Can you upgrade a Dometic thermostat that only has 1 data lead plus ground and 12v

[sonofcy]

I have a 2018 Keystone Montana. It came with a thermostat that does NOT have the usual Rc, R, Y, W, G and more HVAC standard connections. It only has a single wire plus ground and +12V for the LCD display. I know there is a control box somewhere that does have the standard HVAC wiring, and you can buy it on Amazon or CW. I have a cheap inspection scope but can't see jack with it, I might try renting a decent one to look behind the wall where the thermostat is located. Has anyone looked in the A/C unit to see if it's up there? I will go and look myself, but I don't like heights nor the wasps that hang around it, so I thought I would ask first.

[chuckster57]

Control box for A/C and furnace is normally located in the return of the roof unit. Wiring diagram is usually on the inside of the lid. No need to go on the roof, just remove the ceiling assembly.

[retiredusps69]

There is a post Problems with 2019 30 rls cougar which shows the wiring Diagram. It also has a rj 45 connector that looks like a phone cable if I'am correct. Maybe this will help some.

[sonofcy]

Control box for A/C and furnace is normally located in the return of the roof unit. Wiring diagram is usually on the inside of the lid. No need to go on the roof, just remove the ceiling assembly.
What ceiling assembly? It's a Montana if that helps. Cold air comes out the round openings, warm air goes in the oblong openings.

[chuckster57]

Ok that means you have the “quiet cool” like the Big Horns and Tiffen motorhomes.

Sorry but you do have to get on the roof and swat wasps. The control box will still be in the same spot I suspect and you should only have to remove the plastic “cover” that covers the return ducting.

[sonofcy]

Ok that means you have the “quiet cool” like the Big Horns and Tiffen motorhomes.

Sorry but you do have to get on the roof and swat wasps. The control box will still be in the same spot I suspect and you should only have to remove the plastic “cover” that covers the return ducting.
Ok, I will screw up my courage and bring a can of wasp killer. Do you have any ideas how I can get the standard HVAC wiring back down to the thermostat area so I can hook up a NEST? The AC unit is about 10 or more ft from where the thermostat is located. The good news is that the thermostat is located on the wall that is the pre-wired solar access with a standard solar gland on the roof. These units have an attic so to speak. That means space between the inside ceiling and the outside roof. It seems possible I can get a fish tape through there but I could mess up the insulation.

[chuckster57]

If you can, try a flat fish tape, that way it won’t pull/push the insulation. If you get it to the other end, just tape the wire(s) to the tape in a couple spots and have someone feed it as you pull the tape.

[sonofcy]

I was just looking a the ceiling and suddenly realized I can fish the wires through the duct work! It appears to pass directly over the service area where all the other wires are now and the future solar. This way sounds real easy, I will get at it in a day or two when I am done my other projects.

[chuckster57]

Let us know how it goes.

[sonofcy]

Let us know how it goes.
Sorry to take so long but life got in the way. I took off the AC cover and do not see what I have seen in other videos. I suspect I know what has to be done but before I screw something up have a look at these pictures and tell me what you think please.

[Daryles]

I read another thread about thermostat issues. This guy added another thermostat. There is wiring diagrams on the web page. I think he mentions Dometic has a proprietary system where the thermostat talks to the system digitally via one of the wires.
http://www.klenger.net/hvac.html

[chuckster57]

The large styrofoam cover should be held on by 4 or 5 small screws (5/16” I think). Once the cover is removed you should see the control box.

[Daryles]

I found the thread
http://www.keystoneforums.com/forums/showthread.php?t=39155

[Steveo57]

I'm the OP in the referenced post above and wanted to do the same thing you are asking about. The original question is can you upgrade to an old style analog control. The answer is yes but you need to replace the thermostat and control board since they communicate over a single communication line.

Read through my post and see what I ended up doing.

[sonofcy]

The large styrofoam cover should be held on by 4 or 5 small screws (5/16” I think). Once the cover is removed you should see the control box.
I didn't notice any screws but I will look again.

