Superheat Target Calculator
Wet and dry bulb to target superheat.
What it does
Charging by feel overfeeds some coils and starves others. Target superheat converts two thermometer readings into the suction superheat a fixed-metering system should show. Gauges then chase that number, not vibes.
The tool above applies the return-bend formula to indoor wet-bulb and outdoor dry-bulb and prints target with a normal-or-recheck band. Fixed pistons and capillaries in cooling only; TXV systems charge by subcooling.
Refrigeration training bodies with contractor standards teach this same (3WB minus DB minus 80) over 2 with steady-airflow prerequisites. Wet-bulb captures latent load that dry-bulb misses, which is why humid days target higher.
Stabilize first: clean filters, correct blower speed, open registers, then read. Charge toward target with leak checks and 608 rules in force, and triangulate with subcooling and pressures before condemning parts.
How to use
- Enter indoor wet-bulb in F.
- Enter outdoor dry-bulb in F.
- Read target superheat.
- Read the normal-or-recheck band.
- Fix airflow before charging.
- Charge toward target with gauges.
Formula
- WB
- Indoor wet-bulb temp in F.
- DB
- Outdoor dry-bulb temp in F.
Return-bend method for fixed-metering cooling with Fahrenheit inputs. Outside roughly 0 to 30 F the readings deserve rechecking rather than charging. Airflow, subcooling, and pressures complete the diagnosis.
Worked example
Example: 67 WB, 95 DB
67 F indoor wet bulb with 95 F outdoor.
- Indoor wet bulb 67 F, outdoor dry bulb 95 F.
- Target: (3 x 67 - 95 - 80) / 2.
- Numerator: 201 - 175 = 26.
- Target: 26 / 2 = 13 F.
- Charge toward 13 F with steady airflow first.
Reference table
| Indoor WB | Target | Note |
|---|---|---|
| 60 F | 2.5 F | Dry indoor air |
| 64 F | 8.5 F | Below typical |
| 67 F | 13 F | Example point |
| 72 F | 20.5 F | Humid load |
| 75 F | 25 F | Very humid |
Limits
- Fixed-metering cooling only.
- Needs steady airflow and stable conditions.
- Band edges deserve rechecks, not charge.
- Refrigerant handling needs certification.
Common mistakes
- Charging TXVs by superheat.
- Ignoring wet-bulb for dry-bulb.
- Charging into dirty filters.
- Chasing targets on mild days.
- Venting refrigerant illegally.
Try it yourself
Quick checks. Answers sit tucked under each question.
Target at 72 WB and 100 DB?
(3 x 72 - 100 - 80) / 2 = (216 - 180) / 2 = 18 F. Normal band.
Target at 60 WB and 85 DB?
(3 x 60 - 85 - 80) / 2 = (180 - 165) / 2 = 7.5 F. Normal band.
Questions
What is target superheat?
Suction-line superheat that keeps liquid out of the compressor while feeding the coil fully: return-bend math gives (3 x indoor WB minus outdoor DB minus 80) over 2. Gauges compare against it.
Which systems use this?
Fixed-metering (piston/capillary) systems in cooling with steady airflow. TXV and EEV systems hold their own superheat mechanically and charge by subcooling instead.
Why fix airflow first?
Dirty filters, slow blowers, and closed registers warp wet-bulb readings and starve coils. Fix airflow first; charging into bad airflow compounds the error.
What makes superheat read wrong?
Low charge, restriction, weak compressor, and bad airflow each move the needle differently. Target plus subcooling plus pressures triangulate; one number never diagnoses alone.
Do mild days skew readings?
Yes: low load, high humidity, mismatched coils, and mild days all bend readings. Recheck in design conditions and weigh those edge readings lightly.
What gear and rules apply?
Recovery machine, scale, leak check, and EPA 608 certification for regulated refrigerants. Venting carries fines; sloppy charging carries callbacks.