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Wiring for cold room

653 Views 11 Replies 2 Participants Last post by  yuri
We’re building a cold room for processing deer and beef with a 240v window air conditioner. I’m wiring in two PID controllers to bypass the units normal thermostat to allow colder temperatures with one and prevent the coils from freezing with the other. I understand the wiring of the PIDs and the SSRs that they run, but I’m not quite sure where to tie these in to the wiring of the AC unit. The black wire goes into the compressor relay and the *********** leg goes directly into the capacitor. Which one do I tie into?
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I suspect it’s the black power wire as the compressor relay is tied into the circuit board where the thermostat is, but if that’s the case, do I splice it in before or after the relay?
Is there a wiring diagram in the unit you can take a pic of with your IPhone and post here?

We don't get a lot of window AC questions here and few techs except appliance repair guys fix them as most HVAC companies are too expensive and it is cheaper to throw them away than fix.

However if it is 240 volt then the black is 120 volts hot and so is the white to get a combined 240. You should also have a ground and I doubt there is another neutral wire for some 120 circuit.

The thermostat should turn on the compressor relay and be in series with it I would imagine. Most furnaces and ACs use a 24 volt step down transformer from the main power to give you a 24 volt circuit for the thermostat/tstat and that turns on the control board.

Your tstat should be line voltage and heavier duty and as long as your control can handle the current rating of it then you may be able to switch it.

You would need a temperature sensor or thermistor to physically attach to the cooling coil to try prevent it from freezing but even then that is a risky proposition. The unit is designed to be operated at no lower than a 50 deg F outdoor temp and probably the same inside.

If you run it too cold you may slug liquid refrigerant to the compressor and wreck it. Use at your own risk. It needs a load to work against and the outdoor temp is part of the load.
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Diagram Plan Text Technical drawing Drawing

Here’s the wiring diagram. I know many farmers who have used this technique, so I know it works. How long it will last I can’t say but I know a few that have run them for several seasons keeping various products cool such as flowers or vegetables. Each PID has a sensor, and one goes in the room and the other is attached to the coil and set to around 34 F to prevent freezing. Of course, it’s better if this can be wired in to only cut off the compressor, so the fan can continue to run and let the coils warm up a bit before letting it cut back in at around 37. I only need to keep the room around 40f, not looking for freezing.
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The SSRs are rated for 40amps 240v, the PIDs run off of DC 32v from the SSRs.
It has a circuit board and transformer which looks like it has a low voltage tstat probably that CN5 ambiant temp sensor on the lower right side of the diagram.

It also has some UI board on selected models? Maybe that means user interface board?

If you have that model then it looks like the tstat is low voltage. It looks to me like the ambiant temp sensor is just a switch so you can remove it and put your own in.
If you have a CONT/ continuos switch then that will keep the fan running even when the cooling coil is frozen and should thaw it quicker and give you cold air.
Only if the new relays are wired in after the fan switch. Since everything on this unit is digital, I’m guessing I’m going to have to wire in after the circuit board and then disable or fool the original thermostat.
The continous fan should run all the time even when you are not in cooling mode.

Your thermostat/tstat just turns the AC compressor on/off and when in non Continuos fan mode will energize a fan relay.

In Continuos fan that fan relay is overridden and should stay on 24/7
I think I understand everything I need to now. Thank you for your help!
Yur Welcome:smile:
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