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How to Use the Superheat Calculator
Pick fixed orifice or piston, or TXV or EEV, and select the refrigerant from the nameplate. A TXV starts with a 10°F target and a 2°F tolerance, and a fixed orifice starts with a 5°F tolerance.
Type the low-side pressure from the manifold and the suction line temperature from a clamp probe near the compressor. Choose psig, psia or bar, and degrees Fahrenheit or Celsius.
For a fixed orifice, enter the indoor wet-bulb and outdoor dry-bulb temperatures for an approximate target, or type the manufacturer chart value. For a TXV, enter the valve setting or the manufacturer value.
Check superheat against the target, the deviation and the checks suggested. Add the project details, then use PDF, CSV, Copy or Print.
Superheat: the vapour margin that protects the compressor
Superheat is the number of degrees the vapour leaving the evaporator sits above its boiling point at the suction pressure. A little means the coil is doing its job and every drop of liquid has boiled off before the compressor. None, or a negative value, means liquid is on its way to the compressor. A great deal means the evaporator is starved. The calculator finds the boiling point from your suction pressure, subtracts it from the line temperature, and compares the answer with a target you set or one it estimates from the indoor and outdoor conditions.
What to measure and where
Read the suction pressure at the service port and clamp a probe on bare, clean copper of the suction line near the compressor, then insulate it. That gives total superheat, which includes any heat the line picked up on the way. Let the system run until the readings stop moving before you record them, since a unit that has just started gives numbers that mean little. A fixed-orifice target also needs the air entering the evaporator, and the figure to use is the wet-bulb temperature in the return duct a few feet before the coil. If you only have a dry-bulb reading and the humidity, the wet-bulb calculator gives the wet-bulb temperature. In the first example, R-410A at 118 psig has a dew point of 39.8°F, and a suction line at 56°F gives 16.2°F of superheat.
Charging a fixed-orifice system by superheat
A piston or capillary tube cannot adjust itself, so the charge is set by superheat, and the right superheat moves with the weather. ACHR News on superheat charging curves explains that a higher outdoor temperature lowers the target, because the head pressure is higher, and that a higher indoor load raises it. It treats a reading within 5°F of the chart as acceptable, tells technicians to wait at least 15 minutes after adding or recovering refrigerant, and warns against adding charge to a system that reads low superheat at a cool outdoor temperature, since the compressor can flood when it warms up. The chart assumes the airflow the manufacturer specifies, and the article recommends consulting the manufacturer because some use different curves for different models. For that reason the calculator starts the fixed-orifice tolerance at 5°F, and it lets you type the manufacturer value over its own estimate.
The approximate target formula
When no chart is at hand, a rule often quoted for fixed-orifice systems estimates the target as three times the indoor wet-bulb temperature, minus 80, minus the outdoor dry-bulb temperature, all divided by two, with temperatures in °F. AC Service Tech describes it as an approximation that gets close to the chart results but may not match them exactly, and it measures the outdoor dry bulb about a foot from the condenser coil inlet, in the shade. At 67°F wet bulb the target falls from 23°F at 75°F outdoors to 18°F at 85°F, 13°F at 95°F and 8°F at 105°F, so each 10°F of outdoor temperature takes 5°F off. Each degree of indoor wet bulb adds 1.5°F. The formula goes low in hot, dry conditions, and the calculator flags any result under 5°F as too low to rely on.
Airflow, load and cool weather
Superheat responds to anything that changes the heat reaching the coil. Low airflow lowers the evaporating pressure and the coil runs cold, a high indoor load raises the target, and a cool afternoon in spring lowers the outdoor temperature and raises the target the other way. The ACHR chart is based on 400 cfm per ton, and the CFM per ton calculator turns a measured airflow and a capacity into that figure so you can confirm the condition before trusting the target. Do not add refrigerant on a low reading in cool weather without the conditions the chart expects. In the TXV example, R-410A at the same 118 psig with a line at 41.5°F gives 1.7°F of superheat, against a 10°F target, which is a low-superheat verdict and a prompt to check the sensing bulb and the subcooling.
Dew point, glide and gauge error
Superheat is measured from the dew point, because that is where the last liquid is gone. On R-410A the dew and bubble points at 118 psig are 39.8°F and 39.7°F, so the choice hardly matters. On R-407C at 70 psig they are 44.5°F and 33.6°F, and on R-448A 39.2°F and 28.4°F, so a bubble-point lookup would overstate superheat by about 11°F. The same pressure also gives different temperatures for different refrigerants: at 118 psig the dew point is 38.7°F for R-32, 39.8°F for R-410A, 44.9°F for R-454B and 68.4°F for R-22. Check the nameplate before you trust a pressure. The data comes from CoolProp 8.
Frequently Asked Questions
It depends on the load, so use the manufacturer's charging chart. The target falls as outdoor temperature rises and rises as indoor wet-bulb temperature rises. A common approximation is (3 x indoor wet bulb - 80 - outdoor dry bulb) / 2 in °F. At 67°F wet bulb and 85°F outdoors it gives 18°F, and the calculator accepts a reading within 5°F of the target. ACHR News gives that tolerance for chart-based charging and recommends consulting the manufacturer, since some use different curves for different models.
The valve sets it, and ACHR News gives the typical setting as 8°F to 12°F in most residential air conditioning. The calculator starts a TXV with a 10°F target and a 2°F tolerance, which covers that range, and you can replace the target with the manufacturer's value. A TXV system is charged by subcooling, not by superheat, so a superheat outside the band points first at the valve, the bulb or the airflow.
A restriction is the usual suspect. A clogged drier, a partly closed valve or a kinked liquid line starves the evaporator, so the vapour leaving it stays superheated, while liquid stacks up in the condenser and subcooling climbs. More refrigerant does not feed the evaporator past the restriction. Check subcooling: if it is also above target, compare the temperature on each side of the drier and the valves. A high indoor load also raises the target on a fixed-orifice system, so confirm the conditions before adding charge.
A 3 psi error moves the saturation temperature by 1.3°F on R-410A at 118 psig, 2.0°F on R-22 at 70 psig and 3.0°F on R-134a at 35 psig, because low-pressure refrigerants change temperature faster with pressure. A probe that reads 1 to 2°F off adds to that. Zero the gauges to the air, use the elevation field when the site is high, and treat a result within a few degrees of the target band as inside the instrument error.