[sonofcy]

I'm the OP in the referenced post above and wanted to do the same thing you are asking about. The original question is can you upgrade to an old style analog control. The answer is yes but you need to replace the thermostat and control board since they communicate over a single communication line.

Read through my post and see what I ended up doing.
I have seen a picture of the insides of an RV roof AC unit and inside is a box with the standard HVAC wiring all or some of Y1 Y2 G OB RC W1 W2 C * RH. If I have that box I will simply run those wires down to where I want to hook up a Google/Nest Smart thermostat.

[chuckster57]

Only thing is I know RV thermostat and control box run on 12V, don’t residential thermostats work on 24V? I don’t know, that’s why I’m asking.

[Steveo57]

Only thing is I know RV thermostat and control box run on 12V, don’t residential thermostats work on 24V? I don’t know, that’s why I’m asking.

They do but they also have a battery installed that runs it. I've had several in my house and other trailers that ran fine on the battery. You just have to put a new set of batteries in every few years.

I have a WiFi connected one at my house that i ran on batteries for years until I finally got around to putting in a transformer and running the wires to supply it. It ran fine but would drop the WiFi connection frequently on battery only. No problem now.

[sonofcy]

Only thing is I know RV thermostat and control box run on 12V, don’t residential thermostats work on 24V? I don’t know, that’s why I’m asking.
It's been a while since I last worked with HVAC stuff, but my memory is the 24V is to operate the gas valve. There is nothing in an analog thermostat that needs electricity, just a switch. The digital replacements used either the 24V or more commonly a battery to operate the electronics of the thermostat. The Google/Nest I want to install has a rechargeable LiIon. If the recharge circuit MUST be 24VDC then I will get a 12vdc to 24vdc 3A converter for $23 on Amazon.ca

[retiredusps69]

I think some of the TT run the ac with cable like a phone jack. If you want to use normal thermostat wire and not pull all new way not use the old style wire to modular plug adaptor. You can hide it some place close and skip pulling the whole run to the roof. R45j plug to bell wire. I think i have the correct number.

[sonofcy]

I think some of the TT run the ac with cable like a phone jack. If you want to use normal thermostat wire and not pull all new way not use the old style wire to modular plug adaptor. You can hide it some place close and skip pulling the whole run to the roof. R45j plug to bell wire. I think i have the correct number.

I am not sure I understand what you are saying. The existing wiring is one wire (not counting ground and +12VDC) and the thermostat I want to use needs at least 4 plus ground + 12VDC. One for heat, one for A/C, one for fan, one for emergency heat (maybe).
If you are suggesting pulling an RJ45 cable with appropriate plugs and wiring at each end versus pulling half a dozen very small wires (might be in a cable) in a single pull that might be a good idea, but it's a no-brainer either way. I will run the wire through the cold air ceiling ducts and bring it down the central service space that is directly under the A/C duct.

[sonofcy]

The large styrofoam cover should be held on by 4 or 5 small screws (5/16” I think). Once the cover is removed you should see the control box.
No screws, it's just a friction fit. The control box is NOT the one I saw in an earlier post that was clearly labelled with standard HVAC terminals. It seems obvious to me that this control box is capable of much more than what it is currently doing. I have heard there is another box you can get that converts the Dometic to ????. I need to start bookmarking stuff like that. Here is a picture of what my control box might look like, I didn't take mine apart to get details as it was covered in foam that looked glued on. I can easily find several models of the Dometic control box, some with very sophisticated controls like gen start. Here is a link on amazon where you can see one but this one does NOT have standard HVAC terminals.
https://www.amazon.com/Dometic-3109226-005-Comfort-Control-Center/dp/B003BIXYT2/ref=pd_lpo_sbs_263_t_1?_encoding=UTF8&psc=1&refRID=KD5JM17ZJJWMGG86H9A6

[sonofcy]

FYI, here is a good reference on HVAC wiring and standard terminal lettering and even wire colour. It's not standard as in legally, just convention,. but the terminals are industry standard labels.
https://xtronics.com/wiki/Thermostat_signals_and_wiring.html

and here is picture of the thermostat I want to install. I wouldn't be surprised if there is a 24VAC transformer inside the control box as there is 115VAC going into it.