as a device for controlling superheat by regulating refrigerant flow, so superheat says little about charge. Carrier, Trane and Lennox charge these systems by subcooling to the target on the rating plate or charging label. Superheat is then a check that the valve is working; California's Title 24 procedure accepts the manufacturer's range or 4 to 25°F on TXV and EXV systems. Use the subcooling calculator for these.
Worked example
An R-410A split system with a piston metering device. Suppose the manufacturer's superheat charging chart gives a target of 12°F for the indoor wet bulb and outdoor dry bulb measured that day.
- The suction gauge at the outdoor unit reads 118.4 psig.
- On the R-410A chart, that pressure is a dew point of 40°F.
- The clamp on the suction line at the service valve reads 52°F.
- Superheat = 52 - 40 = 12°F, on target.
Had the line read 60°F, superheat would be 20°F, well above target, which on a piston system points to adding refrigerant once airflow is confirmed.
On a blend, use the dew point. An R-454B system with a suction pressure of 117.8 psig has a 45°F dew point. Its bubble point at that pressure is 2.4°F lower, but superheat is measured from the dew point, so a 55°F suction line has 10°F of superheat.