Superheat is the suction-line temperature minus the refrigerant's saturation temperature at the suction pressure; subcooling is the saturation temperature at the liquid-line pressure minus the liquid-line temperature. Systems with a fixed metering device (piston or capillary) are charged to a target superheat read from indoor wet-bulb and outdoor dry-bulb: at 85 °F outdoor and 63 °F return-air wet-bulb the California Energy Commission table gives 10.3 °F and Trane's chart 9 °F. Systems with a TXV or EEV are charged to the subcooling on the nameplate (10 °F on Trane's Runtru A5AC3 condensers). Saturation temperatures come from the refrigerant's pressure-temperature chart; for car A/C refrigerants see the refrigerant P-T chart. Connecting gauges or adding refrigerant requires EPA Section 608 certification in the US.
Superheat and subcooling at a glance
| Superheat | Suction-line temperature − evaporator saturation temperature (from the suction pressure). It is how many degrees the vapor leaving the evaporator has warmed after the last liquid boiled off. |
|---|---|
| Subcooling | Condenser saturation temperature (from the liquid-line pressure) − liquid-line temperature. It is how many degrees the liquid has cooled below the point where it finished condensing. |
| Fixed orifice | Piston or capillary metering. Charge to the target superheat from a table of return-air wet-bulb and outdoor dry-bulb (CEC RA3.2.2.6.1; Trane: "superheat (fixed orifice) per the unit nameplate"). Too much superheat: add refrigerant. Too little: remove refrigerant (CEC). |
| TXV / EEV | Thermostatic or electronic expansion valve. Charge to the target subcooling given by the manufacturer (nameplate or service facts). Superheat is then checked only to confirm the valve works: CEC accepts the maker's range or 4–25 °F for installers. Subcooling above target: remove refrigerant; below: add (CEC). |
| Below 55 °F | The CEC procedures need 55 °F or warmer outdoors, and Trane recommends its subcooling method only above 55 °F outdoors. Otherwise weigh in the charge by line length (CEC RA3.2.3) and recheck in warm weather. |
| °F to kelvin | Superheat and subcooling are temperature differences: multiply °F by 5/9 for K (or °C difference). 10 °F = 5.6 K; 5 °F = 2.8 K. |
How to measure (CEC RA3.2.2 and Trane)
- Run it first. Run in cooling for at least 15 minutes before measuring (CEC), and 10–15 minutes after any refrigerant or airflow change before taking the final superheat (Trane). Trane waits 20 minutes between adjustments when charging by subcooling. The return air must be at least 70 °F dry-bulb (CEC).
- Suction pressure and line temperature. Read the suction (low-side) pressure, look up its saturation temperature on the refrigerant's P-T chart, and clamp a pipe temperature sensor on the suction line near the low-side service valve, in good contact with the pipe (CEC).
- Liquid pressure and line temperature. Read the liquid (high-side) pressure, look up its saturation temperature, and clamp a sensor on the liquid line near the high-side service valve (CEC).
- Indoor wet-bulb (fixed orifice only). Measure the return-air wet-bulb in the return plenum just before the indoor coil, with a wetted-wick sensor or a digital psychrometer (CEC; Trane). This is the wet-bulb, not the dry-bulb or the relative humidity.
- Outdoor dry-bulb. Measure the air entering the outdoor (condenser) coil (CEC: "condenser air entering dry-bulb").
- Instrument accuracy required by the CEC: air and pipe temperature sensors ±2 °F (0.2 °F resolution); digital refrigerant gauges ±3.5 psi on the suction side and ±7.0 psi on the liquid side.
- Blends. Zeotropic blends such as R-454B condense over a small temperature range, so a P-T chart lists a bubble point (liquid) and a dew point (vapor). Trane's R-454B instructions say to use the bubble point when calculating subcooling; use the column the refrigerant maker's chart specifies for each reading.
- Then compare. Superheat (or subcooling) minus target. Installer pass bands from the CEC are in the tolerance table below; the equipment maker's own instructions govern for warranty and operation.
Worked examples
- Superheat, R-22 piston system: suction gauge 68.5 psig (83.2 psia). NIST gives R-22 saturation at 83.23 psia as 39.98 °F, so the evaporator is at 40 °F. The suction line reads 52 °F: superheat = 52 − 40 = 12 °F. Outdoors 85 °F, return wet-bulb 63 °F: CEC target 10.3 °F (12 is within ±5 °F, so it passes); Trane's chart (for its R-454B units) reads 9 °F.
- Subcooling, R-454B TXV system: liquid gauge 323 psig. Trane's Table 9 puts 323 psig at 95 °F liquid with 10 °F subcooling, i.e. a bubble point of 105 °F. The liquid line reads 95 °F: subcooling = 105 − 95 = 10 °F, matching the A5AC3's 10 °F design value.
- Wet-bulb out of range: outdoors 95 °F with return air at 60 °F wet-bulb shows a dash in both tables (target under 5 °F). The CEC says severely undercharged units will still show over 9 °F of superheat, but an overcharge cannot be detected then: warm the house (heat or open windows) or retest later.
Target superheat chart (California Energy Commission Table RA3.2-2)
The CEC table is used for Title 24 refrigerant-charge verification of split systems with fixed metering devices in California; it does not name a refrigerant. Its own rule: use this table or the manufacturer's superheat chart. Find your outdoor temperature row in the slice that holds your wet-bulb.
