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Superheat and subcooling at startup, in plain words
A unit can blow cold air and still be charged wrong. Superheat and subcooling are the two numbers that show whether the refrigerant side is right, and they take two minutes once the unit has settled. Here is what they mean, how to take them, and what they point to.
What they are
Superheat is how many degrees the refrigerant vapor has warmed past its boiling point by the time it leaves the evaporator. You measure it on the suction line.
Subcooling is how many degrees the liquid has cooled below its condensing point by the time it leaves the condenser. You measure it on the liquid line.
The boiling and condensing points come from the pressure: every refrigerant boils at a set temperature for a given pressure. That is what a pressure and temperature chart (or the scale on your gauges) tells you, and why you need to know which refrigerant is in the unit.
Taking the readings
- Run the unit in cooling long enough for the pressures to settle. The manufacturer's instructions say how long; 10 to 15 minutes is common.
- Note the refrigerant from the nameplate, and record the outdoor temperature and the entering air temperature at the unit.
- Read the suction pressure at the suction service port, and the suction line temperature with a clamp probe on the line near that port. Insulate the probe from the air around it.
- Read the liquid line pressure (or the discharge pressure, if that is where the unit's port is and the manufacturer's charts use it), and the liquid line temperature near the liquid service port.
The math
Superheat = suction line temperature minus the saturation temperature for the suction pressure.
Subcooling = the saturation temperature for the liquid pressure minus the liquid line temperature.
A worked example with R-410A
| Suction pressure | 118 psig |
|---|---|
| Saturation temperature at 118 psig | about 40°F |
| Suction line temperature | 51°F |
| Superheat: 51 minus 40 | 11°F |
| Liquid pressure | 365 psig |
| Saturation temperature at 365 psig | about 110°F |
| Liquid line temperature | 100°F |
| Subcooling: 110 minus 100 | 10°F |
Which one to charge by
It depends on the metering device:
- Fixed orifice or piston: charge by superheat. The target comes from the manufacturer's chart, using the indoor wet bulb and the outdoor temperature.
- TXV or electronic expansion valve: charge by subcooling. The valve holds the superheat steady on its own, so subcooling is the number that moves with the charge. The target is on the nameplate or in the instructions.
Heat pumps are usually checked in cooling mode. When the weather makes that impossible, follow the manufacturer's procedure for heating mode or weighing in the charge.
What the numbers point to
These patterns tell you where to look first. They are not a diagnosis on their own.
| Superheat | Subcooling | Points to |
|---|---|---|
| High | Low | Low charge, or a leak |
| Low | High | Too much charge |
| High | High | A restriction: the filter drier, the liquid line or the metering device |
| Low | Normal | On a fixed orifice, low airflow across the indoor coil: filters, belt, a dirty coil, fan speed |
What a good report shows
All four readings, both results, the refrigerant, and the conditions they were taken in. A report that only says "SH 11, SC 10" asks the commissioning provider to trust the math. A report with the pressures and temperatures lets them check it in ten seconds, and that is the one that gets approved.