How to find target superheat
This is the standard field formula for the fixed-orifice charging chart. It works for indoor wet bulb from 50 to 85°F and outdoor dry bulb from 55 to 115°F. Results are kept between 5 and 35°F.
- Run the system in cooling for 10–15 minutes.
- Measure indoor wet bulb at the return with a psychrometer.
- Measure outdoor dry bulb where air enters the condenser coil, in the shade.
- Calculate the target, then measure actual superheat with the superheat calculator.
- Within ±5°F of target is good. Above target: add refrigerant slowly. Below target: recover some.
Indoor wet bulb is 63°F and outdoor dry bulb is 85°F. Target superheat is (3 × 63 − 80 − 85) ÷ 2 = 12°F. A measured superheat of 14°F is within ±5°F, so the charge is good.
TXV and EEV systems
A TXV or EEV holds superheat steady (FieldLoom uses 10°F), so don't charge by target superheat. Charge TXV systems by subcooling instead.
If the target comes out at the 5°F minimum, conditions are too mild to charge accurately by superheat. Come back on a warmer day or weigh in the charge.
Questions
Why does target superheat need wet bulb?
Indoor wet bulb tells you how much heat and moisture the evaporator is absorbing. A higher heat load boils refrigerant faster, so the target superheat goes up.
How close to target should superheat be?
FieldLoom treats measured superheat within ±5°F of the target as a pass.
Does target superheat apply to heat pumps?
Only in cooling mode, and only for fixed-orifice systems. In heating mode follow the manufacturer's charging instructions.