Return-air wet-bulb 50–58 °F
| Outdoor DB (°F) | WB 50 °F | WB 51 °F | WB 52 °F | WB 53 °F | WB 54 °F | WB 55 °F | WB 56 °F | WB 57 °F | WB 58 °F |
|---|---|---|---|---|---|---|---|---|---|
| 55 | 8.8 | 10.1 | 11.5 | 12.8 | 14.2 | 15.6 | 17.1 | 18.5 | 20.0 |
| 56 | 8.6 | 9.9 | 11.2 | 12.6 | 14.0 | 15.4 | 16.8 | 18.2 | 19.7 |
| 57 | 8.3 | 9.6 | 11.0 | 12.3 | 13.7 | 15.1 | 16.5 | 17.9 | 19.4 |
| 58 | 7.9 | 9.3 | 10.6 | 12.0 | 13.4 | 14.8 | 16.2 | 17.6 | 19.0 |
| 59 | 7.5 | 8.9 | 10.2 | 11.6 | 13.0 | 14.4 | 15.8 | 17.2 | 18.6 |
| 60 | 7.0 | 8.4 | 9.8 | 11.2 | 12.6 | 14.0 | 15.4 | 16.8 | 18.2 |
| 61 | 6.5 | 7.9 | 9.3 | 10.7 | 12.1 | 13.5 | 14.9 | 16.3 | 17.7 |
| 62 | 6.0 | 7.4 | 8.8 | 10.2 | 11.7 | 13.1 | 14.5 | 15.9 | 17.3 |
| 63 | 5.3 | 6.8 | 8.3 | 9.7 | 11.1 | 12.6 | 14.0 | 15.4 | 16.8 |
| 64 | – | 6.1 | 7.6 | 9.1 | 10.6 | 12.0 | 13.5 | 14.9 | 16.3 |
| 65 | – | 5.4 | 7.0 | 8.5 | 10.0 | 11.5 | 12.9 | 14.3 | 15.8 |
| 66 | – | – | 6.3 | 7.8 | 9.3 | 10.8 | 12.3 | 13.8 | 15.2 |
| 67 | – | – | 5.5 | 7.1 | 8.7 | 10.2 | 11.7 | 13.2 | 14.6 |
| 68 | – | – | – | 6.3 | 8.0 | 9.5 | 11.1 | 12.6 | 14.0 |
| 69 | – | – | – | 5.5 | 7.2 | 8.8 | 10.4 | 11.9 | 13.4 |
| 70 | – | – | – | – | 6.4 | 8.1 | 9.7 | 11.2 | 12.7 |
| 71 | – | – | – | – | 5.6 | 7.3 | 8.9 | 10.5 | 12.1 |
| 72 | – | – | – | – | – | 6.4 | 8.1 | 9.8 | 11.4 |
| 73 | – | – | – | – | – | 5.6 | 7.3 | 9.0 | 10.7 |
| 74 | – | – | – | – | – | – | 6.5 | 8.2 | 9.9 |
| 75 | – | – | – | – | – | – | 5.6 | 7.4 | 9.2 |
| 76 | – | – | – | – | – | – | – | 6.6 | 8.4 |
| 77 | – | – | – | – | – | – | – | 5.7 | 7.5 |
| 78 | – | – | – | – | – | – | – | – | 6.7 |
| 79 | – | – | – | – | – | – | – | – | 5.9 |
| 80 | – | – | – | – | – | – | – | – | – |
| 81 | – | – | – | – | – | – | – | – | – |
| 82 | – | – | – | – | – | – | – | – | – |
| 83 | – | – | – | – | – | – | – | – | – |
| 84 | – | – | – | – | – | – | – | – | – |
| 85 | – | – | – | – | – | – | – | – | – |
| 86 | – | – | – | – | – | – | – | – | – |
| 87 | – | – | – | – | – | – | – | – | – |
| 88 | – | – | – | – | – | – | – | – | – |
| 89 | – | – | – | – | – | – | – | – | – |
| 90 | – | – | – | – | – | – | – | – | – |
| 91 | – | – | – | – | – | – | – | – | – |
| 92 | – | – | – | – | – | – | – | – | – |
| 93 | – | – | – | – | – | – | – | – | – |
| 94 | – | – | – | – | – | – | – | – | – |
| 95 | – | – | – | – | – | – | – | – | – |
| 96 | – | – | – | – | – | – | – | – | – |
| 97 | – | – | – | – | – | – | – | – | – |
| 98 | – | – | – | – | – | – | – | – | – |
| 99 | – | – | – | – | – | – | – | – | – |
| 100 | – | – | – | – | – | – | – | – | – |
| 101 | – | – | – | – | – | – | – | – | – |
| 102 | – | – | – | – | – | – | – | – | – |
| 103 | – | – | – | – | – | – | – | – | – |
| 104 | – | – | – | – | – | – | – | – | – |
| 105 | – | – | – | – | – | – | – | – | – |
| 106 | – | – | – | – | – | – | – | – | – |
| 107 | – | – | – | – | – | – | – | – | – |
| 108 | – | – | – | – | – | – | – | – | – |
| 109 | – | – | – | – | – | – | – | – | – |
| 110 | – | – | – | – | – | – | – | – | – |
