Subcooling calculator

How it works

Subcooling is the condensing saturation temperature minus the liquid line temperature. CEC Title 24 RA3.2 writes it as Actual Subcooling = T(condenser, sat) − T(liquid).

The tool converts the liquid line pressure to a saturation temperature with the PT data for the refrigerant you choose, then subtracts your line temperature. For blends it reads the bubble point (saturated liquid) column. That matches the Chemours PT guide, which marks saturated liquid as the value to “calculate Subcooling”, and Honeywell’s procedure: pressure and temperature at the condenser outlet, bubble column.

The inputs:

• Refrigerant: any of the 62 on the site.
• Liquid line pressure in psig.
• Liquid line temperature in °F, at the same point as the pressure.
• Target subcooling: the manufacturer’s value from the nameplate or installation manual. The tool has no built-in targets because they differ by unit.
• Elevation in feet, so a local gauge reading is converted to the sea-level basis the PT chart uses.

Worked example

The tool opens with R-410A at 340 psig and a 95°F liquid line.

1. In the R-410A data the bubble pressure is 337.11 psig at 104°F and 341.88 psig at 105°F.
2. Interpolate: 104 + (340 − 337.11) ÷ (341.88 − 337.11) = 104 + 2.89 ÷ 4.77 = 104.6°F bubble.
3. Subcooling = 104.6 − 95 = 9.6°F.

If the nameplate target were 10°F, the deviation is −0.4°F. That is inside ±3°F, so the charge passes.

On a blend, the wrong column changes the answer. Honeywell’s R-448A example uses 196 psig and a 75°F liquid line. Our R-448A data give a bubble point of 85.2°F at 196 psig, so subcooling is 85.2 − 75 = 10.2°F. The dew point at the same pressure is 94.8°F. Use it by mistake and you get 19.8°F, which looks like a heavy overcharge.

R-454B, the common A2L replacement for R-410A (see R-410A vs R-454B), has a smaller glide but the same rule applies. At 300 psig and a 92°F liquid line, the bubble point is 99.2°F, giving 7.2°F of subcooling. The dew point would give 9.9°F.

Reading the result

CEC’s tolerance for TXV and EEV systems is subcooling within ±3°F of the manufacturer’s target for the installer, and ±6°F (with at least 2°F of subcooling) for the HERS/ECC Rater. Test conditions: outdoor air between 55°F and 120°F, return air at 70°F dry bulb or warmer, and 15 minutes of running before measuring.

• Above target: points to an overcharge, or a restriction in the liquid line. CEC says to remove refrigerant. Recover in small amounts and recheck.
• Below target: points to an undercharge. Add refrigerant in small amounts. Charge blends as liquid (see the glide calculator for why).
• Negative subcooling: the line is warmer than saturation. That means flash gas in the liquid line or a bad reading. Recheck the refrigerant selection, the probe contact and the gauge.

After the subcooling is on target, CEC checks the metering device with superheat. The installer check passes if superheat meets the manufacturer’s spec or falls between 4°F and 25°F. Use the superheat calculator with the TXV/EEV setting.

Fixed-orifice systems are charged by superheat against a target, not by subcooling. Use the target superheat calculator for those.

Common mistakes

• Dew column on a blend. On zeotropic blends this overstates subcooling by about the glide. Bubble point is always the subcooling column.
• Guessing the target. Subcooling targets are set by the manufacturer for each unit. Use the nameplate or manual. If you added refrigerant for a long line set with the line set charge calculator, confirm the final charge by subcooling.
• Gauge tolerance. CEC requires liquid line gauges accurate to ±7.0 psi. On R-410A near 105°F, 1°F of saturation is about 4.8 psi, so that tolerance alone is worth about ±1.5°F.
• Probe placement. A clamp with poor pipe contact, or one placed far from the pressure port, gives a false line temperature. CEC also calls for a monthly ice-bath check of temperature sensors at 32°F.
• Not correcting for elevation at high-altitude jobs. See how to read a PT chart.