On a cooling call, superheat answers one question: is the evaporator coil getting the right amount of liquid refrigerant? Liquid boils off as it moves through the coil. Once it has all turned to vapor, any extra heat it picks up raises its temperature above the boiling point. That extra rise, in degrees Fahrenheit, is the superheat.
Superheat in plain English
Think of a pot of water at a full boil. It stays at 212°F until the last of the water is gone. Only then can the steam get hotter. Refrigerant in an evaporator works the same way, except it boils at around 40°F because of the low pressure on the suction side.
If the coil is getting too little refrigerant, it all boils off early and the vapor keeps warming for the rest of the coil: superheat goes up. If the coil is getting too much, some liquid never boils and can head back toward the compressor: superheat goes down toward zero.
DOE's Building America measure guideline defines it as the difference between the suction line refrigerant temperature and the vapor saturation temperature at the measured pressure1.
How do you calculate superheat?
You need two readings taken at the same spot on the suction line, near the outdoor unit's service valve: the pressure and the pipe temperature.
- 1
Run the system 15 minutes
ENERGY STAR's commissioning checklist and DOE's guide both call for 15 minutes of run time before readings, so pressures and temperatures settle.
- 2
Read suction pressure
Connect the low-side gauge to the suction service port.
- 3
Convert pressure to saturation temperature
Use the pressure-temperature (PT) chart for the refrigerant on the nameplate. Digital gauges do this for you once you select the refrigerant.
- 4
Clamp a thermometer on the suction line
Clean the pipe and clamp the probe near the service valve, then insulate it so ambient air doesn't skew it.
- 5
Subtract
Suction line temperature minus saturation temperature is the actual superheat.
Formula
Worked example (example numbers)
An R-32 heat pump in cooling mode. NIST's saturation data puts R-32 at about 40°F when the gauge reads about 121 psig (roughly 135.8 psia at sea level)2.
| Reading | Value |
|---|---|
| Suction pressure | 121 psig |
| Saturation temperature from PT data | 40°F |
| Suction line temperature | 52°F |
| Superheat | 52 − 40 = 12°F |
Whether 12°F is right depends on the target, covered next.
Common mistakes in the math
- Using the wrong PT chart. R-410A, R-32 and R-454B all run at different pressures. Check the nameplate first.
- Using bubble point on a blend. Blends like R-454B have a small glide (Chemours lists about 2°F for its R-454B3), so their PT charts show a bubble and a dew temperature. Superheat is measured against vapor saturation, which is the dew side. ACCA's advice is to follow the manufacturer's guidance for each refrigerant4.
- Reading the line at the coil and the pressure at the condenser. Take both at the same place.
- Skipping the airflow check. Low airflow changes superheat, so a correct-looking number can hide a dirty coil.
What should superheat be?
It depends on the metering device.
Fixed orifice (piston). The target changes with conditions. You read it from the manufacturer's superheat chart using indoor return-air wet bulb and outdoor dry bulb. DOE's guide says the measured value should be within ±5°F of that target: remove refrigerant if superheat is more than 5°F below target and add if it is more than 5°F above1. If the chart gives a target under 5°F, conditions are too mild to verify charge this way. ENERGY STAR's commissioning checklist uses the same ±5°F pass line for fixed-orifice systems5.
TXV or EEV. The valve's job is to hold superheat steady, so you charge by subcooling instead and use superheat to confirm the valve is working. DOE's guide says that without a manufacturer range, 4°F to 25°F may be acceptable1.
There's no single "normal" superheat that applies to every system, so be wary of anyone quoting one number.
What causes high or low superheat?
| Reading | Likely causes | Check before touching the charge |
|---|---|---|
| High superheat | Undercharge, restriction (filter drier, kinked line, TXV stuck closed), high indoor heat load | Subcooling (low on undercharge, high on a restriction), filter drier temperature drop |
| Low superheat | Overcharge on a piston system, low indoor airflow, dirty evaporator, TXV stuck open or loose bulb | Filter, blower speed, coil, static pressure, TXV bulb mounting |
| Normal superheat, poor cooling | Coil circuits fouled on a TXV system, so the valve throttles back | Coil pressure drop; DOE's guide says a clean, properly sized evaporator should drop no more than 0.25 in. w.c.1 |
DOE's guide stresses measuring both superheat and subcooling, because faults other than charge can make either one look like a charge problem on its own1.
Superheat vs subcooling
| Superheat | Subcooling | |
|---|---|---|
| What it measures | Vapor temperature above saturation | Liquid temperature below saturation |
| Where | Suction line (low side) | Liquid line (high side) |
| Formula | Line temp − saturation temp | Saturation temp − line temp |
| Main charging method for | Fixed orifice (piston) | TXV or electronic expansion valve |
| ENERGY STAR tolerance | ±5°F of manufacturer target | ±3°F of manufacturer target |
| High reading usually means | Starved evaporator: undercharge or restriction | Overcharge or restriction |
Source: ENERGY STAR National HVAC Commissioning Checklist, Rev. 14
For the liquid side of the story, see subcooling.
When can you trust a superheat reading?
DOE's guide also says airflow should be at least 300 CFM per ton before charge diagnostics mean anything1. Check the filter, the blower and the static pressure first.
How superheat shows up in TradesWork
The TradesWork technician app includes superheat, subcooling and fixed-orifice target superheat calculators that work on the phone with no signal. PT data covers R-410A, R-22, R-32 and R-454B (R-454B is flagged as approximate), and superheat, subcooling and temperature split readings can be saved to the equipment record. The next tech can then compare today's reading with last season's, which is how slow leaks get caught. The manufacturer's charging chart always wins over any calculator.
Footnotes
-
D. Springer and B. Dakin, Measure Guideline: Air Conditioner Diagnostics, Maintenance, and Replacement, U.S. DOE Building America, DOE/GO-102013-3766, March 2013, pp. 15–31. https://www1.eere.energy.gov/buildings/publications/pdfs/building_america/measure_guide_air_cond_diagnostics.pdf ↩ ↩2 ↩3 ↩4 ↩5 ↩6 ↩7
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NIST Chemistry WebBook, Thermophysical Properties of Fluid Systems, difluoromethane (R-32) saturation properties, retrieved Oct. 15, 2026. Gauge pressure assumes 14.7 psi atmospheric. https://webbook.nist.gov/chemistry/fluid/ ↩
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Chemours, Opteon XL41 (R-454B) product information (manufacturer data). https://www.opteon.com/en/-/media/files/opteon/opteon-xl41-product-information.pdf?la=en ↩
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ACCA HVAC Blog, "Hey Ed, what is refrigerant glide?" https://hvac-blog.acca.org/hey-ed-what-is-refrigerant-glide ↩
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ENERGY STAR Single-Family New Homes, National HVAC Commissioning Checklist, Version 3.1 / 3.2 / 3.3 (Rev. 14), Section 2, revised Jan. 15, 2025. https://www.energystar.gov/sites/default/files/2025-01/National%20HVAC%20Commissioning%20Checklist_Rev%2014.pdf ↩ ↩2
