The primary role of setting an appropriate level of subcooling is to ensure that we deliver a full line of liquid refrigerant to the metering device.

We want to do this at:

• A pressure differential required by the metering device
• At a temperature and pressure no higher than required for maximum capacity and efficiency

But most important is that it is 100% liquid with no “flashing” or bubbles when it hits that metering device. Any amount of refrigerant that is already vapor when it hits the metering device is wasted energy and unwanted turbulence leading to noise and additional pressure drop.

We are generally safe to set the subcooling level listed on the system data tag or the old 10° rule of thumb when you have nothing else to go on.

We need to consider adjusting the target subcooling in the following cases:

• Long lines or tall risers
• Liquid lines running through high-temperature environments

As soon as the pressure or temperature of the refrigerant in the liquid line hits the saturation point, bubbles will begin to form, and then there's the dreaded “flashing.”

Let's consider an example.

We've got an R410a system with a 110° liquid line saturation pressure (368 PSIG) and 5° of subcooling at the condenser, so the liquid line is 105°, but it's a 100′ run of line with 20′ of rise, and then it goes through a hot attic that is 120°.

First, we can estimate the pressure drop of the rise based on the York/Johnson Controls rule of 1/2 PSI of drop per ft of rise, so this means we would see a 10 PSI pressure drop in the riser alone. Depending on the size of the liquid line, there would be an additional pressure drop, but it would not be significant, so let's just estimate a 15 PSI total pressure drop and 2° of sensible heat gain into the liquid line due to the hot attic.

That would mean the liquid line would now be 107°, and the liquid saturation temperature would also be at 107° due to the pressure drop from 368 to 353 PSI.

In other words, the refrigerant could now begin flashing.

In long long-line applications, Carrier instructs you to charge to 10° of subcooling or the listed subcooling, whichever is greater. That's because, at 10°, you have enough wiggle room to deal with most residential/light commercial situations.

In heavy commercial applications, there are routinely longer line runs. The actual field pressure drops and temperature gains must be calculated to ensure flashing will not occur in the liquid line. That often requires a higher subcooling.

—Bryan