Superheat Calculator — Free Online Calculator
Superheat tells you how much the suction gas has warmed above its saturation temperature — the primary charge check on fixed-orifice systems and a useful cross-check on TXV systems. Enter the saturation temperature at suction pressure and the suction line temperature, and this tool gives your measured superheat, whether it's in the healthy band, and the most likely causes when it isn't.
Enter suction-side temperatures
From suction gauge converted on a PT chart
Pipe clamp thermometer on the suction line
Measured superheat
Reading
Healthy
Ideal target
8–12°F
Likely causes
Superheat in the TXV charging band — the evaporator is fully fed without flooding the suction line.
See the breakdown
Fixed-orifice systems need the target superheat chart for your wet-bulb and outdoor temps — this calculator uses a general band. Always follow the manufacturer charging method.
What your reading means (TXV systems)
Measured superheat at the suction line. Fixed-orifice targets are broader — use the target superheat chart for piston meters.
| Superheat | Reading | Most likely cause |
|---|---|---|
| Under 4°F | Very low | Overcharge, TXV stuck open, or non-condensables |
| 4–8°F | Low | Overcharge, restricted airflow, or TXV feeding too much |
| 8–12°F | Healthy | Proper TXV charge and airflow |
| 12–18°F | High | Undercharge, restricted metering, or low indoor airflow |
| Over 18°F | Very high | Severe undercharge — compressor overheating risk |
Sources & standards: HVAC School — Superheat charging fundamentals · EPA Section 608 — refrigerant handling · Manufacturer charging procedures (TXV vs fixed orifice).
The formula, explained in plain English
Superheat is how many degrees the suction gas has warmed past the boiling point at that pressure. It confirms the evaporator is fed without sending liquid to the compressor.
What superheat tells you
Whether the evaporator is receiving the right amount of refrigerant — enough to boil fully, not so much that liquid reaches the compressor.
Where to measure
Suction line a few inches from the service valve with a pipe clamp thermometer. Saturation temp comes from the suction gauge on a PT chart for your refrigerant.
TXV vs fixed orifice
TXV systems hold a tighter 8–12°F band because the valve adjusts flow. Piston meters need the target superheat chart for your indoor wet-bulb and outdoor dry-bulb.
Pair with subcooling
On TXV systems, subcooling is the primary charge method — superheat confirms the evaporator side. On fixed orifice, superheat is primary and subcooling is the cross-check.
Worked examples
Three readings showing healthy TXV superheat, a low-superheat overcharge, and high superheat from undercharge.
Healthy TXV — 45°F sat, 55°F suction line
Result: 10°F sits in the 8–12°F TXV band — charge and airflow are well matched.
Overcharge — 42°F sat, 44°F suction line
Result: nearly no superheat — liquid slugging risk. Recover excess refrigerant before returning to service.
Undercharge — 38°F sat, 58°F suction line
Result: the evaporator is starved — check for leaks and charge to manufacturer spec after confirming airflow.
Frequently asked questions
Common questions about measuring and interpreting HVAC superheat.
What should superheat be on my AC?
For a TXV system, target 8–12°F of superheat at the suction line. For a fixed-orifice (piston) meter, the band is broader — typically 5–15°F — and should be matched to the manufacturer target superheat chart for your indoor wet-bulb and outdoor dry-bulb temperatures.
How do I calculate superheat?
Clamp a pipe thermometer on the suction line a few inches from the service valve, read the suction pressure and convert it to saturation temperature on a PT chart, then subtract: superheat = suction line temp − saturation temp.
What does low superheat mean?
Low superheat means the suction line is too close to saturation — often from overcharge, a TXV stuck open, or excessive airflow. Very low readings risk liquid slugging the compressor.
What does high superheat mean?
High superheat means the suction gas is too hot above saturation — usually undercharge, a restricted metering device, or low indoor airflow starving the evaporator. Prolonged high superheat overheats the compressor.
Do I use superheat or subcooling to charge?
Use superheat for fixed-orifice systems and subcooling for TXV systems as the primary charging method. Many techs still check both as a cross-check once the system is stable.
When should I take a superheat reading?
Take readings after the system has run in cooling for 10–15 minutes with stable indoor and outdoor conditions. Avoid charging during startup, defrost, or when the filter is dirty — airflow skews the result.
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