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How to Use the Subcooling Calculator
Select the refrigerant from the nameplate and say whether the system has a TXV or EEV or a fixed orifice. A TXV starts with a 10°F subcooling target, and you can replace it with the nameplate value.
Type the high-side pressure and the liquid line temperature near the condenser outlet. Choose psig, psia or bar, and degrees Fahrenheit or Celsius, and add the site elevation if it is high.
If the metering device is above the condenser, enter the vertical rise and any friction or drier pressure loss. The page works out the subcooling needed to arrive as liquid.
Check subcooling against the target, the margin over the rise and the pattern suggested. Add the project details and use PDF, CSV, Copy or Print.
Subcooling: liquid at the metering device
Subcooling is the number of degrees the liquid line sits below the temperature at which that liquid would start to boil at the same pressure. It matters for two reasons. It shows how much liquid the condenser is holding, which makes it the main charge indicator on a TXV system, and it shows whether the liquid will reach the metering device without turning back into vapour on the way. This page does both checks. It compares your reading with a target you set, and it works out how much subcooling a liquid line rise and its friction losses use up.
What the reading tells you
The condenser outlet is the last place the refrigerant is a mix of liquid and vapour. Below that point everything that matters is liquid, and subcooling is the margin it carries. With too little charge the condenser runs short of liquid, so subcooling falls to near zero, and a liquid line that is warmer than its bubble point gives a negative value, which means vapour is in the line. Because a persistent low reading can come from a slow leak, the refrigerant leak finder is the next step when the charge keeps dropping after you add refrigerant. In the example, R-410A at 360 psig has a bubble point of 108.7°F, and a liquid line at 100°F gives 8.7°F of subcooling.
Charging a TXV system by subcooling
ACHR News on charging methods explains why a TXV system is charged to subcooling: the valve holds superheat, so the liquid leaving the condenser is the quantity that moves with the charge. A low charge gives low subcooling and an overcharge gives high subcooling together with a high compression ratio. The article also notes that capacity rises 0.5% for each degree of subcooling at the same condensing pressure, and that a fixed-orifice system is charged by superheat instead. The target comes from the nameplate or the installation manual, and no universal number exists. The calculator fills in 10°F with a 2°F tolerance for a TXV only as a starting value, and it leaves the target blank for an orifice.
Subcooling needed for a liquid line that rises
A vertical liquid line costs pressure, and pressure is what keeps the liquid from boiling. The Professor column in ACHR News works an R-22 example with a 40 ft lift, which equates to a static loss of 20 psig, plus 1 psig for a drier and 2 psig for friction. Pressure falls from 211 to 188 psig, the saturation temperature falls from about 105°F to 97°F, and the liquid needs at least 8°F of subcooling. The calculator repeats this from the saturated liquid density of the refrigerant, which it takes from CoolProp, and its figures match: 105.1°F falling to 97.4°F, or 7.7°F needed. The need is not the same for every refrigerant. A 30 ft rise at a condensing temperature of 105°F needs 2.7°F on R-410A, 3.6°F on R-404A, 4.9°F on R-22 and 7.0°F on R-134a, because low-pressure refrigerants change temperature faster with pressure.
Friction, driers and valves
The rise is only part of the loss. A filter-drier, a solenoid valve, a sight glass and the pipe itself each take pressure, and that pressure comes out of the same subcooling. Enter their total in the friction field, in psi or bar to match the pressure unit you chose. If you have to estimate the pipe loss yourself, the Moody chart calculator gives the Darcy friction factor from the Reynolds number and the pipe roughness, and you can apply it to the density and velocity of the liquid refrigerant. The refrigerant maker's line-sizing tables are a quicker check. When the loss is uncertain, the ACHR column advises making sure there is enough subcooling to handle an extreme pressure drop. The usual remedy for a shortfall is to subcool the liquid before the rise, for example with a liquid-suction heat exchanger or a mechanical subcooler, both of which ACHR News names.
Bubble point, glide and the reading you trust
The calculator uses the bubble point because subcooling is measured from saturated liquid. R-410A and R-22 give almost the same answer from either point. R-407C does not: at 260 psig its bubble point is 109.9°F and its dew point 118.5°F, so a dew-point lookup would claim an extra 9°F of subcooling. Gauge error matters as well. A 3 psi error moves the bubble point of R-410A near 360 psig by 0.6°F, of R-22 near 211 psig by 1.0°F and of R-134a near 150 psig by 1.2°F. Read the pressure at the liquid line service valve, put the probe on clean bare copper and insulate it, and let the system settle before you record anything.
Frequently Asked Questions
There is no single number, so use the value on the unit's nameplate or in the installation manual. ACHR News explains that a TXV system is charged to the subcooling of the liquid leaving the condenser, and that a low charge gives low subcooling and an overcharge gives high subcooling. The calculator starts with 10°F and a 2°F tolerance as a placeholder that you replace. Subcooling that is correct on one model can be wrong on another, because the amount needed depends on the line set and the lift.
Enough to cover the pressure lost to the rise and to friction before the metering device. A liquid column loses about 0.5 psi per foot on R-22, and every 30 ft on R-22 at 105°F needs about 4.9°F of subcooling, against 2.7°F for R-410A and 7.0°F for R-134a at the same condensing temperature. In the example, a 40 ft rise and 3 psi of friction on R-22 at 211 psig need 7.7°F, and the system has 7.1°F, so the margin is negative.
Subcooling is the temperature below the point where the liquid would begin to boil, which is the bubble point. Blends with glide have a higher dew point at the same pressure. At 260 psig, R-407C has a bubble point of 109.9°F and a dew point of 118.5°F, so using the dew point would add about 9°F of subcooling that is not there. For R-410A at 360 psig the two points are 108.7°F and 108.9°F, so the choice hardly matters. The table shows both points for every refrigerant.
Both high together point to a restriction in the liquid line. A clogged drier, a partly closed valve or a kinked line starves the evaporator, so the vapour leaving it is more superheated, and it holds liquid back in the condenser, so subcooling rises. Adding refrigerant does not fix it. Compare the temperature on each side of the drier and the valves, and clear the restriction before you touch the charge.