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32° Saturation (Evap Temperature)
Evaporator temperatures below 32° are common and acceptable in refrigeration; that's why there is a defrost sequence.
In a heat pump running in heat mode, it's the same; freezing is a part of the process, and defrost is necessary.
In comfort cooling, we can't allow the evaporator to get below 32°, or it will freeze.
I can't tell you how many times I look back at technician notes and can see in plain black and white that the system will freeze.
That is not OK.
Freezing causes flood back, no cooling, water damage, and biological growth.
We cannot leave a system that is just going to freeze.
The image above is of the Danfoss refrigerant slider app, and it shows that when suction pressure drops below 102 PSIG on an R410a system, the coil hits 32° and will start to freeze.
This means that we need to set up equipment not to freeze during normal operating conditions.
A typical residential A/C system should be set up so that the return temp can get all the way down to 68° and still be just above freezing.
Let's say it's 78° in a house on an R410a system, and your suction pressure is 108 PSIG (as shown).
Your suction saturation (coil temperature) is 35°. So, the coil won't freeze.
The coil temperature will drop approximately 1° for every degree the return temperature drops. So, if the customer sets it down to 74°, the saturation will now be 31°, and the coil will freeze.
It's pretty basic stuff, but it's very important if you don't want to leave a problem for your customers.
Many things can cause this (low airflow, restrictions, low refrigerant), but step #1 is having the wherewithal to catch it.
Keep in mind that this is only once the system has run long enough to stabilize. Don't start making changes until the system has run at least 10 minutes and leveled off.