| 111 | – | – | – | – | – | – | – | – | – |
| 112 | – | – | – | – | – | – | – | – | – |
| 113 | – | – | – | – | – | – | – | – | – |
| 114 | – | – | – | – | – | – | – | – | – |
| 115 | – | – | – | – | – | – | – | – | – |
Return-air wet-bulb 59–67 °F
| Outdoor DB (°F) | WB 59 °F | WB 60 °F | WB 61 °F | WB 62 °F | WB 63 °F | WB 64 °F | WB 65 °F | WB 66 °F | WB 67 °F |
|---|---|---|---|---|---|---|---|---|---|
| 55 | 21.5 | 23.1 | 24.6 | 26.2 | 27.8 | 29.4 | 31.0 | 32.4 | 33.8 |
| 56 | 21.2 | 22.7 | 24.2 | 25.7 | 27.3 | 28.9 | 30.5 | 31.8 | 33.2 |
| 57 | 20.8 | 22.3 | 23.8 | 25.3 | 26.8 | 28.3 | 29.9 | 31.3 | 32.6 |
| 58 | 20.4 | 21.9 | 23.3 | 24.8 | 26.3 | 27.8 | 29.3 | 30.7 | 32.1 |
| 59 | 20.0 | 21.4 | 22.9 | 24.3 | 25.7 | 27.2 | 28.7 | 30.1 | 31.5 |
| 60 | 19.6 | 21.0 | 22.4 | 23.8 | 25.2 | 26.6 | 28.1 | 29.6 | 31.0 |
| 61 | 19.1 | 20.5 | 21.9 | 23.3 | 24.7 | 26.1 | 27.5 | 29.0 | 30.4 |
| 62 | 18.7 | 20.1 | 21.4 | 22.8 | 24.2 | 25.5 | 27.0 | 28.4 | 29.9 |
| 63 | 18.2 | 19.6 | 20.9 | 22.3 | 23.6 | 25.0 | 26.4 | 27.8 | 29.3 |
| 64 | 17.7 | 19.0 | 20.4 | 21.7 | 23.1 | 24.4 | 25.8 | 27.3 | 28.7 |
| 65 | 17.1 | 18.5 | 19.9 | 21.2 | 22.5 | 23.8 | 25.2 | 26.7 | 28.2 |
| 66 | 16.6 | 18.0 | 19.3 | 20.7 | 22.0 | 23.2 | 24.6 | 26.1 | 27.6 |
| 67 | 16.0 | 17.4 | 18.8 | 20.1 | 21.4 | 22.7 | 24.1 | 25.6 | 27.1 |
| 68 | 15.5 | 16.8 | 18.2 | 19.5 | 20.8 | 22.1 | 23.5 | 25.0 | 26.5 |
| 69 | 14.8 | 16.3 | 17.6 | 19.0 | 20.3 | 21.5 | 22.9 | 24.4 | 26.0 |
| 70 | 14.2 | 15.7 | 17.0 | 18.4 | 19.7 | 20.9 | 22.3 | 23.9 | 25.4 |
| 71 | 13.6 | 15.0 | 16.4 | 17.8 | 19.1 | 20.3 | 21.7 | 23.3 | 24.9 |
| 72 | 12.9 | 14.4 | 15.8 | 17.2 | 18.5 | 19.7 | 21.2 | 22.8 | 24.3 |
| 73 | 12.2 | 13.7 | 15.2 | 16.6 | 17.9 | 19.2 | 20.6 | 22.2 | 23.8 |
| 74 | 11.5 | 13.1 | 14.5 | 15.9 | 17.3 | 18.6 | 20.0 | 21.6 | 23.2 |
| 75 | 10.8 | 12.4 | 13.9 | 15.3 | 16.7 | 18.0 | 19.4 | 21.1 | 22.7 |
| 76 | 10.1 | 11.7 | 13.2 | 14.7 | 16.1 | 17.4 | 18.9 | 20.5 | 22.1 |
| 77 | 9.3 | 11.0 | 12.5 | 14.0 | 15.4 | 16.8 | 18.3 | 20.0 | 21.6 |
| 78 | 8.5 | 10.2 | 11.8 | 13.4 | 14.8 | 16.2 | 17.7 | 19.4 | 21.1 |
| 79 | 7.7 | 9.5 | 11.1 | 12.7 | 14.2 | 15.6 | 17.1 | 18.8 | 20.5 |
| 80 | 6.9 | 8.7 | 10.4 | 12.0 | 13.5 | 15.0 | 16.6 | 18.3 | 20.0 |
| 81 | 6.0 | 7.9 | 9.7 | 11.3 | 12.9 | 14.3 | 16.0 | 17.7 | 19.4 |
| 82 | 5.2 | 7.1 | 8.9 | 10.6 | 12.2 | 13.7 | 15.4 | 17.2 | 18.9 |
| 83 | – | 6.3 | 8.2 | 9.9 | 11.6 | 13.1 | 14.9 | 16.6 | 18.4 |
| 84 | – | 5.5 | 7.4 | 9.2 | 10.9 | 12.5 | 14.3 | 16.1 | 17.8 |
| 85 | – | – | 6.6 | 8.5 | 10.3 | 11.9 | 13.7 | 15.5 | 17.3 |
| 86 | – | – | 5.8 | 7.8 | 9.6 | 11.3 | 13.2 | 15.0 | 16.7 |
| 87 | – | – | 5.0 | 7.0 | 8.9 | 10.6 | 12.6 | 14.4 | 16.2 |
| 88 | – | – | – | 6.3 | 8.2 | 10.0 | 12.0 | 13.9 | 15.7 |
| 89 | – | – | – | 5.5 | 7.5 | 9.4 | 11.5 | 13.3 | 15.1 |
| 90 | – | – | – | – | 6.8 | 8.8 | 10.9 | 12.8 | 14.6 |
| 91 | – | – | – | – | 6.1 | 8.1 | 10.3 | 12.2 | 14.1 |
| 92 | – | – | – | – | 5.4 | 7.5 | 9.8 | 11.7 | 13.5 |
