Flaky Furnace Thermostat

BFL13 wrote:
"
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"

Does a voltmeter confirm you got 12v to the new Tstat? ISTR on my furnace- only Tstat, the two wires are "positive" from the furnace and are switched by the Tstat. Not sure where "negative" is.

Note the fan switch position. It is confusing that you have a heat pump, but this is for the furnace.

EDIT--the furnace operates its own fan, so not sure what that is about.

Not clear on whether taking the red and white from old Tstat gets you the wires to put on W and R with the new. Also the new has its own battery, does not use 12v from the RV, not sure where that applies in this case.

Here is how the original one is supposed to work --if the wires from that are in cut -out with the heat pump even with that white one removed, not clear what is going on.

http://old.rvcomfort.com/rvp/pdf_documents/t_stat_electric_heat_function.pdf

Thanks for the help. Your mentioning that the CT87K had an internal battery caused me to re-read the owner's manual (more closely this time). It is for 24 VAC systems - not 12 VDC systems. So I think my plan is sound, but I need to get a compatible thermostat.

Here is how I understand the OEM tstat works:

It connects to the furnace via three wires. It gets 12 VDC on one wire, ground on another wire, and uses the 3rd wire to control the furnace. (The 12-volts and ground are used by the tstat for the digital display as well as other funtions so I have to leave them connected to the tstat. That's why am tapping off of the 12 volt line to go to the new tstat.)

The OEM tstat "energizes" the white wire from the tstat which connects to the furnace's control wire. Basically, (I think) it receives 12 volts from the furnace and when heat is called for, it returns the 12 volts to the furnace via the control wire. It essentially shorts the red wire from the furnace to the control wire from the furnace to turn it on.

As for the fan mode and fan speed switches on the OEM tstat, they control the air conditioner's fan both in cooling mode and in heat pump mode.

I'll let you know how it goes after I obtain the right tstat.

"I'll let you know how it goes after I obtain the right tstat"

The new one you have might work if you want to isolate the furnace as the heat source. But the actual furnace wires that would go to its
Tstat need to be identified.

On my old time Atwood furnace it is two blue wires that go to the Tstat and are switched there. On my new digital Honeywell, one goes to the W and one goes to R and it doesn't matter which goes to which.

On yours, you need to find the two wires, whichever they are, that come from the furnace that the Tstat switches. Then put them on W and R and it ought to work.

They should start the furnace just by touching them together, so I suppose you could take them all off the OEM and start touching them until you find the two that make the furnace start. (Not sure that is such a good idea in case it hurts some thing else)

On mine you can see the two blue wires leaving the power end of the furnace and going through a wall as a pair, and they come out at the Tstat.

Perhaps your furnace has something visible like that for the wires that go off to the Tstat. Perhaps a wiring diagram exists for that furnace in your rig.

OP, you mentioned 3 wires so have a question. Is your heatpump a Dometic and if so you might have the " communicating " tstat & if so that's what you must use.

sayoung wrote:
OP, you mentioned 3 wires so have a question. Is your heatpump a Dometic and if so you might have the " communicating " tstat & if so that's what you must use.

The 3 wires I mentioned are in a sheath and come from the gas furnace. There are 4 wires in another sheath that come from the Coleman Air Conditioner/Heat Pump.

BFL13 wrote:
"I'll let you know how it goes after I obtain the right tstat"

The new one you have might work if you want to isolate the furnace as the heat source. But the actual furnace wires that would go to its
Tstat need to be identified.

On my old time Atwood furnace it is two blue wires that go to the Tstat and are switched there. On my new digital Honeywell, one goes to the W and one goes to R and it doesn't matter which goes to which.

On yours, you need to find the two wires, whichever they are, that come from the furnace that the Tstat switches. Then put them on W and R and it ought to work.

They should start the furnace just by touching them together, so I suppose you could take them all off the OEM and start touching them until you find the two that make the furnace start. (Not sure that is such a good idea in case it hurts some thing else)

On mine you can see the two blue wires leaving the power end of the furnace and going through a wall as a pair, and they come out at the Tstat.

Perhaps your furnace has something visible like that for the wires that go off to the Tstat. Perhaps a wiring diagram exists for that furnace in your rig.

The new tstat is designed to work on 24 VAC systems. I doubt that I could get it to work on my 12 VDC system.

As long as the backup batteries are good the tstat doesn't care if it's 24 or 12

CarnationSailor wrote:

sayoung wrote:
OP, you mentioned 3 wires so have a question. Is your heatpump a Dometic and if so you might have the " communicating " tstat & if so that's what you must use.

The 3 wires I mentioned are in a sheath and come from the gas furnace. There are 4 wires in another sheath that come from the Coleman Air Conditioner/Heat Pump.

With only 4 wires back to roof AC , you don't have a heatpump based on the Coleman wireing diagrams I have seen.

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)

sayoung wrote:

CarnationSailor wrote:

sayoung wrote:
OP, you mentioned 3 wires so have a question. Is your heatpump a Dometic and if so you might have the " communicating " tstat & if so that's what you must use.

The 3 wires I mentioned are in a sheath and come from the gas furnace. There are 4 wires in another sheath that come from the Coleman Air Conditioner/Heat Pump.

With only 4 wires back to roof AC , you don't have a heatpump based on the Coleman wireing diagrams I have seen.

Trust me, I have a heat pump.

Check out the online manual for my thermostat which is the RVComfort.HP Model 8530. Seven wires are attached to it and are identified as follows:

These 3 connect to trailer wires that are in the same sheath so undoubtedly go to the same place, the furnace:

Red (+12 vdc)
Blue (-12 vdc)
White (Furnace)

These 4 connect to trailer wires that are in another sheath so they go to the same place, the Air Conditioner/Heat Pump:

Gray (Fan Lo)
Green (Fan Hi)
White/Black (Heat Pump)
Yellow (Compressor)

According to Figure 6 here, the Tstat gets two blue wires (same as my Atwood's)

https://manuals.heartlandowners.org/manuals/Heating-Cooling/Furnace/Suburban/SF-20-25-30-35-42_Furnace_Suburban.pdf