The two subtractions, and what each one is looking at
Superheat is how many degrees the suction line is above the saturation temperature that corresponds to the suction pressure. Because a mixture of liquid and vapour sits at its saturation temperature, any reading above it means the liquid is gone and only vapour is left. Superheat is therefore a statement about the low side and the evaporator.
Subcooling is the mirror image on the high side: how many degrees the liquid line is below the saturation temperature for the liquid pressure. A reading below saturation means the vapour is gone and only liquid is left, which says something about the condenser and the liquid seal.
Both need a pressure-temperature relationship to turn a gauge reading into a saturation temperature, and that relationship belongs to one specific refrigerant. This page does not include it. You read it from the chart for the refrigerant that is actually in that system, which is the only chart that is correct for it.
Why this page will not tell you what the target is
Because there is no such number in general. The target superheat for a fixed-orifice system depends on the indoor wet bulb and the outdoor dry bulb at that moment, and manufacturers publish it as a two-dimensional chart rather than a figure. On a system with a thermostatic or electronic expansion valve the valve holds superheat itself and subcooling becomes the reference instead. Different equipment, different refrigerant, different chart.
The page therefore takes the chart figure from you, restates it beside what you measured, and reports the difference without calling it anything. That is not caution for its own sake — a number published as fact on a web page and applied to the wrong system is worse than no number at all.
The part nobody accounts for: what your instruments are worth
This is the genuine teaching point of the page. Superheat is a subtraction between a temperature you measured and a temperature you derived from a pressure you measured, so both instruments land in the answer.
Near the default chart rows here — 45°F at 76 psig and 52°F at 90 psig — the slope is 0.5 degrees of saturation temperature for each psig. A gauge accurate to 2 psig is therefore worth a whole degree of superheat before the thermometer has said anything. Add a thermometer good to a degree and the pessimistic band is 2 degrees. If your measured superheat is 13 and the chart says 12, the difference of 1 degree is inside the band and means nothing.
The slope steepens as pressure falls. On the low side of a system running at low load, a psig can be worth appreciably more than half a degree, and the same gauge becomes a worse instrument exactly when the reading matters. Entering two chart rows close to your actual operating pressure is what makes the band on this page honest.
| Slope near your point | Gauge error | Thermometer error | Band on superheat |
|---|---|---|---|
| 0.5°F per psig | ±2 psig | ±1°F | ±2.0°F |
| 0.5°F per psig | ±1 psig | ±0.5°F | ±1.0°F |
| 0.8°F per psig | ±2 psig | ±1°F | ±2.6°F |
Measurement points, and the errors that hide in them
The suction line temperature has to be taken at the same point as the pressure, on a clean line, with the sensor insulated from the surrounding air. A probe strapped to a bare line in a hot condenser cabinet reads somewhere between the refrigerant and the ambient, and reads high. The pressure drop along a suction line is real, so a pressure taken at the condensing unit and a temperature taken at the evaporator are not describing the same point at all.
The liquid line has the same problem in the other direction: a long run in the sun gains heat, so subcooling measured at the indoor end is not what it was leaving the condenser. Neither of these is an arithmetic problem and neither shows up as anything strange in the result.
What this page is not
Work on a sealed refrigerant system is federally regulated and needs certification. Nothing on this page is a procedure for connecting gauges, adding refrigerant, recovering it or opening a system. Refrigerant is stored under pressure and freezes skin on contact, venting it is illegal, and a sealed system is not a homeowner repair. This page does arithmetic on numbers you already have.
There is no procedure here for putting gauges on a system, for adding or removing refrigerant, or for recovering it, and there will not be. If your readings are a long way from the chart, the next step is a certified technician, not this page. What the page is genuinely useful for is the office half of the work: recording readings, converting them into the two numbers, and knowing how much of the difference from the chart is real and how much is instrument.
For the air-side measurements that belong on the same service sheet, the external static pressure calculator and the coil enthalpy calculator. For what the indoor blower is drawing while all this is happening, the blower watt draw calculator. For the load the equipment was chosen against, the heat loss calculator and the mini split sizing calculator.
Questions people ask
What should my superheat be?
There is no general answer, and this page will not invent one. On a fixed-orifice system the target comes from a manufacturer chart read at the indoor wet bulb and outdoor dry bulb at the moment of the reading, and it moves substantially across that chart. On a system with an expansion valve the valve controls superheat, so subcooling is the figure the charging chart references instead. Read the target from the chart for that equipment, type it into the field provided, and the page will restate it beside your measurement without judging either.
Why does the page ask for two chart rows separately?
To work out how many degrees of saturation temperature one psig is worth near your operating point, which is what converts a gauge accuracy specification into an error in superheat. Two neighbouring rows from the same pressure-temperature chart give the slope. It matters because the slope is not constant: at low suction pressures a psig buys more degrees than it does at higher pressure, so the same gauge is a worse instrument at low load. Without the two rows the page can only show the thermometer half of the uncertainty.
My superheat is negative. What does that mean?
Arithmetically it means the suction line read colder than the saturation temperature you entered, which is not a state the arithmetic can produce honestly. In practice it is nearly always a measurement problem: the pressure and temperature came from different points on the system, the chart row belongs to a different refrigerant, or the two readings were taken minutes apart while conditions moved. Take both again at the same point and the same moment before reading anything into it. This page states no cause and offers no procedure.
Can I use this to charge a system?
No. Work on a sealed refrigerant system is federally regulated and requires certification, and nothing on this page is a procedure for connecting gauges, adding refrigerant, recovering it or opening a system. The page is arithmetic on numbers you already have — useful for a service record, for spotting that a difference is smaller than your instrument error, and for keeping readings from different visits comparable. What to do about a reading is a certified technician question.
Which matters more, superheat or subcooling?
It depends on what is metering the refrigerant, which is why charging charts reference one or the other rather than both equally. A fixed-orifice system does not control superheat, so superheat responds to charge and is the figure the chart uses. An expansion valve holds superheat roughly constant by design, so superheat stops being informative about charge and subcooling takes over. Reading the wrong one for the metering device is a common way to chase a number that was never going to move. Both are calculated here because a service sheet normally records both.