| 93 | – | – | – | – | – | 6.8 | 9.2 | 11.1 | 13.0 |
| 94 | – | – | – | – | – | 6.2 | 8.7 | 10.6 | 12.5 |
| 95 | – | – | – | – | – | 5.6 | 8.1 | 10.0 | 12.0 |
| 96 | – | – | – | – | – | – | 7.5 | 9.5 | 11.4 |
| 97 | – | – | – | – | – | – | 7.0 | 8.9 | 10.9 |
| 98 | – | – | – | – | – | – | 6.4 | 8.4 | 10.4 |
| 99 | – | – | – | – | – | – | 5.8 | 7.9 | 9.9 |
| 100 | – | – | – | – | – | – | 5.3 | 7.3 | 9.3 |
| 101 | – | – | – | – | – | – | – | 6.8 | 8.8 |
| 102 | – | – | – | – | – | – | – | 6.2 | 8.3 |
| 103 | – | – | – | – | – | – | – | 5.7 | 7.8 |
| 104 | – | – | – | – | – | – | – | 5.2 | 7.2 |
| 105 | – | – | – | – | – | – | – | – | 6.7 |
| 106 | – | – | – | – | – | – | – | – | 6.2 |
| 107 | – | – | – | – | – | – | – | – | 5.7 |
| 108 | – | – | – | – | – | – | – | – | 5.2 |
| 109 | – | – | – | – | – | – | – | – | – |
| 110 | – | – | – | – | – | – | – | – | – |
| 111 | – | – | – | – | – | – | – | – | – |
| 112 | – | – | – | – | – | – | – | – | – |
| 113 | – | – | – | – | – | – | – | – | – |
| 114 | – | – | – | – | – | – | – | – | – |
| 115 | – | – | – | – | – | – | – | – | – |
Return-air wet-bulb 68–76 °F
| Outdoor DB (°F) | WB 68 °F | WB 69 °F | WB 70 °F | WB 71 °F | WB 72 °F | WB 73 °F | WB 74 °F | WB 75 °F | WB 76 °F |
|---|---|---|---|---|---|---|---|---|---|
| 55 | 35.1 | 36.4 | 37.7 | 39.0 | 40.2 | 41.5 | 42.7 | 43.9 | 45.0 |
| 56 | 34.6 | 35.9 | 37.2 | 38.5 | 39.7 | 41.0 | 42.2 | 43.4 | 44.6 |
| 57 | 34.0 | 35.3 | 36.7 | 38.0 | 39.2 | 40.5 | 41.7 | 43.0 | 44.2 |
| 58 | 33.5 | 34.8 | 36.1 | 37.5 | 38.7 | 40.0 | 41.3 | 42.5 | 43.7 |
| 59 | 32.9 | 34.3 | 35.6 | 36.9 | 38.3 | 39.5 | 40.8 | 42.1 | 43.3 |
| 60 | 32.4 | 33.7 | 35.1 | 36.4 | 37.8 | 39.1 | 40.4 | 41.6 | 42.9 |
| 61 | 31.8 | 33.2 | 34.6 | 35.9 | 37.3 | 38.6 | 39.9 | 41.2 | 42.4 |
| 62 | 31.3 | 32.7 | 34.1 | 35.4 | 36.8 | 38.1 | 39.4 | 40.7 | 42.0 |
| 63 | 30.7 | 32.2 | 33.6 | 34.9 | 36.3 | 37.7 | 39.0 | 40.3 | 41.6 |
| 64 | 30.2 | 31.6 | 33.0 | 34.4 | 35.8 | 37.2 | 38.5 | 39.9 | 41.2 |
| 65 | 29.7 | 31.1 | 32.5 | 33.9 | 35.3 | 36.7 | 38.1 | 39.4 | 40.8 |
| 66 | 29.1 | 30.6 | 32.0 | 33.4 | 34.9 | 36.3 | 37.6 | 39.0 | 40.4 |
| 67 | 28.6 | 30.1 | 31.5 | 33.0 | 34.4 | 35.8 | 37.2 | 38.6 | 39.9 |
| 68 | 28.0 | 29.5 | 31.0 | 32.5 | 33.9 | 35.3 | 36.8 | 38.1 | 39.5 |
| 69 | 27.5 | 29.0 | 30.5 | 32.0 | 33.4 | 34.9 | 36.3 | 37.7 | 39.1 |
| 70 | 27.0 | 28.5 | 30.0 | 31.5 | 33.0 | 34.4 | 35.9 | 37.3 | 38.7 |
| 71 | 26.4 | 28.0 | 29.5 | 31.0 | 32.5 | 34.0 | 35.4 | 36.9 | 38.3 |
| 72 | 25.9 | 27.4 | 29.0 | 30.5 | 32.0 | 33.5 | 35.0 | 36.5 | 37.9 |
| 73 | 25.4 | 26.9 | 28.5 | 30.0 | 31.5 | 33.1 | 34.6 | 36.0 | 37.5 |
| 74 | 24.8 | 26.4 | 28.0 | 29.5 | 31.1 | 32.6 | 34.1 | 35.6 | 37.1 |
| 75 | 24.3 | 25.9 | 27.5 | 29.1 | 30.6 | 32.2 | 33.7 | 35.2 | 36.7 |
| 76 | 23.8 | 25.4 | 27.0 | 28.6 | 30.1 | 31.7 | 33.3 | 34.8 | 36.3 |
| 77 | 23.2 | 24.9 | 26.5 | 28.1 | 29.7 | 31.3 | 32.8 | 34.4 | 36.0 |
| 78 | 22.7 | 24.4 | 26.0 | 27.6 | 29.2 | 30.8 | 32.4 | 34.0 | 35.6 |
| 79 | 22.2 | 23.8 | 25.5 | 27.1 | 28.8 | 30.4 | 32.0 | 33.6 | 35.2 |
