Target Subcooling Chart? There Is No Chart, Here’s Why

One of our readers asked us if there is a target subcooling chart. Namely, we do have the most extensive target superheat chart here; shouldn’t there be a similar target subcooling chart as well?

Well, there is none. That’s because the target subcooling doesn’t significantly depend on the outdoor temperature (like the target superheat).

Example: 3-ton 16 SEER air conditioner using R-410A refrigerant has target subcooling of 11.8°F at 80°F outdoor temperature. At 100°F outdoor temperature, the target superheat rises to 12.1°F. This is a pretty insignificant increase.

Nonetheless, when measuring subcooling (here’s how to do it in 9 steps), we do have to compare that measurement with target subcooling. That’s the only way we can check if we have low subcooling, normal subcooling, or high subcooling.

So, we do need to know what our target subcooling temperature is. Here is how to determine target subcooling without using a target subcooling chart:

How To Determine Target Subcooling?

We measure and calculate target superheat. This is not the case with subcooling. For subcooling, we have to get the metric from the manufacturer. Here are two places you will likely find the specified target subcooling for your air conditioner:

• Specification plate on the outdoor unit. You will likely see something like “TXV Subcooling 12°F”.
• Shroud on the outdoor AC unit, on the back side of the shroud, should have something like “Target Subcooling 12°F”.

In limited cases, the subcooling won’t be just one number. You can get a list of subcooling temperatures, depending on the outdoor temperature. Here is an example:

• Target subcooling at 70°F = 11.7°F.
• Target subcooling at 80°F = 11.8°F.
• Target subcooling at 90°F = 11.9°F.
• Target subcooling at 100°F = 11.9°F.
• Target subcooling at 100°F = 12.1°F.

You can see that this range (from 11.7°F to 12.1°F) is very tight; only 0.4°F difference between very low 70°F outdoor temperature and very high 110°F outdoor temperature. This is why most manufacturers just specify a single subcooling temperature.

Most specified subcooling temperatures (on the rating place) range between 8°F and 12°F. You might see subcooling figures as high as 18-19°F.

Example: In the Carrier 38C Series 1.5 – 5 AC units specification sheet (these units use R-410A refrigerant), we can find “Systems should be charged to 10°F subcooling or the rating plate subcooling, whichever is greater”. This is the specified target subcooling by a manufacturer; Carrier in this case.

What happens if you can’t read the subcooling from the specification sheet or rating place?

In HVAC, we use this rule of thumb:

If you don’t know the subcooling, presume the subcooling is 11°F.

It turns out that using 11°F target subcooling is a fairly good estimate for most air conditioning units.

Now, what happens if the measured subcooling deviates from the target subcooling? Let’s look at some of these scenarios:

Target Subcooling Below, The Same As, Or Above Measured Subcooling

To diagnose if our AC unit works properly, we need to compare target subcooling (from the rating plate) to measured subcooling (here is how we calculate subcooling based on suction line and condenser coil temperatures).

We are faced with these 3 distinct situations:

1. Target subcooling is higher than measured subcooling (low subcooling situation). Example: In an R-22 system, we see that target subcooling should be 11°F, but when we measure subcooling, we only get 5°F. In this case, we usually have to add R-22 refrigerant (charge the system).
2. Target subcooling is lower than measured subcooling (high subcooling situation). Example: In an R-410A system, we have a 10°F target subcooling (specified on the rating plate), but our measured subcooling is 18°F. In this case, we usually have to remove the R-410A refrigerant (recover the freon).
3. Target subcooling is the same as measured subcooling (with +/- 3°F deviation; normal subcooling situation). Example: In an R-134A system, we have a 12°F, and our measured subcooling is 14°F. Since the difference between the target and measured (actual) subcooling is 2°F (less than 3°F), we consider that we have normal subcooling. We don’t have to add or remove the R-134A refrigerant; the unit is adequately charged.

As we see, both subcooling and superheat are used to diagnose AC units. With superheat, we can measure superheat (or total superheat) AND calculate target superheat. With subcooling, we can measure subcooling at TXV, but we cannot calculate target subcooling; target subcooling is specified by the manufacturer.

Hence we don’t have a target subcooling chart as we do for target superheat. We have to check the rating place or, alternatively, use the 11°F target subcooling rule of thumb.