Superheat calculator

What is superheat and why does it matter for AC charging?

Superheat is the number of degrees Fahrenheit a refrigerant vapor is above its saturation temperature at the same pressure. In a working air conditioner or heat pump, refrigerant enters the evaporator as a saturated liquid and vapor mix, boils off as it absorbs heat from the air, and leaves the evaporator as pure vapor. The amount of additional heat absorbed after full vaporization is the superheat. It is measured at the suction line near the outdoor unit service valve and tells you exactly how the refrigerant charge and airflow are interacting inside the evaporator.

Superheat is the primary diagnostic number for charging fixed-orifice systems (piston or capillary tube metering devices) and a verification number for TXV systems. Too low and liquid refrigerant is reaching the compressor (slugging risk). Too high and the evaporator is starving for refrigerant, the system is undercharged, or there is a restriction. Within ±3°F of target means the charge is correct under current conditions.

How to calculate target superheat for a fixed orifice system

The industry-standard target superheat formula for fixed orifice systems is:

Target SH = [(Indoor Wet Bulb × 3) − Outdoor Dry Bulb − 80] ÷ 2

Plug your real conditions in. On a standard 95°F outdoor / 67°F IDWB day, the math works out to: (67 × 3) − 95 − 80 = 26, divided by 2 = 13°F target superheat. On a humid 95°F day with 72°F IDWB: (72 × 3) − 95 − 80 = 41, divided by 2 = 20.5°F target. The formula scales with humidity because wet bulb captures both temperature and moisture load on the evaporator, which is what actually determines how much heat needs to be absorbed.

The formula is valid for indoor wet bulb 55 to 80°F and outdoor dry bulb 60 to 120°F. Outside that range, manufacturer charging tables override the formula. Goodman, Rheem, Carrier, and Trane publish target superheat tables in their installation manuals; this calculator's output matches those tables within 1°F across the valid range.

The actual superheat measurement procedure

Actual superheat is suction line temperature minus saturation temperature converted from suction pressure. Step by step:

• Run the system for 10 to 15 minutes with steady indoor and outdoor conditions before measuring. Charge changes need time to stabilize.
• Connect the manifold low-side gauge to the vapor line service port at the outdoor unit. Read pressure in PSIG.
• Convert pressure to saturation temperature using the PT chart for your refrigerant. For R-454B and other glide refrigerants, use the dew (vapor) saturation, not the bubble.
• Clamp a thermocouple on bare copper at the same vapor line location, downstream of the suction service port. Insulate the sensor so it reads pipe temperature, not ambient.
• Wait for the thermocouple to stabilize (1 to 2 minutes), then read.
• Subtract saturation temperature from line temperature. That is actual superheat.

Example with R-410A: suction pressure reads 130 PSIG, which is 47°F saturation. Suction line temperature reads 60°F. Actual superheat = 60 − 47 = 13°F. If target on this same system is 13°F, the charge is correct. If actual is 18°F and target is 13°F, the system is undercharged by roughly 4 oz of refrigerant.

How to read the diagnosis: undercharged, overcharged, or correct

Compare actual superheat to target. Three outcomes:

• Within ±3°F of target: charge is correct. Verify with subcooling between 5 and 25°F to confirm the orifice is sized right.
• Actual is more than 3°F above target (high superheat): the evaporator is starving for refrigerant. Most likely the system is undercharged. Add refrigerant in 4 to 8 oz increments and re-measure after stabilization. Also rule out a refrigerant leak (oil traces near fittings), restricted filter dryer, or low outdoor airflow before adding charge.
• Actual is more than 3°F below target (low superheat): too much refrigerant is in the evaporator. Most likely the system is overcharged. Recover refrigerant in 4 to 8 oz increments. Rule out dirty evaporator coil, low indoor airflow, undersized return duct, or oversized orifice.
• Actual below 5°F (critical): liquid refrigerant is reaching the compressor. Stop the system. Liquid slugging can destroy a compressor in minutes. Recover refrigerant aggressively and inspect for stuck-open TXV or grossly oversized orifice.

Why TXV systems use a fixed superheat target instead

A thermostatic expansion valve self-regulates the refrigerant flow into the evaporator based on suction line temperature. The TXV holds superheat at a fixed value (typically 8 to 12°F) across a wide range of outdoor temperatures and indoor loads, which is why TXV systems don't use the variable target superheat formula. Instead, TXV systems are charged by subcooling on the liquid line, and superheat is checked as a verification number.

If a TXV system has superheat outside the 8 to 12°F range, the TXV itself is suspect. Check the sensing bulb mounting and insulation. A bulb that is loose, exposed to ambient air, or installed at the wrong angle on the suction line will hunt or stick. If the TXV is functioning correctly and superheat is still off, the system either has a refrigerant problem (charge, leak, restriction) or an airflow problem (dirty coil, blower, ducts).

Common superheat measurement mistakes

Five mistakes cause most bad superheat measurements:

• Reading pressure before the system stabilizes. Suction pressure swings widely in the first 5 to 10 minutes of operation. Always run the system for at least 10 minutes at steady-state conditions before measuring.
• Skipping thermocouple insulation. An uninsulated clamp reads partial ambient. Insulate the sensor with foam or putty so it reads true pipe temperature.
• Using bubble point instead of dew point for R-454B. The 1.5°F glide error pushes superheat readings outside ±3°F of target even when charge is actually correct. Use dew for suction-side measurements.
• Measuring wet bulb in the wrong location. Indoor wet bulb must be measured at the return grille, not the supply, and not at the thermostat. Use a digital psychrometer that reports IDWB directly.
• Ignoring airflow problems. A clean correctly-sized system that suddenly drifts high on superheat after a filter change usually has an airflow restriction, not a charge problem. Always check static pressure with our static pressure calculator before adjusting charge.

Refrigerant differences: R-410A, R-454B, R-32, and R-22 superheat

The superheat math is identical across refrigerants, but the saturation conversion is not. R-410A reads ~144 PSIG at 47°F saturation. R-454B reads ~129 PSIG at the same saturation on the dew side. R-32 reads ~125 PSIG. R-22 reads ~76 PSIG. Reading the wrong chart while charging a system is one of the fastest ways to overcharge or undercharge by a pound or more. This calculator pulls the saturation temperature from the correct chart automatically based on the refrigerant you select.

R-454B specifically requires the dew (vapor) saturation reading because the refrigerant has 1.5°F of temperature glide. At the suction line measurement point, the refrigerant is fully vaporized, so the dew point is the correct reference. The bubble point applies to the liquid side for subcooling measurements. This is the single most common source of "off but I cannot explain why" superheat readings on R-454B service calls.

How to use superheat alongside subcooling for full charge verification

A complete refrigerant charge verification uses both superheat and subcooling. Each number reveals different failure modes:

• Superheat correct + subcooling correct: charge is right, metering device is sized correctly, system is healthy.
• Superheat high + subcooling low: undercharged. Both numbers move the same direction because there is simply too little refrigerant.
• Superheat low + subcooling high: overcharged. Liquid is backing up into the condenser.
• Superheat high + subcooling high: restriction in the liquid line, filter dryer, or metering device. Refrigerant cannot flow fast enough through the orifice.
• Superheat low + subcooling low: TXV stuck open, oversize orifice, or evaporator airflow far too low to boil off refrigerant.

Always check both before declaring a system charged. Run our subcooling calculator next to complete the diagnostic.