Do you know what’s easy? Calculating subcooling. It’s a piece-of-cake calculation. We are going to explain exactly how to calculate subcooling in much the same manner as we explained superheat calculation with R-22, R-410A, and R-134A examples here.
In this article, you will similarly get 3 key resources that will explain everything to know about subcooling calculation, including:
- Subcooling Formula. We only need to subtract the subcooled liquid refrigerant temperature from the saturated-state temperature.
- Subcooling Calculator. Here, you just insert 2 measured temperatures (gauge and thermometer temperatures), and the calculator will determine subcooling temperature automatically.
- 3 Subcooling Calculation Examples For R-22, R-410A, And R-134A. These are the 3 most commonly used refrigerants in HVAC systems today.
Alright, let’s first look at what subcooling actually is before we use a formula and calculator to quantify it:
Subcooling is the temperature decrease of liquid refrigerant that happens in the 2nd part of the condenser coils (outdoor unit). To calculate subcooling, we have to measure only these 2 temperatures (more about how to measure subcooling here; it’s a 9 step-by-step process):
- Temperature of the liquid refrigerant in the high side discharge line (this temperature decreases since liquid can be cooled below saturation temperature point).
- Temperature of the saturated refrigerant (liquid and vapor mixture; this temperature is quite constant).
Let’s start with the general subcooling formula and introduce the practical subcooling formula we use in HVAC for AC diagnostics (you can check why we measure subcooling and superheat here):
To calculate subcooling, we use this general formula:
Subcooling = Tsaturated refrigerant – Tliquid after condenser coil exit
Here we calculate subcooling by subtracting the temperature of the liquid refrigerant after the condenser coil exits from the saturated refrigerant temperature we find at the beginning of the condenser coil.
In practice, we use an HVAC manifold gauge (red part, attached to the discharge line service port) to measure the saturated refrigerant temperature.
To measure the temperature of the subcooled refrigerant, we usually use a clamp-on thermometer attached to the liquid line. That’s why we use this more practical subcooling formula:
Subcooling = TGauge – TCramp-On Thermometer
Quick Example: Let’s say we have a 4-ton 16 SEER AC unit that uses R-22 refrigerant. With a manifold gauge (and via R-22 PT chart), we measure the saturated refrigerant temperature of 89°F. The clamp-on thermometer reads 77°F. Here is how we use the subcooling formula to manually calculate the subcooling in this system:
Subcooling (R-22) = 89°F – 77°F = 12°F
We see that the calculated subcooling for R-22 refrigerant in this system is 12°F.
This is the manual calculation. You can freely use this online subcooling calculator that will determine the subcooling immediately (we are also going to use in the R-22, R-410A, and R-134A subcooling calculation examples below to calculator):
We can use the R-22 example above to illustrate how this works. You slide the 1st slider to ’89’ and the 2nd slider to ’77’ and get the result:
The calculated subcooling for this R-22 AC unit is 12°F.
Let’s solve another R-22 example and show how to calculate subcooling for R-410A and R-134A as well:
R-22 Subcooling Calculation (1st Example)
Alright, at different outdoor temperatures and for different air conditioners (Lennox, Carrier, etc.), you will almost always have different subcooling temperatures.
Let’s take a very efficient 3-ton 20 SEER AC unit that runs on R-22 refrigerant. We attach our manifold gauge and the thermometer to the discharge line and service port. The manifold gauge gives a 93°F temperature reading, and the digital thermometer gives us 87°F reading. Here is how we can calculate R-22 subcooling for this system:
Subcooling (R-22) = 99°F – 87°F = 6°F
We see that in this system, the R-22 subcooling is equal to 6°F.
R-410A Subcooling Calculation (2nd Example)
Due to the R-22 phase-out, the R-410A is becoming increasingly more popular.
How to calculate R-410A subcooling? Alright, let’s take a 3.5-ton 18 SEER unit, attach the gauge and thermometer. We measure that the temperature of the saturated refrigerant is 95°F (via pressure and referencing R-410A PT chart) and the thermometer tells us that the subcooled R-410A liquid temperature is 87°F. Let’s calculate R-410A subcooling:
Subcooling (R-410A) = 95°F – 87°F = 8°F
In this system, we have 8°F R-410A subcooling.
R-134A Subcooling Calculation (3rd Example)
The subcooling calculation procedure is the same for R-134A as well.
Suppose we have a 5-ton 16 SEER unit, charged with R-134A refrigerant. The attached manifold charge tells us that the saturated refrigerant within the condenser has an 88°F temperature. The subcooled R-134A liquid freon temperature, as read by a digital or IR thermometer, is 82°F. Let’s calculate R-134A subcooling:
Subcooling (R-134A) = 88°F – 82°F = 6°F
We see that in this system, we have 6°F R-134A subcooling.
With this subcooling formula and calculator, you can pretty much calculate subcooling for any refrigerant. The R-22, R-410A, and R-134A subcooling calculation examples are here to help you out. If you need a bit of help, you can always use the comment section below, gives us some figures, and we can help you out.