| 80 | 21.7 | 23.3 | 25.0 | 26.7 | 28.3 | 29.9 | 31.6 | 33.2 | 34.8 |
| 81 | 21.1 | 22.8 | 24.5 | 26.2 | 27.9 | 29.5 | 31.2 | 32.8 | 34.4 |
| 82 | 20.6 | 22.3 | 24.0 | 25.7 | 27.4 | 29.1 | 30.7 | 32.4 | 34.0 |
| 83 | 20.1 | 21.8 | 23.5 | 25.2 | 26.9 | 28.6 | 30.3 | 32.0 | 33.7 |
| 84 | 19.6 | 21.3 | 23.0 | 24.8 | 26.5 | 28.2 | 29.9 | 31.6 | 33.3 |
| 85 | 19.0 | 20.8 | 22.6 | 24.3 | 26.0 | 27.8 | 29.5 | 31.2 | 32.9 |
| 86 | 18.5 | 20.3 | 22.1 | 23.8 | 25.6 | 27.3 | 29.1 | 30.8 | 32.6 |
| 87 | 18.0 | 19.8 | 21.6 | 23.4 | 25.1 | 26.9 | 28.7 | 30.4 | 32.2 |
| 88 | 17.5 | 19.3 | 21.1 | 22.9 | 24.7 | 26.5 | 28.3 | 30.1 | 31.8 |
| 89 | 17.0 | 18.8 | 20.6 | 22.4 | 24.3 | 26.1 | 27.9 | 29.7 | 31.5 |
| 90 | 16.5 | 18.3 | 20.1 | 22.0 | 23.8 | 25.6 | 27.5 | 29.3 | 31.1 |
| 91 | 15.9 | 17.8 | 19.7 | 21.5 | 23.4 | 25.2 | 27.1 | 28.9 | 30.8 |
| 92 | 15.4 | 17.3 | 19.2 | 21.1 | 22.9 | 24.8 | 26.7 | 28.5 | 30.4 |
| 93 | 14.9 | 16.8 | 18.7 | 20.6 | 22.5 | 24.4 | 26.3 | 28.2 | 30.1 |
| 94 | 14.4 | 16.3 | 18.2 | 20.2 | 22.1 | 24.0 | 25.9 | 27.8 | 29.7 |
| 95 | 13.9 | 15.8 | 17.8 | 19.7 | 21.6 | 23.6 | 25.5 | 27.4 | 29.4 |
| 96 | 13.4 | 15.3 | 17.3 | 19.2 | 21.2 | 23.2 | 25.1 | 27.1 | 29.0 |
| 97 | 12.9 | 14.9 | 16.8 | 18.8 | 20.8 | 22.7 | 24.7 | 26.7 | 28.7 |
| 98 | 12.4 | 14.4 | 16.4 | 18.3 | 20.3 | 22.3 | 24.3 | 26.3 | 28.3 |
| 99 | 11.9 | 13.9 | 15.9 | 17.9 | 19.9 | 21.9 | 24.0 | 26.0 | 28.0 |
| 100 | 11.4 | 13.4 | 15.4 | 17.5 | 19.5 | 21.5 | 23.6 | 25.6 | 27.7 |
| 101 | 10.9 | 12.9 | 15.0 | 17.0 | 19.1 | 21.1 | 23.2 | 25.3 | 27.3 |
| 102 | 10.4 | 12.4 | 14.5 | 16.6 | 18.6 | 20.7 | 22.8 | 24.9 | 27.0 |
| 103 | 9.9 | 11.9 | 14.0 | 16.1 | 18.2 | 20.3 | 22.4 | 24.5 | 26.7 |
| 104 | 9.3 | 11.5 | 13.6 | 15.7 | 17.8 | 19.9 | 22.1 | 24.2 | 26.3 |
| 105 | 8.8 | 11.0 | 13.1 | 15.2 | 17.4 | 19.5 | 21.7 | 23.8 | 26.0 |
| 106 | 8.3 | 10.5 | 12.6 | 14.8 | 17.0 | 19.1 | 21.3 | 23.5 | 25.7 |
| 107 | 7.9 | 10.0 | 12.2 | 14.4 | 16.6 | 18.7 | 21.0 | 23.2 | 25.4 |
| 108 | 7.4 | 9.5 | 11.7 | 13.9 | 16.1 | 18.4 | 20.6 | 22.8 | 25.1 |
| 109 | 6.9 | 9.1 | 11.3 | 13.5 | 15.7 | 18.0 | 20.2 | 22.5 | 24.7 |
| 110 | 6.4 | 8.6 | 10.8 | 13.1 | 15.3 | 17.6 | 19.9 | 22.1 | 24.4 |
| 111 | 5.9 | 8.1 | 10.4 | 12.6 | 14.9 | 17.2 | 19.5 | 21.8 | 24.1 |
| 112 | 5.4 | 7.6 | 9.9 | 12.2 | 14.5 | 16.8 | 19.1 | 21.5 | 23.8 |
| 113 | – | 7.2 | 9.5 | 11.8 | 14.1 | 16.4 | 18.8 | 21.1 | 23.5 |
| 114 | – | 6.7 | 9.0 | 11.4 | 13.7 | 16.1 | 18.4 | 20.8 | 23.2 |
| 115 | – | 6.2 | 8.6 | 10.9 | 13.3 | 15.7 | 18.1 | 20.5 | 22.9 |
Target superheat in °F (suction-line temperature minus evaporator saturation temperature). Rows: outdoor air entering the condenser, dry-bulb. Columns: return air entering the evaporator, wet-bulb. A dash means the target is below 5 °F; the CEC says a valid charge check cannot be made then (warm the house or retest another time). Source: California Energy Commission, 2025 Reference Appendices, Table RA3.2-2; the 2022 edition prints the same values.
