Superheat Calculator

Enter the suction pressure and suction line temperature to get superheat, using the dew point for blends like R-454B. On a fixed orifice or piston system, add the outdoor and return air readings to see the target superheat and whether the charge passes.

Check the unit's nameplate. R-454B and R-410A are not interchangeable.

At the suction service valve port. For a vacuum, pick inHg.

Clamp the probe on the suction line near the service valve and insulate it.

Superheat

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Fill in the boxes above to see the answer.

Saturation temperatureDew point at the suction pressure
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Temperature glideDew minus bubble point at this pressure
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Target superheatTitle 24 Table RA3.2-2
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Compared to target
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Air entering the condenser, with the sensor shaded. Fill in all three readings to get Title 24's target superheat.

In the return plenum, before the coil, after 15 minutes of running.

Title 24 needs at least 70°F (21.1°C) for this test.

From the manufacturer's charging chart, if it gives one. It's used instead of Title 24's table.

What is superheat?

Superheat is how many degrees the refrigerant vapor has warmed above its saturation temperature. In the evaporator, refrigerant boils at the saturation temperature for its pressure. Once the last of the liquid has boiled off, the vapor keeps picking up heat, and every degree it gains past that point is a degree of superheat.

Measured at the suction line, it shows how well the evaporator is being fed. Some superheat means all the liquid boiled off before it could reach the compressor and damage it. Too much usually means the coil is starved of refrigerant, from a low charge or a restriction, and too little means it's getting more refrigerant than the load can boil off.

How it's calculated

Absolute pressure = gauge pressure + 14.696 psi (101.325 kPa, one standard atmosphere)
Superheat = suction line temperature − dew point at the suction pressure
Glide = dew point − bubble point at the same pressure (blends only)
Target superheat (fixed orifice) = Title 24 Table RA3.2-2 at (outdoor dry bulb, return wet bulb), each rounded to the nearest whole °F
Superheat − target within ±5°F passes. Above: add refrigerant. Below: remove refrigerant
Saturation data: CoolProp 8.0.0, interpolated from tables every 0.5 °C; R-448A from Arkema's Forane PT chart and R-449A from Chemours' Opteon XP40 tables, every 0.5 °F

Sources

If a number doesn't match your chart, email me what you entered and what you expected, and I'll check it against the source. You can also read how the numbers are checked.

Checked by Paul Williams · Updated

Questions

How do you calculate superheat?

Find the saturation temperature for the suction pressure on a PT chart, then subtract it from the suction line temperature. For example, R-410A at 118.4 psig saturates at 40.0°F, so a suction line at 52°F has 12.0°F of superheat.

Why does superheat use the dew point?

Superheat is how far the vapor has warmed past the point where the last drop of liquid boiled off, and that point is the dew point. For a blend with a lot of glide the difference is big: R-407C at 63.2 psig has a dew point of 40.0°F but a bubble point of about 29°F, so reading the wrong column would add about 11°F of false superheat. Chemours' PT guide says to use saturated vapor pressures for superheat.

What does negative superheat mean?

On a single-component refrigerant like R-32 or R-22, a suction line with liquid in it sits at the saturation temperature, so it reads about 0, not below. On a blend it can read below zero by up to the glide (about 11°F on R-407C, 2.7°F on R-454B), which means liquid is reaching the compressor. Anything lower than that is a reading problem: check the refrigerant choice, the pressure port and the probe.

Where do I take the readings?

Read pressure at the suction service valve port and clamp the temperature probe on the suction line near that valve, insulated from the outside air. Let the system run first: manufacturers ask for 10 to 20 minutes.

What should my superheat be?

It depends on the metering device. Fixed orifice and piston systems are charged to a target superheat from the manufacturer's chart, or California Title 24's table, which use indoor wet bulb and outdoor dry bulb temperatures. Fill in the fixed orifice charge check and this calculator reads that table for you, or use the target superheat calculator for the target on its own. On TXV and EEV systems the valve controls superheat, so the charge is set by subcooling instead.

How close to the target does superheat have to be?

Title 24's installer check passes when the superheat is within 5°F of the target (an ECC-Rater allows 8°F). More than 5°F above the target means add refrigerant, and more than 5°F below means remove some. For example, at 95°F outdoors and a 66°F return wet bulb the target is 10.0°F, so 12°F of superheat passes and 16°F doesn't.

Does altitude change the reading?

It can. PT charts, and this calculator, assume sea level (14.696 psia). At 5,000 ft the air is about 2.5 psi lighter, so a gauge zeroed there reads high and superheat comes out about 1°F low for R-410A. Gauges with absolute pressure sensors correct for this themselves.

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