Superheat vs. Subcooling: What Each Tells You and When to Charge by Which

Superheat and subcooling are the two readings that tell you what's happening inside a running system. Each one alone can mislead you; together they point to the problem.

What superheat measures

Superheat = suction line temperature − evaporator saturation temperature

Once all the liquid refrigerant in the evaporator has boiled off, any more heat the vapor picks up is superheat. Superheat tells you how much of the evaporator is being used:

• High superheat: the evaporator is starved. The refrigerant boils off early and the end of the coil does little work. Common causes are an undercharge, a restriction or a metering device that's underfeeding.
• Low superheat: the evaporator is flooded. Liquid makes it almost to the end of the coil, and in the worst case liquid floods back to the compressor. Causes include an overcharge, low indoor airflow or a metering device that's overfeeding.

Measure suction line temperature with a clamp near the outdoor service valve, and read saturation temperature from suction pressure on the PT chart (most digital gauges do this for you).

What subcooling measures

Subcooling = condenser saturation temperature − liquid line temperature

After the vapor condenses fully to liquid, the condenser keeps cooling it below saturation. Subcooling tells you how much liquid is backed up in the condenser:

• High subcooling: extra liquid in the condenser, from an overcharge or a restriction downstream (like the filter drier or metering device).
• Low subcooling: little liquid in the condenser, from an undercharge, or when the metering device is passing more than the condenser can supply.

Which one to charge by

Metering device	Charge by	Target
TXV or EEV	Subcooling	Manufacturer's target (data plate or installation manual), often around 8–12 °F
Fixed orifice (piston, cap tube)	Superheat	From the target superheat chart for current indoor wet bulb and outdoor dry bulb

Why? A TXV holds superheat roughly constant on its own, so superheat won't tell you much about charge on a TXV system. On a fixed orifice, superheat moves with charge, so it's the right reading. Work out a fixed-orifice target with the target superheat calculator.

Airflow first. Both numbers assume correct indoor airflow. Low airflow lowers superheat and can make an undercharged system look right.

Reading them together

Superheat	Subcooling	Most likely
High	Low	Undercharge
Low	High	Overcharge
High	High	Restriction in the liquid line or metering device
Low	Low	Low indoor airflow or an overfeeding metering device. Also suspect an inefficient compressor if suction is high and head is low

Blends and glide

For refrigerants with temperature glide (like R-454B, R-407C or R-448A), use the dew point for superheat and the bubble point for subcooling. Most digital gauges handle this automatically once you select the right refrigerant. See how to read a PT chart.

FAQ

What is a normal superheat?

There's no single normal value. On a fixed-orifice system the target changes with indoor wet bulb and outdoor dry bulb, often somewhere between about 5 and 20 °F. On TXV systems superheat often sits around 8–12 °F, but you charge those by subcooling.

What is a normal subcooling?

Use the manufacturer's number. Many residential TXV systems specify around 8–12 °F, but some go higher or lower.

Can I check charge with the system running only a few minutes?

No. Let the system run 10–15 minutes so pressures settle, and confirm airflow before you judge the charge.

Sources

• HVAC School: how similar are A2Ls to R-410A
• AC Direct: R-454B vs. R-410A pressures