Manufacturer chart: Trane fixed-orifice superheat table
From Trane's installation manual for the Runtru A5AC3 condensing units (R-454B, models 018B to 060B; the A5AC4 manual prints the same table). Trane: measure return-air wet-bulb at the unit just before the coil and the outdoor dry-bulb; add refrigerant to lower superheat, remove it to raise superheat; do not charge outside the table.
Indoor wet-bulb 50–59 °F
| Outdoor DB (°F) | WB 50 °F | WB 51 °F | WB 52 °F | WB 53 °F | WB 54 °F | WB 55 °F | WB 56 °F | WB 57 °F | WB 58 °F | WB 59 °F |
|---|---|---|---|---|---|---|---|---|---|---|
| 55 | 7 | 9 | 10 | 11 | 12 | 14 | 15 | 17 | 18 | 20 |
| 60 | 5 | 7 | 8 | 9 | 10 | 12 | 13 | 15 | 16 | 18 |
| 65 | – | – | 4 | 6 | 8 | 10 | 11 | 13 | 14 | 16 |
| 70 | – | – | – | – | 5 | 7 | 8 | 10 | 11 | 13 |
| 75 | – | – | – | – | – | – | 5 | 6 | 7 | 9 |
| 80 | – | – | – | – | – | – | – | – | 4 | 6 |
| 85 | – | – | – | – | – | – | – | – | – | – |
| 90 | – | – | – | – | – | – | – | – | – | – |
| 95 | – | – | – | – | – | – | – | – | – | – |
| 100 | – | – | – | – | – | – | – | – | – | – |
| 105 | – | – | – | – | – | – | – | – | – | – |
| 110 | – | – | – | – | – | – | – | – | – | – |
| 115 | – | – | – | – | – | – | – | – | – | – |
Indoor wet-bulb 60–69 °F
| Outdoor DB (°F) | WB 60 °F | WB 61 °F | WB 62 °F | WB 63 °F | WB 64 °F | WB 65 °F | WB 66 °F | WB 67 °F | WB 68 °F | WB 69 °F |
|---|---|---|---|---|---|---|---|---|---|---|
| 55 | 21 | 23 | 24 | 26 | 27 | 29 | 30 | – | – | – |
| 60 | 19 | 21 | 22 | 24 | 25 | 27 | 28 | 30 | 31 | – |
| 65 | 17 | 18 | 19 | 21 | 22 | 24 | 25 | 27 | 28 | 27* |
| 70 | 14 | 16 | 17 | 18 | 19 | 21 | 22 | 24 | 25 | 27 |
| 75 | 10 | 12 | 14 | 16 | 18 | 19 | 21 | 22 | 24 | 26 |
| 80 | 7 | 9 | 10 | 11 | 12 | 14 | 16 | 18 | 19 | 21 |
| 85 | 4 | 6 | 7 | 9 | 10 | 13 | 14 | 16 | 18 | 20 |
| 90 | – | – | 4 | 6 | 8 | 10 | 11 | 13 | 14 | 16 |
| 95 | – | – | – | – | 4 | 6 | 8 | 10 | 13 | 14 |
| 100 | – | – | – | – | – | – | 6 | 8 | 10 | 12 |
| 105 | – | – | – | – | – | – | 4 | 6 | 7 | 9 |
| 110 | – | – | – | – | – | – | – | – | 4 | 7 |
| 115 | – | – | – | – | – | – | – | – | – | – |
Indoor wet-bulb 70–78 °F
| Outdoor DB (°F) | WB 70 °F | WB 71 °F | WB 72 °F | WB 73 °F | WB 74 °F | WB 75 °F | WB 76 °F | WB 77 °F | WB 78 °F |
|---|---|---|---|---|---|---|---|---|---|
| 55 | – | – | – | – | – | – | – | – | – |
| 60 | – | – | – | – | – | – | – | – | – |
| 65 | 31 | – | – | – | – | – | – | – | – |
| 70 | 28 | 30 | 31 | – | – | – | – | – | – |
| 75 | 28 | 29 | 31 | 32 | – | – | – | – | – |
| 80 | 23 | 25 | 26 | 28 | 29 | 31 | 33 | – | – |
| 85 | 21 | 23 | 24 | 26 | 28 | 29 | 30 | 31 | 32 |
| 90 | 18 | 20 | 22 | 24 | 25 | 27 | 28 | 30 | 31 |
| 95 | 16 | 18 | 20 | 22 | 23 | 25 | 26 | 28 | 29 |
| 100 | 13 | 16 | 18 | 20 | 21 | 23 | 25 | 27 | 29 |
| 105 | 11 | 13 | 15 | 18 | 20 | 22 | 24 | 26 | 28 |
| 110 | 9 | 11 | 13 | 16 | 18 | 21 | 23 | 26 | 28 |
| 115 | 6 | 9 | 12 | 14 | 16 | 19 | 21 | 24 | 26 |
Target superheat in °F, as printed in Trane's Table 10 (whole degrees, outdoor dry-bulb in 5 °F steps). A dash means outside the table: Trane says not to charge by superheat under those conditions. * At 65 °F outdoor and 69 °F wet-bulb the manual prints 27, lower than the 28 beside it while every other row rises steadily, so it is probably a misprint; it is shown as printed.
