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Open Roads Forum  >  Tech Issues

 > Flaky Furnace Thermostat

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BFL13

Victoria, BC

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Posted: 11/09/19 02:36pm Link  |  Quote  |  Print  |  Notify Moderator

I meant Operation Number 16 where they are both on. Anyway, once you have time to poke around some more with it, you will probably find the problem. Interesting puzzle!


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sayoung

Tx

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Posted: 11/09/19 03:12pm Link  |  Quote  |  Print  |  Notify Moderator

Which 8530-xxx do you have as there are a couple of different series ?
From your original post, you said something about turning on the batteries so I have to ask are you turning off a battery disconnect and killing the 12volts to the rv ?

CarnationSailor

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Posted: 11/09/19 04:23pm Link  |  Quote  |  Print  |  Notify Moderator

sayoung wrote:

Which 8530-xxx do you have as there are a couple of different series ?
From your original post, you said something about turning on the batteries so I have to ask are you turning off a battery disconnect and killing the 12volts to the rv ?


It's either the 8530-345 or 8530-348. Doesn't matter which one as they both are wired the same. (Actually, I think the 8530-345 is white and the 8530-348 is black. Or vice versa.)

No, I am not turning the disconnect switch on or off. I only mentioned that in my first post so as to be clear what I meant by the tstat's "default setting"; that is, when it is first powered up, it starts at (defaults to) a setting of 72 degrees.


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sayoung

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Posted: 11/09/19 05:27pm Link  |  Quote  |  Print  |  Notify Moderator

Check the 4wire bundle, red 12+ & the White is furnace. The 3 wire bundle does the fan speeds & the HP reversing valve. If mfg followed colemans specs for OEM install.

CarnationSailor

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Posted: 11/09/19 07:43pm Link  |  Quote  |  Print  |  Notify Moderator

sayoung wrote:

Check the 4wire bundle, red 12+ & the White is furnace. The 3 wire bundle does the fan speeds & the HP reversing valve. If mfg followed colemans specs for OEM install.


Are you referring to wire bundles up at the air conditioner?

I ask because the 3 wire bundle at the tstat connects to tstat terminals identified as +12 vdc, -12 vdc, and Furnace. The 3 wire bundle is all about the furnace. It's the 4 wire bundle that goes to the AC/heat pump.

I think we might be rehashing ground already covered. I believe I've proven that to turn on the furnace, the OEM tstat connects the red +12 vdc wire to the white "Furnace" wire - both of which are in a 3-wire bundle at the tstat. I think I just need to find a new tstat that will do the same thing - allowing me to move furnace control from the OEM tstat to the new tstat.

Getting ready to head south tomorrow so I may have to put this on hold until we get to S. Cal.

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sayoung

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Posted: 11/09/19 08:22pm Link  |  Quote  |  Print  |  Notify Moderator

Per the Coleman OEM install spec I was looking at, the 3 wire does not go to the furnace. Both 3 & 4 bundles go back to AC control box then a wire to furnace, but that's the way it's SUPPOSEd to be wired. To turn on furnace the white in the 4 wire gets connected to the red (12+). Do it with a meter 1st to check . Why they can't put a tstat wire in that will work all the contacts is beyond me.

CarnationSailor

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Posted: 12/10/19 04:59pm Link  |  Quote  |  Print  |  Notify Moderator

CarnationSailor wrote:

OK, I'm ready to give up on finding the cause of the tstat resetting. I am think of installing a mechanical, 2-wire thermostat next to the existing tstat, but I want to be sure to wire it up correctly.

The existing tstat is supplied with 12-volts via a red wire and there is a white wire that is labeled "Furnace" in the tstat manual. My assumption is that the white wire supplies 12-volts to the furnace whenever the tstat calls for heat.

Leaving the red wire connected to the existing tstat, I plan on tapping off the red wire to provide 12-volts to the new tstat. Then I would disconnect the white wire from the existing tstat and connect it to the new tstat.

Sound like the right way to do it?

Thanks


I quoted my plan above just to refresh everyone's memory.

Well, I finally got around to executing my plan, and it worked perfectly - AFTER I bought a compatible tstat. According to Honeywell, the CT87K was not going to work in a 12VDC system. (It was designed for a 24VAC system.)

I exchanged the CT87K for a CT53K which is about as old tech as you can go. Completely mechanical with the old bi-metallic coil.

So now I have two tstats mounted side-by-side: the OEM which controls only the Air Conditioner / Heat Pump, and the CT53K which controls only the furnace.

I would have preferred to solve the original problem without having to use a workaround, but tracking down why 12VDC to the tstat was being interrupted was beyond my knowledge level.

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