How To Measure Subcooling? Easy 9 Step-By-Step Guide

Let’s learn exactly how to measure subcooling. Measuring subcooling follows a very similar procedure as measuring superheat (detailed here) but with important differences. We are still going to need 3 key tools; a manifold gauge (red high side part), a clamp-on thermometer, and the easiest subtraction math.

Subcooling is measured at the high side liquid line. In most basic terms, subcooling measures by how many degrees a liquid refrigerant is cooled below the saturation temperature. All of this subcooling is happening in the 2nd part of the condenser coils (outdoor unit). You can read all about subcooling and superheat here.

Let’s look at one example that will help illustrate what we need to measure to calculate subcooling:

Suppose you have a 3-ton 16 SEER AC unit that runs on R-134A refrigerant. Here are the 2 temperatures we need to measure for subcooling:

  • We used a manifold gauge (high side red part) with listed R-134A temperatures. When connected to the liquid line service port (this is where we measure subcooling), we measured 35 psi pressure. Checking the gauge meter for R-134A refrigerant, we can read that the temperature is 40°F.
  • We used a clamp-on digital thermometer on a suction line and measured 34°F temperature.

From these 2 temperature measurements, we can calculate the subcooling using this equation (this is the ‘easiest subtraction math’ part):

Subcooling = TGauge – TCramp-On Thermometer

Now we just insert our two temperature measurements (34°F and 40°F) and calculate subcooling like this:

Subcooling = 40°F – 34°F = 6°F

We see that subcooling is 6°F. That means that the refrigerant liquid in the discharge line (smaller liquid line) is 6°F below the R-134A saturation temperature in our system.

Alright, here we can clearly see that we only need to measure 2 temperatures in order to measure subcooling.

how to measure subcooling
For subcooling, we are measuring the temperature of liquid line with the red gauge, and the saturation temperatured with a clamp-on thermometer at the suction line.

Here is the step-by-step guide on how to get these 2 temperatures and measure / calculate subcooling:

How To Measure Subcooling Step-By-Step

Subcooling is measured at the outdoor unit. You will need an HVAC gauge (we’ll only be using the red gauge) and a clamp-on thermometer. Here’s the step-by-step guide:

  1. Prior to subcooling measurement, the AC unit should be off for at least 30 minutes. This will equalize the refrigerant pressure throughout the unit (lines, coils) since the refrigerant is in a saturated state (mixture of vapor and liquid).
  2. Let the air out of the HVAC gauge. Open the vent, listen to that “psssss” air makes when it is pushed out of the gauge, and screw the vent tightly. This is a necessary step; the air inside the gauge is sucked backed into the liquid line and that’s exactly what we want to avoid by letting the air out.
  3. Connect the red HVAC gauge to the smaller high side discharge line’s service port. If you are at the outdoor unit, you will notice 2 lines coming from the unit. For subcooling, we need to measure the temperature of the liquid line (also called the discharge line). You will find a discharge line service port; unscrew it, and screw the red gauge line on it. This will give us that 40°F measurement in the example above.
  4. Attach the clamp-on thermometer to the smaller discharge line. This line is the 2nd line coming from the outdoor unit; it has a bit smaller diameter than the bigger suction line. Here we will measure that 34°F measurement from the example above.
  5. When the gauge and thermometer are attached, start the AC unit in cooling mode and wait for about 15 minutes. You should be seeing the pressure and temperature in the red gauge decrease. Wait until the temperature of the clamp-on thermometer stabilizes.
  6. Read the red gauge pressure. If you are using a gauge with a temperature meter for your specific freon, you should also be able to read the temperature. Note the gauge temperature down.
  7. Read and note the temperature measured by the clamp-on thermometer.
  8. Once you have both temperatures, just subtract the temperature on the thermometer from the gauge temperature.
  9. The result is the subcooling temperature.

That’s it.

Let’s do one example with R-410A refrigerant:

How To Measure R-410A Subcooling

Now let’s say we have a 4-ton 18 SEER unit that uses R-410A freon. As we have seen, we have to make these 2 temperature measurements:

  1. With the red gauge on the discharge line port, we measure the 102 psi pressure. Looking at the meter for an R-410A on the gauge, we can see this correlates to about 32°F temperature.
  2. With the thermometer clamped on the smaller discharge line, we measure the temperature of 25°F.

Now, we can simply calculate the R-410A subcooling in this AC unit by 32°F – 25°F subtraction. The subcooling is 7°F.

Hopefully, you can now measure subcooling for any AC unit yourself. If you have any questions about how to measure subcooling, you can use the comment section below, and we can help you.

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