Calculate superheat and subcooling from your gauge readings for R410A, R454B, R32, R22, R134a, R407C, R404A, R290 and R1234yf. The calculator uses the correct dew and bubble temperatures for blends, gives the target superheat for fixed-orifice systems, and tells you whether the charge looks low, high or right.
Always follow the equipment manufacturer's charging chart where one is provided. Let the system run 15 minutes before taking readings, and check airflow and coil cleanliness before adjusting the charge.
| Reading | Pressure | Dew (vapour) | Bubble (liquid) | Used |
|---|
Superheat = suction line temperature − saturation (dew) temperature at suction pressure
Subcooling = saturation (bubble) temperature at liquid pressure − liquid line temperature
Superheat tells you how far the vapour leaving the evaporator is above its boiling point, which shows whether the evaporator is being fed the right amount of refrigerant. Subcooling tells you how far the liquid leaving the condenser is below its condensing point, which shows how much liquid is stacked in the condenser. Together they are the standard way to check the refrigerant charge of an air conditioner or heat pump.
Example (R-410A): 118 psig suction gives a dew temperature of 39.8 °F. The suction line reads 52 °F, so superheat = 52 − 39.8 = 12.2 °F. With 63 °F indoor wet bulb and 85 °F outdoors, the target superheat for a fixed-orifice system is (3 × 63 − 80 − 85) ÷ 2 = 12 °F, so the charge is right.
Read the liquid line pressure at the liquid service valve and clamp a thermometer next to it. Convert the pressure to the bubble temperature and subtract the line temperature. Example (R-410A): 317 psig gives a bubble temperature of 99.7 °F, the line reads 90 °F, so subcooling = 99.7 − 90 = 9.7 °F.
For systems with a fixed orifice, piston or capillary tube, the correct superheat depends on the indoor heat load and the outdoor temperature. A widely used approximation of the manufacturers' charts is:
| Indoor WB ↓ / Outdoor DB → | 65 °F | 70 °F | 75 °F | 80 °F | 85 °F | 90 °F | 95 °F | 100 °F | 105 °F | 110 °F | 115 °F |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 56 °F | 12 | 9 | 6 | — | — | — | — | — | — | — | — |
| 58 °F | 14 | 12 | 10 | 7 | — | — | — | — | — | — | — |
| 60 °F | 18 | 15 | 12 | 10 | 8 | 5 | — | — | — | — | — |
| 62 °F | 20 | 18 | 16 | 13 | 10 | 8 | 6 | — | — | — | — |
| 63 °F | 22 | 20 | 17 | 14 | 12 | 10 | 7 | — | — | — | — |
| 64 °F | 24 | 21 | 18 | 16 | 14 | 11 | 8 | 6 | — | — | — |
| 66 °F | 26 | 24 | 22 | 19 | 16 | 14 | 12 | 9 | 6 | — | — |
| 68 °F | 30 | 27 | 24 | 22 | 20 | 17 | 14 | 12 | 10 | 7 | — |
| 70 °F | 32 | 30 | 28 | 25 | 22 | 20 | 18 | 15 | 12 | 10 | 8 |
| 72 °F | 36 | 33 | 30 | 28 | 26 | 23 | 20 | 18 | 16 | 13 | 10 |
| 74 °F | 38 | 36 | 34 | 31 | 28 | 26 | 24 | 21 | 18 | 16 | 14 |
"—" means the target is below 5 °F: do not add refrigerant under these conditions. Measure the indoor wet bulb in the return air and the outdoor dry bulb at the condenser air inlet. Where the unit has its own charging chart, use that instead.
Systems with a TXV or electronic expansion valve are charged by subcooling, because the valve holds superheat roughly steady whatever the charge. Choose "TXV or EEV" above, enter the target subcooling from the outdoor unit's nameplate (often 8–12 °F), and the liquid line pressure and temperature. Add refrigerant if subcooling is low, recover if it is high, and allow 10–15 minutes for the system to settle after each change.
