Getting Dometic Heater to run without an AC unit on roof

[offgridtod]

We have a 2014 5th wheel where the AC units were removed due to a fire in the master bedroom. We are in the process of remodeling and the main technical issues is getting the central furnace to run. We got the thermostat in the kitchen to power up but the furnace does not power on. Apparently there AC unit has a relay or something that controls the call for heat.

If anyone knows (we are in Coastal Oregon, AC is simply never needed) but heat is key. Can we bypass the AC and run the heater directly from the thermostat.

There is a four wire plug at the ceiling - Red, White/Red - connected they power up the thermostat. A black and an Orange wire remain. Suggestion to get furnace to power up?

[offgridtod]

Thanks! We got the heater working and now have a semi-good understanding of the wiring. Now I'm trying to brainstorm an automatic ceiling fan for when the LCD sends the signal for AC. Heat when heat is needed and cieling fan when cooling is needed. Just an idea.

[time2roll]

offgridtod wrote:

Wild Card wrote:
Supply 12v DC to the blue wire and ground to the white.

Please clarify, I don't have a blue wire or white.

These wires come right off the furnace.
Where are you looking? While you are in the furnace compartment look for and post the model number.

[dougrainer]

1. The roof top AC units(excluding the penguin model) do NOT have a control system
2. The control system is purchased separate from the Roof AC.
3. As I stated, you can just use the Control system module box without the roof AC to run the Furnace and tstat.
4. You need to post the P/n of your existing Wall tstat or post a PIC of it. Doug

PS, Your furnace is NOT Dometic. It is either a Atwood or a Suburban.

[wa8yxm]

More than likely the A/C powers the Thermostat when the furnace is running.

Two suggestions.. one requires technical knowledge I do not have (Specifically the diagrams for the Thermostat including some they likely do not publish)

But if this is the case (and there may be a way to test this.. Okk how to test it)

IF there is a 12 volt wire where the A/C was that is "HOT'. and another that rund down to the Thermostat, Hook a Automotive TEST LAMP between those two leads See if that makes the furnace work. Just a standard test lamp or if you wish get a license plate lamp with pigtalis (Auto store) stick a dash indicator lamp in the socket.. or a Marker light assembly works too. I use one of these as a "Self Resetting Fuse" (In the event of a short the light comes on, clear the short and the light goes out) INCANDESCENT lamps only folks, NO LED's for this job.

IF that does not work.

Identify the Furnace T-Stat leads and swap your digital T-Stat for something like this.

Honeywell Heat Only 20 bucks at True Value Hardware[/ur]

I did set the "Anticipator" setting to the m...

[offgridtod]

Here is a manual which goes into the details on wiring.

http://www.dyersonline.com/downloadfile/download/aitfile/aitfile_id/3355/

The late model system uses an Orange comms wire then the blue wire at the SZLCD control box.

So I need a SZLCD control box or relay board - not sure if those are synonymous

[offgridtod]

I guess we better drill down to part numbers:

Here is one option: Dometic Duo-Therm AC 3106996.022 Heat & Cool Relay Control Board

Another via Walmart - Dometic 3316230.000 White Duotherm Single Zone Thermostat with Control Kit

It remains a little unclear if the board or kit will be superior, as in add a thermostat or just add the board.

[offgridtod]

Ok, so I think I get it. Either install a Dometic AC or possibly get a control board. I found AC unit dent and ding for $399, or $569 new - that's somewhat attractive though I'll never use it in Oregon. I'll look into a control board.

[2chiefsRus]

I know nothing about your situation. I do know from reading many posts on here, that I would do what dougrainer recommends. He gives the best advice in many, many situations in my opinion.

[dougrainer]

Without the Dometic AC control board installed, NO you cannot get the furnace function to work. You do NOT have to have the 120 connected to the AC control board to get the Tstat and furnace functions to work if you still have the Dometic AC control board. If NOT, then the simple Tstat I gave the link to is the cheapest best bet. Doug

[dougrainer]

Yes, the AC upper control board has a relay that controls the Furnace on and off thru the wall tstat. The EASIEST thing to do is buy this Tstat and run the 2 BLUE wires AT the furnace to this tstat. Just run 2 wires from the 2 Blue wires at the furnace to a convenient location in the RV to sense the heat and operate the furnace. Doug

https://www.amazon.com/Atwood-38453-Thermostat/dp/B007HRWNBS/ref=sr_1_4?ie=UTF8&qid=1518282195&sr=8-...

[offgridtod]

Wild Card wrote:
Supply 12v DC to the blue wire and ground to the white.

Please clarify, I don't have a blue wire or white.

[BFL13]

If you have a "heat only" furnace and a new thermostat, the stat manual will explain how to wire it up. I have a new Honeywell RTH111 digital stat that was confusing, but tech support and some guys on here got me straight. With that one, you take the two furnace wires and put one on W and one on R-- doesn't matter which ones, and good to go. The manual does have advice about heat pump connections too.

[Wild_Card]

Supply 12v DC to the blue wire and ground to the white.

[MrWizard]

its probable the oem unit was a 'heat pump'
it was the primary heating source, and the furnance was secondary and came on if/when the heat pump could not provide the needed amount of heat

the fire probably destroyed the wiring, but if you can find the thermostat control wires and connect the correct wire back together, the furnace should work

OR just run new 'stat' wires

do some reasearch find out what 'heat pump' is normal for that RV
visit an owners group and ask questions about the heat pump operation and wiring
look up the MFG specs and wiring diagrams for the heat pump