Flaky Furnace Thermostat

[CarnationSailor]

I have a Suburban gas furnace in my 4-year old 5th-wheel. The thermostat name on the front panel is "RVComfort.HP". When heating on propane only (that is, not using the heat pump), four times in the past two days the thermostat has changed the temp setting from our daytime setting of 69 or our night time setting of 66 degrees to 72 degrees. All by itself.

I believe that 72 degrees is the thermostat's default setting; i.e., when I first turn on the batteries, the thermostat sets itself to 72 degrees. I suspect that something is causing the thermostat to reset to it's default setting. It could be a momentary loss of 12-volts to it, but I doubt that is the reason because I can't think of anything that would interrupt the voltage from the batteries. (We are dry camping.)

Has any seen this before?

[CarnationSailor]

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.

[sayoung]

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]

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.

Thanks

[sayoung]

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]

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.

[sayoung]

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 ?

[BFL13]

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!

[CarnationSailor]

BFL13 wrote:
Doug's link to the Tstat shows that on "Gas"- the WF (white) is energized.
Heat pump has white/black energized.

I take it that the Gas switch position connects the two parts of the white wire so that acts like connecting the two ends of the blue wires.

I assume the red wire is just to power the Tstat. When you go to the Heat pump position, that energizes both the gas and the HP so they do that "gap" routine.

Your mystery is why when on Gas only and you have set 66F, it sometimes sets itself to 72F, the default after a power interruption to the Tstat.

The Red wire power to the Tstat is not from the furnace as such (which should have its own 12v power supply that the Tstat switches), but it might come from the same connection the furnace gets its power from. Say the Tstat's connection is loose but the furnace's connection is tight.

Or not making contact inside one of those wire nuts mentioned?

Assumes the furnace keeps running after the Tstat jumps to 72 and now the problem is just that it gets too warm in the RV. Assumes the furnace will cycle off at 72.

You said the problem does not happen when in heat pump mode where both the HP and the furnace are energized and you get to where you are on furnace assist. Mystery why a loose red wire would not lose its contact sometimes then too.

Yes, however, heat pump mode does not energize both the heat pump and the furnace. It energizes only the heat pump except when the tstat is calling for a 5 or more degree increase.

[BFL13]

Deleted-- got that wrong about the single white wire being in two parts. So not a clue! Unfortunately post was quoted next, so ignore that in there too! 😞

[CarnationSailor]

BFL13,

"It seems your OEM's brain isn't working right for how it switches between heat pump and furnace."

It works as specified. If I am in heat pump mode and increase the setting by 5 or more degrees above the current room temp, it automatically turns the furnace on to augment the heat pump. That is the only heat pump/furnace interaction that is supposed to happen.

I just put a meter on the OEM tsat. With the system OFF, I have 12 vdc on the red wire's connection point inside the tstat and 0 vdc on the white wire's connection point. (I am reasonably sure these wires come from/to the furnace.) Then when I switch to "Gas Heat" mode, the white (and red) wire's connection point has 12 vdc on it, and the furnace turns on. This is consistent with the tstat's manual which says that it "energizes" the furnace via the WF (White Furnace) wire.

So I seem to have a red wire and a white wire coming from the furnace that are "shorted" together by the tstat - much as your two blue wires are shorted together by a heat-only tstat. I still think the problem has to do with the tstat I bought from Lowe's. However, as you say, things could be much more complicated than this.

Tracing wires and/or replacing the wires between the furnace and the tstat is going to be a last resort option as my furnace is in a very inaccessible area.

[BFL13]

The two wires that go on the furnace-only Tstat are not pos and neg. It is one pos 12v wire that is cut with the Tstat inserted so that it switches that one wire.

The two blue wires heading from the power end of the furnace to the Tstat is like that. I am guessing your two blues go to some complicated connection place that has different colour wires coming out of it to that complicated OEM Tstat that runs all those things, so you can't tell which is each end of the "one" wire to use with the new furnace-only Tstat.

IMO put the OEM wires back where they were, and start over by grabbing the two blues closer to the furnace where you can tell which they are, cut them there, add more wire to each blue where you cut them going to the new Tstat location. That should get you furnace only.

Now the trick would be whether the OEM Tstat will still run the air and the heat pump properly, except now it won't go to furnace when its "gap" gets that way. It seems your OEM's brain isn't working right for how it switches between heat pump and furnace. That leaves you to pick the heat source, if the OEM Tstat still runs the heat pump ok.

