Guide · By DueTrack Studio · Updated 2026-10-29
Superheat vs. subcooling: what they mean and when to use each
Superheat is how far the suction vapor is above its saturation (boiling) temperature: suction-line temperature minus saturation temperature at suction pressure. Subcooling is how far the liquid is below its saturation (condensing) temperature: saturation temperature at liquid pressure minus liquid-line temperature. Fixed-orifice (piston) systems are usually charged by superheat; TXV systems by subcooling — always follow the manufacturer.
How do you calculate superheat?
- Read suction pressure at the service valve.
- Convert it to saturation temperature with a PT chart for that refrigerant.
- Measure suction-line temperature near the same point.
- Superheat = line temperature − saturation temperature.
How do you calculate subcooling?
- Read liquid-line pressure.
- Convert it to saturation (condensing) temperature.
- Measure liquid-line temperature.
- Subcooling = saturation temperature − line temperature.
Which one do you charge by?
Systems with a fixed metering device (piston) are typically charged to a target superheat that depends on indoor wet-bulb and outdoor temperature. Systems with a TXV regulate superheat themselves, so they are typically charged to the manufacturer's target subcooling. The equipment's nameplate and installation manual come first.
What do high or low readings suggest?
Readings point to possibilities — undercharge, overcharge, restriction, low airflow — rather than a single answer. Confirm with airflow, temperatures and the manufacturer's charging chart before adding or recovering refrigerant.
Questions
I always mix up superheat and subcooling. Is there a simple way to remember?
Superheat is on the suction (vapor) side, above boiling; subcooling is on the liquid side, below condensing.
What is a normal superheat?
It depends on the system and conditions. For fixed-orifice systems, use a target superheat chart; for TXV systems, use the manufacturer's subcooling target.