Target superheat calculator
Enter the indoor return air wet bulb and the outdoor air temperature entering the condenser to get the target superheat from CEC Table RA3.2-2. Add your measured superheat to see whether the charge is within ±5°F.
Target superheat chart (CEC Table RA3.2-2)
An extract of the official table every 5°F of outdoor dry bulb and every 2°F of indoor wet bulb; the calculator above uses all 1,647 cells of the full 1°F table. A dash means the target is below 5°F, where the superheat method cannot give a valid result.
| DB \ WB | 50 | 52 | 54 | 56 | 58 | 60 | 62 | 64 | 66 | 68 | 70 | 72 | 74 | 76 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 55°F | 8.8 | 11.5 | 14.2 | 17.1 | 20.0 | 23.1 | 26.2 | 29.4 | 32.4 | 35.1 | 37.7 | 40.2 | 42.7 | 45.0 |
| 60°F | 7.0 | 9.8 | 12.6 | 15.4 | 18.2 | 21.0 | 23.8 | 26.6 | 29.6 | 32.4 | 35.1 | 37.8 | 40.4 | 42.9 |
| 65°F | – | 7.0 | 10.0 | 12.9 | 15.8 | 18.5 | 21.2 | 23.8 | 26.7 | 29.7 | 32.5 | 35.3 | 38.1 | 40.8 |
| 70°F | – | – | 6.4 | 9.7 | 12.7 | 15.7 | 18.4 | 20.9 | 23.9 | 27.0 | 30.0 | 33.0 | 35.9 | 38.7 |
| 75°F | – | – | – | 5.6 | 9.2 | 12.4 | 15.3 | 18.0 | 21.1 | 24.3 | 27.5 | 30.6 | 33.7 | 36.7 |
| 80°F | – | – | – | – | – | 8.7 | 12.0 | 15.0 | 18.3 | 21.7 | 25.0 | 28.3 | 31.6 | 34.8 |
| 85°F | – | – | – | – | – | – | 8.5 | 11.9 | 15.5 | 19.0 | 22.6 | 26.0 | 29.5 | 32.9 |
| 90°F | – | – | – | – | – | – | – | 8.8 | 12.8 | 16.5 | 20.1 | 23.8 | 27.5 | 31.1 |
| 95°F | – | – | – | – | – | – | – | 5.6 | 10.0 | 13.9 | 17.8 | 21.6 | 25.5 | 29.4 |
| 100°F | – | – | – | – | – | – | – | – | 7.3 | 11.4 | 15.4 | 19.5 | 23.6 | 27.7 |
| 105°F | – | – | – | – | – | – | – | – | – | 8.8 | 13.1 | 17.4 | 21.7 | 26.0 |
| 110°F | – | – | – | – | – | – | – | – | – | 6.4 | 10.8 | 15.3 | 19.9 | 24.4 |
| 115°F | – | – | – | – | – | – | – | – | – | – | 8.6 | 13.3 | 18.1 | 22.9 |
How it works
Target superheat is the superheat a correctly charged fixed-metering system should show at today’s indoor and outdoor conditions. It applies only to fixed metering devices: capillary tubes and piston (fixed orifice) types. TXV and EEV systems are charged by subcooling.
The tool looks up CEC Table RA3.2-2, “Target Superheat (Suction Line Temperature − Evaporator Saturation Temperature)”. The table’s columns are return air wet bulb from 50°F to 76°F. Its rows are condenser entering air dry bulb from 55°F to 115°F. Both run in 1°F steps, 1,647 cells in all. The values did not change between the 2022 and 2025 Reference Appendices. Between whole degrees the tool interpolates linearly in both directions (bilinear).
The inputs:
- Indoor return air wet bulb (°F WB), measured in the return plenum. CEC uses a 5/8 in “Title 24 Return Plenum Measurement Access” hole.
- Outdoor air entering condenser (°F DB), measured with a shaded sensor at the average entering air.