The liquid line temperature that gives each subcooling value at common liquid line pressures:
| Liquid pressure | Bubble temp. | Line temp. for 8 °F SC | for 10 °F SC | for 12 °F SC |
|---|---|---|---|---|
| 225 psig | 76.8 °F | 69 °F | 67 °F | 65 °F |
| 250 psig | 83.6 °F | 76 °F | 74 °F | 72 °F |
| 275 psig | 89.9 °F | 82 °F | 80 °F | 78 °F |
| 300 psig | 95.8 °F | 88 °F | 86 °F | 84 °F |
| 325 psig | 101.4 °F | 93 °F | 91 °F | 89 °F |
| 350 psig | 106.7 °F | 99 °F | 97 °F | 95 °F |
| 375 psig | 111.7 °F | 104 °F | 102 °F | 100 °F |
| 400 psig | 116.4 °F | 108 °F | 106 °F | 104 °F |
| 425 psig | 121.0 °F | 113 °F | 111 °F | 109 °F |
| 450 psig | 125.3 °F | 117 °F | 115 °F | 113 °F |
| Liquid pressure | Bubble temp. | Line temp. for 8 °F SC | for 10 °F SC | for 12 °F SC |
|---|---|---|---|---|
| 225 psig | 80.3 °F | 72 °F | 70 °F | 68 °F |
| 250 psig | 87.2 °F | 79 °F | 77 °F | 75 °F |
| 275 psig | 93.6 °F | 86 °F | 84 °F | 82 °F |
| 300 psig | 99.6 °F | 92 °F | 90 °F | 88 °F |
| 325 psig | 105.3 °F | 97 °F | 95 °F | 93 °F |
| 350 psig | 110.6 °F | 103 °F | 101 °F | 99 °F |
| 375 psig | 115.7 °F | 108 °F | 106 °F | 104 °F |
| 400 psig | 120.6 °F | 113 °F | 111 °F | 109 °F |
| 425 psig | 125.2 °F | 117 °F | 115 °F | 113 °F |
R-454B uses the bubble temperature for subcooling. Its pressures run about 4–5% below R-410A at the same temperature, so never use an R-410A chart on an R-454B system.
| System | Charge by | Typical superheat | Typical subcooling |
|---|---|---|---|
| Fixed orifice / piston | Superheat | Target from chart, about 5–20 °F (3–11 K) | about 5–15 °F (3–8 K) |
| TXV / EEV | Subcooling | about 8–12 °F (4–7 K), held by the valve | Nameplate value, often 8–12 °F (4–7 K) |
| Superheat | Subcooling | Most likely cause |
|---|---|---|
| High | Low | Low refrigerant charge (find the leak before adding) |
| Low | High | Overcharge |
| High | High | Restriction in the liquid line, filter drier or metering device; TXV underfeeding |
| Low | Low | Metering device overfeeding (TXV bulb loose or stuck open) or an inefficient compressor |
| Low | Normal | Low indoor airflow: dirty filter or coil, slow blower |
| Normal | High | Overcharge, or non-condensables; check condenser airflow and cleanliness |
Zeotropic blends boil and condense over a temperature range called glide. At the same pressure the liquid starts to boil at the bubble temperature and the last liquid disappears at the higher dew temperature. Superheat is measured on vapour, so it uses the dew temperature; subcooling is measured on liquid, so it uses the bubble temperature. R-454B has about 2 °F of glide, R-407C about 10 °F. R-410A's glide is so small that it is treated as a single value. The full tables are in the Refrigerant PT Chart.
Measure the suction pressure and the suction line temperature. Convert the pressure to the saturation temperature with a PT chart (the dew temperature for blends) and subtract it from the line temperature. Superheat = suction line temperature − saturation temperature.
Measure the liquid line pressure and temperature at the condensing unit. Convert the pressure to the saturation temperature (the bubble temperature for blends) and subtract the liquid line temperature from it.
On a TXV system superheat is usually 8 to 12 °F and the charge is set by subcooling. On a fixed-orifice system it depends on conditions: target superheat = (3 × indoor wet bulb − 80 − outdoor dry bulb) ÷ 2, typically between 5 and 20 °F.
Use the target on the outdoor unit's nameplate, usually 8 to 12 °F (4 to 7 K) for TXV systems. Charge until the measured subcooling is within about 3 °F of that target. Fixed-orifice systems typically run 5 to 15 °F but are charged by superheat.
Charge fixed-orifice, piston and capillary systems by superheat. Charge TXV and EEV systems by subcooling, because the valve holds superheat roughly constant whatever the charge.
The evaporator is starved of refrigerant. With low subcooling it usually means low charge; with high subcooling it points to a restriction in the liquid line, filter drier or metering device.
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