I don't know where that brain is located. You replaced the OEM Tstat and nothing changed, so the broken brain must be elsewhere. Not a clue, but could be a control board in the mix that is not in the Tstat. Or as was suggested, there is bad wire connection to the OEM's brain that did not get fixed by replacing the OEM Tstat.

Some of those boards have wire connections that slide on to the edge of the board where there are contacts. Sometimes pulling them off the edge and cleaning the contacts and putting them back on is all it takes to get things going again.

[CarnationSailor]

DrewE wrote:

CarnationSailor wrote:

BFL13 wrote:
The Suburban gas furnace Installation manual should explain which wires to use with the thermostat. ( Any link to the manual on Google? We could check that out.)

Of the three wires you need two with this Tstat AFAIK, but which two? IMO you can find out by touching them together.

Yes, the Tstat is just a switch and has its own battery to power it, so it does not matter if the wire ends on W and R are 12v.

Do you mean to go without the air conditioner and heat pump now, and just have the furnace? (Where we camp on the Island here we never need air conditioning, but YMMV)

From the tstat manual, the only thing that makes sense is to connect the red (+12 vdc) to the white (Furnace) via the additional tstat. But this didn't work and you seem sure that it doesn't matter that the 2nd tstat is made for a 24 vac system. Maybe that means that the tstat that I bought from Lowe's is bad?

Does it work if you manually connect the wires together (no thermostat involved)? If not, does it work to connect the white to the other (ground) wire? One pair of them should make the furnace run, and those are the two that get connected to the thermostat. Since it has a battery, ignore the 24VAC power connections entirely. If manually jumping the wires makes the furnace run but connecting them to the thermostat does not, then it would seem the thermostat is indeed bad or you're misunderstanding the connections to it--which admittedly is not at all hard to do with some thermostats as the conventions and instructions for wiring them seem to have been invented by insane Martians.

If no combination of wires makes the furnace run, then perhaps you accidentally shorted the wrong ones together at some point and blew the furnace fuse (there may well be a smallish value one on the furnace control board).

The furnace still works when controlled via the OEM tstat so no blown fuse.

Although it seems logical to me that the only wires from the furnace to use are the red (+12 vdc) and the white ("Furnace"), I will test that by shorting them together. (I'm pretty sure if I involve the one labeled "-12 vdc", I'm going to end up send 12 volts to ground and blowing a fuse somewhere.)

Since we are leaving for Palm Springs in the morning, I'll postpone any further troubleshooting until we get there. (It's upper 80's there now so I can risk accidentally disabling both my furnace and my heat pump AFTER we get there!)

[DrewE]

CarnationSailor wrote:

BFL13 wrote:
The Suburban gas furnace Installation manual should explain which wires to use with the thermostat. ( Any link to the manual on Google? We could check that out.)

Of the three wires you need two with this Tstat AFAIK, but which two? IMO you can find out by touching them together.

Yes, the Tstat is just a switch and has its own battery to power it, so it does not matter if the wire ends on W and R are 12v.

Do you mean to go without the air conditioner and heat pump now, and just have the furnace? (Where we camp on the Island here we never need air conditioning, but YMMV)

From the tstat manual, the only thing that makes sense is to connect the red (+12 vdc) to the white (Furnace) via the additional tstat. But this didn't work and you seem sure that it doesn't matter that the 2nd tstat is made for a 24 vac system. Maybe that means that the tstat that I bought from Lowe's is bad?

Does it work if you manually connect the wires together (no thermostat involved)? If not, does it work to connect the white to the other (ground) wire? One pair of them should make the furnace run, and those are the two that get connected to the thermostat. Since it has a battery, ignore the 24VAC power connections entirely. If manually jumping the wires makes the furnace run but connecting them to the thermostat does not, then it would seem the thermostat is indeed bad or you're misunderstanding the connections to it--which admittedly is not at all hard to do with some thermostats as the conventions and instructions for wiring them seem to have been invented by insane Martians.

If no combination of wires makes the furnace run, then perhaps you accidentally shorted the wrong ones together at some point and blew the furnace fuse (there may well be a smallish value one on the furnace control board).

[CarnationSailor]

BFL13 wrote:
Ok, I thought you disabled the OEM Tstat by removing the white wire and moving it over to the new Furnace only Tstat.

I don't have a clue about the heat pump side and all that. Doug has some ideas earlier in the thread--he does have a clue!

Yes, Doug seems to be very knowledgeable. He did say that my problem can be difficult to troubleshoot. That is why I have abandoned trying to fix the problem and am trying to make this work around work.

Thanks for your time helping me.