Where the two tables differ
| Outdoor DB (°F) | Return WB (°F) | CEC target (°F) | Trane target (°F) | CEC minus Trane (°F) |
|---|---|---|---|---|
| 75 | 60 | 12.4 | 10 | +2.4 |
| 75 | 63 | 16.7 | 16 | +0.7 |
| 75 | 67 | 22.7 | 22 | +0.7 |
| 85 | 63 | 10.3 | 9 | +1.3 |
| 85 | 67 | 17.3 | 16 | +1.3 |
| 85 | 70 | 22.6 | 21 | +1.6 |
| 95 | 67 | 12.0 | 10 | +2.0 |
| 95 | 70 | 17.8 | 16 | +1.8 |
| 105 | 70 | 13.1 | 11 | +2.1 |
How to read the difference
- Across the conditions above the CEC target runs 0.7 to 2.4 °F above Trane's, inside the CEC installer band of ±5 °F. Both tables have the same shape: the target rises with indoor wet-bulb and falls as the outdoor temperature rises.
- The equipment manufacturer's chart governs for its own units. The CEC table is the California code reference and the fallback when no maker's chart is available.
Pass/fail tolerances (CEC Table RA3.2-1)
| Metering device | Who tests | Test conditions (°F) | Pass criterion (°F) |
|---|---|---|---|
| Fixed metering device | Installer | 55 °F ≤ outdoor dry-bulb ≤ 115 °F; return dry-bulb ≥ 70 °F; return wet-bulb ≤ 76 °F | Superheat within ±5 °F of the target |
| Fixed metering device | Rater (HERS / ECC) | 55 °F ≤ outdoor dry-bulb ≤ 115 °F; return dry-bulb ≥ 70 °F; return wet-bulb ≤ 76 °F | Superheat within ±8 °F of the target |
| TXV / EEV | Installer | 55 °F ≤ outdoor dry-bulb ≤ 120 °F; return dry-bulb ≥ 70 °F | Subcooling within ±3 °F of the manufacturer target; superheat meets the manufacturer spec or 4–25 °F |
| TXV / EEV | Rater (HERS / ECC) | 55 °F ≤ outdoor dry-bulb ≤ 120 °F; return dry-bulb ≥ 70 °F | Subcooling within ±6 °F of the manufacturer target and at least 2 °F; superheat meets the manufacturer spec or 3–26 °F |
TXV systems: charging by subcooling (Trane R-454B chart)
For a TXV or EEV indoor unit, Trane says subcooling is the only recommended charging method above 55 °F outdoors. Take the design subcooling from the nameplate or the service facts (10 °F for every A5AC3 size, per Trane's product data), correct it for line length and lift as the manual's tables direct, then adjust the charge until the liquid gauge pressure matches the chart for the measured liquid-line temperature. Add refrigerant if the pressure is below the chart value; recover refrigerant if above (Trane). Other refrigerants and other makers have their own charts.
R-454B liquid gauge pressure (psig) by liquid-line temperature and design subcooling (SC)
| Liquid line (°F) | 8 °F SC (psig) | 9 °F SC (psig) | 10 °F SC (psig) | 11 °F SC (psig) | 12 °F SC (psig) | 13 °F SC (psig) | 14 °F SC (psig) |
|---|---|---|---|---|---|---|---|
| 55 | 170 | 172 | 175 | 178 | 181 | 184 | 187 |
| 60 | 184 | 187 | 190 | 194 | 197 | 200 | 203 |
| 65 | 200 | 203 | 206 | 210 | 213 | 217 | 220 |
| 70 | 217 | 220 | 223 | 227 | 230 | 234 | 238 |
| 75 | 234 | 238 | 241 | 245 | 249 | 252 | 256 |
| 80 | 252 | 256 | 260 | 264 | 268 | 272 | 276 |
| 85 | 272 | 276 | 280 | 284 | 288 | 292 | 297 |
| 90 | 292 | 297 | 301 | 305 | 309 | 314 | 318 |
| 95 | 314 | 318 | 323 | 327 | 332 | 336 | 341 |
| 100 | 336 | 341 | 346 | 351 | 355 | 360 | 365 |
| 105 | 360 | 365 | 370 | 375 | 380 | 385 | 390 |
| 110 | 385 | 390 | 396 | 401 | 406 | 412 | 417 |
| 115 | 412 | 417 | 422 | 428 | 433 | 439 | 445 |
| 120 | 439 | 445 | 450 | 456 | 462 | 468 | 474 |
| 125 | 468 | 474 | 480 | 486 | 492 | 498 | 504 |
Source: Trane, Runtru A5AC3 IOM, Table 9 (R-454B). Find the row for the measured liquid-line temperature and the column for the unit's design subcooling; the system is charged when the liquid gauge reads that pressure. Each pressure is the R-454B bubble-point pressure at liquid temperature + subcooling (for example 95 °F + 10 °F = 105 °F at 323 psig); Trane says to use the bubble point when calculating subcooling.