- Indoor return air dry bulb (optional). The tool warns if it is below 70°F, because the test is not valid there.
- Measured superheat (optional), from the superheat calculator.
The tool also shows the field formula (3 × WB − 80 − DB) ÷ 2, published by ACCA. ACCA does not name who came up with it. It is an approximation, not the CEC table.
Worked example
The tool opens at 63°F indoor wet bulb and 85°F outdoor dry bulb.
- CEC Table RA3.2-2, row 85°F, column 63°F: 10.3°F.
- Formula: (3 × 63 − 80 − 85) ÷ 2 = (189 − 165) ÷ 2 = 12.0°F.
At 63.5°F wet bulb and 85°F outdoors the tool interpolates between the 63°F cell (10.3°F) and the 64°F cell (11.9°F): (10.3 + 11.9) ÷ 2 = 11.1°F.
Say the superheat calculator gave 12.2°F at the 63/85 conditions. 12.2 − 10.3 = +1.9°F, inside ±5°F, so the charge passes.
Where the formula and table disagree:
| Outdoor DB / indoor WB | CEC table | Formula |
|---|---|---|
| 80°F / 67°F | 20.0°F | 20.5°F |
| 95°F / 67°F | 12.0°F | 13.0°F |
| 85°F / 63°F | 10.3°F | 12.0°F |
| 70°F / 62°F | 18.4°F | 18.0°F |
| 100°F / 72°F | 19.5°F | 18.0°F |
| 95°F / 63°F | dash (below 5°F) | 7.0°F |
The last row is the important one. At 95°F outdoors and 63°F wet bulb the formula still gives a number, but CEC says no valid superheat test exists.
Reading the result
CEC Table RA3.2-1 tolerances for the superheat method are ±5°F of target for the installer and ±8°F for the HERS/ECC Rater. Test conditions are outdoor dry bulb from 55°F to 115°F, return dry bulb of 70°F or warmer, and return wet bulb of 76°F or less, after 15 minutes of running in cooling.
- Measured above target: likely undercharged, or a restriction. Confirm indoor airflow, then add refrigerant.
- Measured below target: likely overcharged, or low indoor airflow.
- Dash (target below 5°F): no valid test. In CEC’s words, a severely undercharged unit “will show over 9°F of superheat”, but an overcharge cannot be detected. The usual cause is a hot day and a cool house. CEC’s fixes: turn on space heating or open windows to warm the house, or come back another time.
- Outside the table (wet bulb outside 50–76°F or outdoor outside 55–115°F): no target. Below 55°F outdoors, CEC says installers weigh in the charge (RA3.2.3), which is valid at any outdoor temperature.
The manufacturer’s own charging chart takes priority where one exists. CEC allows either its table or the manufacturer’s superheat chart.
Common mistakes
- Using it on a TXV or EEV system. The valve holds superheat steady whatever the charge. Use the subcooling calculator.
- Dry bulb in the wet bulb box. Indoor needs wet bulb; outdoor needs dry bulb.
- Treating the formula as the standard. It runs up to about 2°F off the table and gives numbers where CEC prints a dash.
- Measuring before the system settles. CEC wants 15 minutes of runtime, with the wet-bulb sensor in place by minute 12.
- Reading superheat off the wrong PT column. For blends, actual superheat uses the dew point. See how to read a PT chart.
Related
Sources
- CEC 2025 Reference Appendices (CEC-400-2025-010-AP), Table RA3.2-1 and Table RA3.2-2 energy.ca.gov
- CEC 2022 Reference Appendices (CEC-400-2022-010-AP), RA3.2.2.6.1 superheat method energy.ca.gov
- CEC 2022 LMCV-MCH-25a, Refrigerant Charge: Superheat Method (instructions) energy.ca.gov
- ACCA HVAC Blog (Matt Akins): How do you calculate the target superheat of a residential AC and heat pump? hvac-blog.acca.org
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