Air conditioning electrical advice requested please...

James77MCI8 · July 16, 2010, 05:36:02 PM

A regular household digital thermostat, 24 VAC transformer and some 24VAC relays and you would have a nice control system

Len Silva · July 16, 2010, 05:57:45 PM

I have had some experience with those old style controls and would not recommend them. I had two on an old Duotherm basement unit. The problem is that the thermostat carries the compressor load and the thermostat itself heats up, causing the unit to run much colder than you want. I solved it with some relays.

James has the right idea. A digital household thermostat, a 24 vac transformer and a few relays will fix you up. If you get a two stage cooling thermostat, you could use it to control both air conditioners, though I would strongly recommend separate thermostats.

Find out what the coil voltage is on the compressor start relay to begin with.

If you decide to go that route, I could help you figure out the wiring.

gus · July 16, 2010, 06:32:49 PM

Or, find another controller on ebay.

miles2go · July 16, 2010, 06:59:29 PM

It sounds like it isn't easy but there is a better way than the old rotary controls. Thanks Len for alerting me that the older controls are not the way to go.

James, I think, as Len says, what you propose is the thing to do. Thank you.

Gus, do you mean replace the controller board and the touch pad or "knobs" with another, or just look for another type of touch pad that will work? I have tried that and there are just no touch pads available.

Len, the specs say:
Compressor 12.4 FLA, 63.5 LRA, Min. Circuit Ampacity 20.
Fan 3.10 FLA, 8.5 LRA and 1/4 HP.

I don't know if that has anything to do with the coil voltage of the compressor?
Thanks.

Len Silva · July 17, 2010, 06:42:51 AM

You will have to get right down into the guts of the unit and find the start relay (shown on the schematic) On the relay itself, it should tell the coil voltage. It might be 120 vac or 12 vdc or something else. We would need that info to figure out the wiring to replace your controller.

James77MCI8 · July 17, 2010, 07:35:07 AM

looks as though the start relay drops the start capacitor out of the circuit once the compressor is up to speed. Can you check the voltage at terminal 5. I'm bet'n it is 110V.

miles2go · July 17, 2010, 09:24:43 AM

I will get into the unit and check the relay for the coil voltage and I will check the voltage at terminal 5. I was out there on top of the bus just now. A new problem surfaced that I will have to fix first. To check the a/c units I had run a direct connection to the panel in the barn/shop and all worked fine. This morning I rewired the connection into what I had hoped would be permanent. It didn't work.

I have a #6 shore cord hooked directly to a 50 amp double pole breaker in the shop panel (240v). The shop panel has grounds and neutrals on the same bus. The shore cord runs through a Marinco inlet to a sub panel on the bus with a 50 amp double pole GFI BREAKER in it. Only one side is used to run to the main panel (120v). At the main panel there is a regular 50 amp double pole breaker that feeds to a 20 amp breaker that feeds the roof air. When I turn the 20 amp breaker that feeds the roof air on the GFI breaker in the sub panel trips off.

The grounds and neutrals are only bonded at the shop panel, no where in the bus are they bonded. I will have to either run an extension cord to run the a/c to check what you need or re-wire without a GFI breaker in the sub panel. I'll go do it now and report back on the voltage you asked me for... unless I do something RADICALLY wrong and then I probably won't report back.

Len Silva · July 17, 2010, 10:38:00 AM

A couple of things; looking again, I'm sure James is correct, the relay is 110 volts, no need to look at it.

It appears that the neutral is grounded in the AC unit, either intentionally or by accident. Not uncommon for some very old units to be wired that way. Shouldn't be hard to find.

miles2go · July 17, 2010, 10:40:26 AM

I disconnected the neutral pigtail from the GFI breaker, hooked the neutral that was into the GFI straight to the neutral bus defeating the GFI functionality and everything worked fine. The voltage reading on terminal #5 was 119v ac as soon as the compressor cut in. So now I have to defeat the neutral to ground in the unit as soon as I find it, then re=attach the GFI functionality. Thanks!

miles2go · July 17, 2010, 10:59:54 AM

I can't find where the ground and neutral are connected, at least not around the control board. The ground wire comes in with the white and black, it's green, and goes straight to a screw on the chassis of the roof unit. I just looked and it can be seen also in the picture i posted of the control board.

James77MCI8 · July 17, 2010, 01:51:45 PM

The neut. and grounds are connected through the capacitors. Take a close look at the schematics and you will see it.

miles2go · July 17, 2010, 03:24:18 PM

Ok I see where on top of both capacitors there is a jumper and then it goes to a grounding screw... can I just unplug and remove these? Also the motor is grounded here. Should that be left alone?

James77MCI8 · July 17, 2010, 04:12:58 PM

You should be able to disconnect those grounds

miles2go · July 17, 2010, 06:26:58 PM

Thanks James, disconnected the grounds and everything works fine now.

JLL · July 18, 2010, 10:25:19 AM

I also have 2 dometic's that are 1990 vintage. I Use a pulse Air. you can run front and back alternitive 7min. on front then it will switch to 5min. on the back or you can run both. or front only. It works great if you have 30 amp only.
JLL