Safety and certification
- EPA Section 608 (US): EPA regulations at 40 CFR Part 82, Subpart F require technicians who maintain, service, repair or dispose of equipment that could release refrigerants to be certified. A "technician" includes anyone who attaches or detaches gauges to measure pressure, or adds or removes refrigerant. Certification types: Type I (small appliances), Type II (high- or very high-pressure appliances, except small appliances and car A/C), Type III (low-pressure appliances) and Universal (all types); credentials do not expire. Apprentices are exempt only while closely supervised by a certified technician.
- No venting: the same rules prohibit intentionally venting refrigerants (ozone-depleting substances or their substitutes) while servicing; only de minimis releases during good-faith recovery, normal-operation leaks, and specific exempted substitutes are allowed (EPA).
- A2L refrigerants: R-454B is mildly flammable; Trane's manual carries a "Risk of Fire – Flammable Refrigerant" warning and requires approved equipment and procedures for it.
- Pressures in a running system are high (Trane's R-454B chart reaches 504 psig). Wear eye and hand protection and follow the equipment maker's instructions; local codes and the manufacturer's data govern.
Sources
- California Energy Commission, 2025 Reference Appendices for the 2025 Building Energy Efficiency Standards (CEC-400-2025-010-AP), Residential Appendix RA3.2: Table RA3.2-1 and Table RA3.2-2 Target Superheat, 2025 (energy.ca.gov/sites/default/files/2025-07/CEC-400-2025-010-AP.pdf). Used for: the full target-superheat table (55-115 F outdoor, 50-76 F wet-bulb), installer and rater tolerances, measurement steps.
- California Energy Commission, 2022 Reference Appendices (CEC-400-2022-010-AP), Table RA3.2-2, 2022 (energy.ca.gov/sites/default/files/2022-08/CEC-400-2022-010-AP.pdf). Used for: cross-check: every value agrees with the 2025 edition (the 2022 PDF prints many cells cut off after the decimal point).
- Trane, Installation, Operation, and Maintenance: Condensing Units A5AC3018B-A5AC3060B (Runtru by Trane, R-454B), ODR-SVX001C-EN, Table 10 Fixed orifice superheat charging table and Table 9 R-454B charging chart, January 2026 (runtruhvac.com/assets/pdf/shared-new-docs/air-conditioners/A5AC3-Air-Conditioner/A5AC3-Installation-and-Operation-Manual.pdf). Used for: the manufacturer superheat table (55-115 F outdoor in 5 F steps, 50-78 F wet-bulb) and the R-454B subcooling chart; the A5AC4 manual prints the identical Table 10.
- Trane, Product and Submittal Data: Split System Air Conditioner A5AC3 (Runtru by Trane), ODR-PSD030C-EN, January 2026 (runtruhvac.com/assets/pdf/shared-new-docs/air-conditioners/A5AC3-Air-Conditioner/A5AC3-Product-and-Submittal-Data.pdf). Used for: design subcooling 10 F for every A5AC3 size.
- NIST Chemistry WebBook, SRD 69, Thermophysical Properties of Fluid Systems, saturation table for chlorodifluoromethane (R-22), downloaded 2026-10-06: webbook.nist.gov/chemistry/fluid. Used for: the R-22 example (83.230 psia saturates at 39.983 °F).
- U.S. EPA, Section 608 Technician Certification Requirements (updated July 15, 2026) and Stationary Refrigeration – Prohibition on Venting Refrigerants (updated December 8, 2025): epa.gov/section608.
Frequently asked questions
What should superheat be on a fixed-orifice (piston) air conditioner?
It depends on the indoor return-air wet-bulb and the outdoor dry-bulb temperature, so read it from a charging table. At 85 °F outdoor and 63 °F wet-bulb the California Energy Commission table gives 10.3 °F and Trane's chart 9 °F; at 95 °F outdoor and 67 °F wet-bulb they give 12.0 °F and 10 °F. Use the equipment maker's chart where one exists; the CEC lets installers pass within ±5 °F of the target.
What should subcooling be on a TXV system?
Whatever the manufacturer specifies on the unit nameplate or service data; there is no single universal number. For example, Trane lists 10 °F design subcooling for every size of its Runtru A5AC3 condensers. California's Title 24 check passes an installer within ±3 °F of the manufacturer target.
How do you calculate superheat?
Superheat = suction-line temperature minus the saturation temperature for the suction pressure. Example with R-22: a suction gauge reading of 68.5 psig (83.2 psia) is a saturation temperature of 40 °F (NIST), so a suction line at 52 °F has 12 °F of superheat. Measure the suction line near the outdoor unit's service valve after at least 15 minutes of running.
How do you calculate subcooling?
Subcooling = saturation temperature for the liquid-line pressure minus the liquid-line temperature. Example with R-454B from Trane's chart: 323 psig on the liquid line is a bubble point of 105 °F, so a liquid line at 95 °F has 10 °F of subcooling. For blends such as R-454B, Trane says to use the bubble point.
Do I need a license to check or add refrigerant?
In the US, yes. EPA rules under Clean Air Act Section 608 (40 CFR Part 82, Subpart F) define a technician as anyone who attaches gauges to an appliance or adds or removes refrigerant, and require technicians to pass an EPA-approved test for the type of equipment: Type II covers high- and very high-pressure appliances other than small appliances and car A/C, and Universal covers all types. Intentionally venting refrigerant is prohibited.