A guy showed me the pig in a blanket trick, did it for a bit and realized what the hell am I doing! Weather its cold or hot outside all that does is possibly harm the a/c by covering condenser. I can artificially increase pressures all day, won't tell me nothing. Best thing to do is notify the customer to wait till temps are more appropriate, otherwise let the other guy take the heat for misdiagnosing an issue and over/under charging a unit.
Set up a tent. Run the condenser in the tent until the ambient air (in the tent) is at the desired temperature. Then, perform your checks. It adds a bit of set up time, but will more accurately simulate a 90 degree day.
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My company forces us out too soon. I don't like doing it, but I have wrapped a unit or few in a contractors trash bag to get the head up. Only on obvious leakers the customers refuse to fix. I figure if it's leaking that much a diligent charge under better circumstances won't be perfect for long anyway. EPA sanctioned? No. But the EPA doesn't pay the customer's HVAC bill nor sign my paycheck.
I roll my eyes every time I see a low OA recorded on a service record after an early spring clean and check - tells me right away the tech balked at doing the call in low temps and was making damn sure any call back tech knew the score. I do it too. On the ticket and service record.
I wouldn't suggest or recommend you tell the boss your just not going to do it. You could end up unemployed, or laid off.
There is no good method. I have several ways I do it. None 100% guarantied to always get it right.
I often use a remote starter cable an cycle the fan and watch the suction pressure through the rise and fall of the head pressure. It takes a while to get use to what the readings are telling you, and to figure out if they are good readings or not. But if the indoor humidity is too low, it doesn't tell you much.
What you can also do. Is take your leak detector inside with you when you check the air handler. If its an upflow system, stick the probe in the return while you are checking the air handler(or furnace). If its a down flow, then stick it in the supply plenum. If there is a leak inside it will pick it up. Then you can recommend a full leak check to pin point it, without ever having to having put the gauges on. And when you do the repair, you can weigh the charge back in.
My company has alway's done cold weather charging. It's starts with Dispatch telling the homeowner to turn up the heat a couple of degrees while the Technician is on it's way to their house. When I get their. Check Wetbulb. Should be atleast 58. Wrap outside unit and simulate 80 degree day. If I need a higher wetbulb, I keep the heat on and do my checks inside, then get setup outside. We haven't had any problems, as long as you don't take any freon out of system. I don't mind doing the cold weather chargeing, it beats staying home and waiting on the weather to warm up.
I don't trust it at all, but my company has us do it. Certain customers want us out once a year for the furnace AND the a/c maintenance for whatever reason. We check refrigerant charges in January. Frankly it's a sick joke to me. Just a method to say, "yeah, we did your maintenance." Unless it's a heat pump, reschedule for May at least... You have to run their furnace for half an hour to get the indoor temp up to at least 75, then hope the indoor air isn't to humid. No, you're not the only one.
I have done hundreds of a/c tune-ups in cool to cold weather. I remember a few where I washed outdoor coils standing in snow. The only thing about blankets, cardboard garbage can lids is that you are simulating hot weather, but what simulation temperature are you simulating? 80-90-110??? And then there is no load inside. Don't add refrigerant! do your thing, clean drains, coils, filters....... and just hope charge is good. Company's do it to keep tech's busy and try to avoid the big rush the first days its hot out.
As iron sharpens iron so one man sharpens another Proverbs 27:17 NIV84
In my area, going to be cold the rest of March and into the beginning of April. At least we can get going on the commercial PMs. But them homes are usually in the 60s in early spring and the same out doors. But even on days like that you can tell if a system is low on refrigerant or not, but it will not be accurate.
4 x 4 box with a toggle switch and a rotary speed control switch, like for a ceiling fan. Two wires with spade connectors to hook in series with the condenser fan motor. Adjust the dial to get the head pressure where you want it.