How to Check Superheat and Subcooling on an AC System
What the two numbers tell you
Superheat and subcooling tell you where the refrigerant is changing state inside the system. Refrigerant boils in the evaporator at a temperature set by the suction pressure. Once the last drop of liquid has boiled off, the vapor keeps picking up heat, and every degree above that boiling point is superheat. On the other side, hot vapor condenses in the condenser at a temperature set by the head pressure. Once it is all liquid, the liquid keeps cooling, and every degree below that condensing point is subcooling.
So superheat proves the compressor is getting vapor, not liquid. Subcooling proves the metering device is being fed a solid column of liquid with no bubbles. Both are the difference between a line temperature you measure and a saturation temperature you look up from the pressure, which is what the superheat and subcooling calculator does.
Which one you charge by
The metering device decides. A fixed orifice or piston cannot adjust its feed, so the charge sets the superheat. You charge those systems to a target superheat from the manufacturer's chart, which you enter with the indoor wet-bulb temperature at the return and the outdoor dry-bulb temperature at the condenser. The common field formula, (3 × indoor wet-bulb − 80 − outdoor dry-bulb) ÷ 2, gets you close when the chart is missing, but the chart on the unit wins.
A TXV or EEV holds superheat on its own by opening and closing, so superheat says little about the charge until the valve starves. On those systems you charge to subcooling, typically 8 to 14°F, with the exact target printed on the data plate or in the installation manual. Superheat on a TXV system is still worth reading. At the evaporator outlet it normally sits around 8 to 15°F because that is what the valve is set to hold, and at the outdoor unit it reads higher because the suction line picks up heat on the way. It is a diagnostic, not a charging target. A TXV system with good subcooling and bad superheat has a valve, airflow or restriction problem, not a charge problem.
Get the measurement right
Fix airflow first. A dirty filter, a matted blower wheel or an iced coil changes the load on the evaporator, and the readings move with it. Aim for about 400 CFM per ton (350 in humid climates, 450 in dry ones) and confirm it with static pressure and the blower table before you touch the charge. Let the system run 10 to 15 minutes so the pressures settle. Most manufacturers want outdoor air above roughly 55 to 65°F for a charging check; below that you weigh the charge in.
Then take four readings:
- Suction pressure at the suction service valve. Convert it to saturation temperature with the refrigerant P/T chart. For blends such as R-454B and R-407C use the dew point.
- Suction line temperature with a clamp probe about 6 in. from the service valve, on bare, clean copper, strapped tight and shaded from the sun. Superheat = line temperature − saturation temperature.
- Liquid pressure at the liquid service valve, converted to saturation temperature. For blends use the bubble point.
- Liquid line temperature at the same valve. Subcooling = saturation temperature − line temperature.
Get the direction right. Superheat is the line above saturation; subcooling is saturation above the line. A superheat at or below zero means liquid is reaching the probe or the reading is wrong, and a negative subcooling means vapor in the liquid line or a bad reading. Remember too that adding or recovering refrigerant requires EPA Section 608 certification.
What high and low readings mean
Read the two numbers together, with airflow already verified, and they point at a cause.
| Superheat | Subcooling | Points toward |
|---|---|---|
| High | Low | Undercharge, which usually means a leak |
| High | High | Liquid-line restriction: plugged filter-drier, kinked line, or a TXV underfeeding or stuck |
| Low | High | Overcharge on a fixed-orifice system. A TXV holds superheat, so on a TXV system high subcooling by itself usually means overcharge |
| Low | Low | TXV overfeeding from a loose or badly insulated bulb, or very low load on the evaporator |
Low superheat is the one to respect. It means liquid may be leaving the evaporator and heading for the compressor, where it washes out oil and breaks valves. High superheat runs the compressor hot. On a TXV system, low subcooling is normally the first sign of an undercharge; superheat climbs only once the valve runs out of liquid to meter.
Worked example with R-410A
A TXV split system on a 95°F day. The suction gauge reads 118 psig, which on the R-410A chart is a saturation temperature of about 40°F (39.8°F). The suction line clamp reads 52°F. Superheat = 52 − 40 = 12°F. The liquid gauge reads 340 psig, which saturates at about 105°F (104.6°F). The liquid line reads 95°F. Subcooling = 105 − 95 = 10°F, inside the 8 to 14°F range and right on a 10°F data plate target. The charge is correct, and the 12°F superheat at the outdoor unit says the valve is feeding the coil properly. Had the subcooling come back at 3°F with superheat at 25°F, you would be looking for a leak before adding gas.
If this were a piston system, the same 12°F would be compared against the chart instead. At 67°F indoor wet-bulb and 95°F outdoor, the field formula gives (201 − 80 − 95) ÷ 2 = 13°F target, so the charge is within a degree of right and you would leave it alone.
On the roof without a signal
The math is simple. The P/T lookups are where mistakes happen, especially on blends with glide. AirForge keeps the tables for R-410A, R-454B, R-32, R-22 and the rest in your pocket with the superheat and subcooling math built in, and it works with no signal. If you are studying for a certification, the HVAC practice exam has charging and diagnosis questions like these.
Step by step
- Verify airflowCheck the filter, coil and blower wheel, then confirm roughly 400 CFM per ton from static pressure and the blower table. Let the system run 10 to 15 minutes.
- Connect gauges and clamp the probesRead suction and liquid pressure at the service valves. Clamp temperature probes on clean, bare copper about 6 in. from each valve, strapped tight and shaded.
- Convert pressures to saturation temperaturesUse the P/T chart for your refrigerant. On blends, use the dew point for the suction side and the bubble point for the liquid side.
- Calculate superheatSuction line temperature minus saturated suction temperature.
- Calculate subcoolingSaturated condensing temperature minus liquid line temperature.
- Compare to the targetFixed orifice: the manufacturer's superheat chart from indoor wet-bulb and outdoor dry-bulb. TXV or EEV: the data plate subcooling, usually 8 to 14°F.
- Diagnose before you adjustRead the two numbers together. Add or recover refrigerant in small amounts, let it settle, and read again.
Frequently asked questions
Do I charge by superheat or subcooling?
Fixed-orifice and piston systems are charged by superheat using the manufacturer's chart with indoor wet-bulb and outdoor dry-bulb. TXV and EEV systems are charged by subcooling, usually 8 to 14°F, with the target on the data plate.
What is a normal subcooling?
Most residential TXV systems call for 8 to 14°F of subcooling at the liquid service valve. Use the number on the data plate or in the installation manual when there is one.
Can I check the charge in cool weather?
Most manufacturers want outdoor temperatures above roughly 55 to 65°F before you charge by superheat or subcooling. Below that, weigh the charge in by the data plate and line-set length.
Why is my superheat reading zero or negative?
Liquid refrigerant is reaching the probe, or a reading is wrong. Check the probe contact and the refrigerant chart, then look for a flooding evaporator: overcharge, low airflow or a TXV overfeeding.
Written by TapForge Studios, a one-person Android studio run by a tradesman with a background in electrical, HVAC and life-safety work. Reviewed October 7, 2026. This guide is general information, not engineering, legal or tax advice; the adopted code edition, the manufacturer's instructions and the authority having jurisdiction govern.
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