Superheat and Subcooling Explained

Superheat — the evaporator's report card

Inside the evaporator, refrigerant boils at its saturation temperature. Any heat the vapor picks up after it has fully boiled off is superheat. You want some — it proves every drop boiled before heading to the compressor. Too little and liquid can slug the compressor; too much and the coil is starving and capacity falls off.

How to measure:

1. Read the suction pressure at the service valve.
2. Convert it to a saturation temperature with a PT chart — for blends like R-410A and R-454B, use the dew point column.
3. Clamp a thermometer on the suction line near the service valve (clean copper, insulated clamp).
4. Subtract: line temp minus saturation temp = superheat.

Worked example (R-410A): suction pressure 130 psig. The PT chart says R-410A at 130 psig saturates at about 45°F (at 45°F the chart reads 130.6/130.1 psig bubble/dew). Your clamp reads 58°F on the suction line. 58 − 45 = 13°F superheat — a healthy number for a fixed-orifice system on a mild day.

Subcooling — the condenser's report card

In the condenser, refrigerant condenses at its saturation temperature. Any cooling the liquid picks up after it has fully condensed is subcooling. It proves you have a solid column of liquid feeding the metering device. On TXV systems — where the valve holds superheat steady on its own — subcooling is the charge check: compare it to the nameplate target (often around 10°F, but the plate wins).

How to measure:

1. Read the liquid (discharge-side) pressure.
2. Convert to saturation temperature — for blends, use the bubble point column this time.
3. Clamp a thermometer on the liquid line near the condenser outlet.
4. Subtract: saturation temp minus line temp = subcooling.

What the readings mean

Reading	Likely story
High superheat	Evaporator starved — low charge, a restriction (filter drier, kink, clogged screen), or a metering device underfeeding.
Low superheat	Evaporator flooded — overcharge or overfeeding metering device. Floodback risk: liquid reaching the compressor kills bearings and dilutes oil.
High subcooling	Too much liquid stacked in the condenser — classic overcharge on a TXV system (check for non-condensables too).
Low subcooling	Not enough liquid stacked — undercharge on a TXV system; the valve can't hold superheat once it runs out of liquid to meter.
Both high	Think restriction plus overcharge, or a charge added to mask a restriction. Don't just add gas.
Both low	Classic undercharge — find the leak before you top up.

Read them together, never alone — and always against the manufacturer's target for that unit. Superheat targets move with outdoor and indoor conditions on fixed-orifice systems; the charging chart on the unit is the authority.