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Setting & Checking Charges in the Rain
I live in Central Florida, and while it can get pretty hot in the summer, we also tend to get afternoon thunderstorms that come and go in a flash. I have been connecting gauges, checking charges, and even pulling vacuums in the rain (even under umbrellas or cardboard boxes) for most of my career. Only recently did I stop to think if that was a good idea.
Moisture in the System
I am going to go ahead and make the blanket statement that opening the system or connecting gauges while it is actively raining is just a bad idea. It's not because you are “made of sugar,” as the old-timers and my grandpa might claim. It's actually because even a drop of water in the system can do a lot of damage in the age of POE oil. If you have a good shelter or large umbrella, you might be OK. In Florida, however, we get these bursts of crazy weather that you probably aren't going to keep out of an open suction line.
That isn't to say you don't start a compressor just because it looks like it “might rain.” However, I would suggest being prepared with caps or plugs to seal it up quickly if it does start to pour.
If it is actively raining, I would also advise against connecting gauges or opening the panels and testing electrical components unless you have a good umbrella or shelter in place. Electrical testing can damage the components as well as be unsafe, and connecting in the rain can lead to moisture contamination.
Wet Condenser Coil
You will not be able to test or set a charge with any level of accuracy when the condenser coil is wet. The system pressures will be low, and the subcooling will be high due to the evaporative effect and the superior heat transfer of water over air. (If you want to learn about identifying your proper subcooling temperature, check out this article.)
If you simply want to confirm that the unit is functional, you can take an evaporator delta T and measure the liquid line and suction line temps at the evaporator to approximate proper operation. You will not be able to “set the charge” until the condenser coil has been allowed to dry completely.
The liquid line will generally be around the outdoor temperature. It may even a bit lower depending on the SEER of the unit and how wet the coil is. (Wetter/higher SEER = cooler liquid line.)
The suction line will be approximately the return temp minus 40°F (4.44°C) + the desired superheat +/- 5°F (2.75°K). It will tend to be on the lower side of the evaporator temperature scale because of the lower liquid pressure.
If you read our articles, you know that we are huge advocates of taking accurate measurements and not just walking away from a system without doing appropriate testing. However, if it's raining, you're just not going to get good readings. You also risk doing more harm than good to the system by taking those readings. Sometimes, taking fewer readings can be the best call. When it's raining, I would rather have my techs note the delta T, indoor liquid line temp, and suction line temp with a note about “raining” than risk an issue by connecting in the rain.
In cases where we have to set a charge, we will need to return and set the charge once the weather is dry. In cases where we did a simple drain cleaning, replaced a blower wheel, replaced a thermostat, or replaced a capacitor, those indoor readings will suffice. Are they conclusive? NO! Would I rather contaminate a system? Nope. Should we return to every system the next day just because it was drizzling to check the charge with gauges? I say no to this as well.
You may say (as many do) that connecting while raining has never caused issues for you before. To that, I would say: how do you know?
It's not like systems spontaneously combust when moisture enters them. The result is long term damage that could never be traced back the initial cause.
It was also less of an issue when mineral oil was the prevalent oil in use. You can read more about refrigerant oil and why it matters HERE.
Am I saying that you can never check a charge even in a light sprinkle? No.
Just use common sense. Don't be a robot that ALWAYS connects gauges, especially when it'll likely do more harm than good.