Superheat is how many degrees the refrigerant vapor leaving the evaporator is above its boiling point, and subcooling is how many degrees the liquid leaving the condenser is below its condensing point. Technicians use them to check refrigerant charge and the expansion valve. Neither can be judged by feel.
You may hear a technician say the superheat is high or the subcooling is low. These are diagnostic numbers, not settings you manage, but understanding them helps you follow the conversation.
Superheat
Refrigerant boils in the evaporator at a temperature set by its pressure. After it has all turned to vapor, it keeps warming. The number of degrees it has warmed above the boiling point is the superheat. It is measured on the suction line near the evaporator outlet, using a thermometer and the pressure.
- Too high: the coil is starved. Causes include low charge, a restriction or a valve that is not feeding enough.
- Too low: the coil is flooded. Liquid may return to the compressor. Causes include overcharge or a valve that is overfeeding.
Subcooling
On the high side, refrigerant condenses at a temperature set by its pressure. If it cools further before reaching the expansion valve, it is subcooled. The number of degrees below the condensing point is the subcooling. It is measured on the liquid line.
- Too low: possibly low charge, a poor condenser or flashing in the line.
- Too high: possibly overcharge or a restriction.
How they work together
| Reading | Possible meaning |
|---|---|
| High superheat, low subcooling | Low charge |
| High superheat, normal or high subcooling | Restriction at the valve or drier |
| Low superheat, high subcooling | Overcharge |
| Low superheat, normal subcooling | Valve overfeeding or a poor bulb contact |
| Both normal, box warm | Airflow, coil or load problem |
Why you should not guess
Adding refrigerant because the sight glass bubbled is a common mistake. Bubbles can come from a restriction, cold weather or a leak. Measuring superheat and subcooling tells the technician which. Charging by feel or by pressure alone risks overcharging, which damages the compressor. See topping off a leaking system and recharge by weight.
What you can do
Ask the technician to write down the readings on the invoice, together with the outdoor temperature. Comparing readings across visits shows slow changes. Keep them with your records. For the parts involved, read the TXV and the sight glass, and for the whole system see the refrigeration cycle.
Tools technicians use
Superheat and subcooling need accurate pressure gauges or digital manifolds, clamp thermometers and a pressure-temperature chart for the refrigerant. Measurements must be taken with the system stable, not right after start-up or defrost. A good technician waits, checks the readings and repeats them. If a technician adjusts the charge quickly on a system that has just started, ask them to recheck once it has settled.
Questions about this topic
Do I need to measure these myself?
No. A technician does, with gauges and thermometers.
What are normal values?
They depend on the system and refrigerant. The manufacturer or technician gives targets.
Why do they matter?
They show whether the charge and metering device are working correctly.