Dometic furnace wiring details?

[LHaven]

I'm poring over the Dometic furnace wiring manual for my rig.

I understand that the connections to the furnace are one 12V, one ground, and two blue wires that the thermostat opens when the rig is warm and closes when it is cold.

The furnace keeps one of those blue wires at 12V and sends it out to the thermostat, and the result of closing those thermostat wires is to deliver 12V to the other wire, which feeds various boards and sensors in the furnace. Meanwhile, the 12V feed to the furnace is independent of the 12V coming back from the thermostat wiring because it has to continue to operate the blower for a period after the thermostat opens the blue circuit (temperature achieved).

Assuming I got all this correct, I understand it.

What isn't mentioned here is: when is the main 12V line at the furnace live? Is it live all the time, i.e., connected directly to the fusebox? Or does it go through one of the interface/gateway boxes and get switched off under certain circumstances?

I can't figure out whether the black and red leads on the left hand plug of the interface box (image) are delivering 12V+ and ground to the furnace, or thermostat signals to the A/C, or what they do. I don't even know if they're inputs or outputs.

I'm trying to determine if my interface box is good or blown. I see the blue connections to the furnace, but I don't know if furnace power runs through this board.

I hear a relay on this board click when I call for heat... and that's all that happens. I don't know whether I should see 12V+ at the red furnace wire all the time, independent of any possible fault in this interface box. Should I?

[LHaven]

I found a section that explains that the black and red leads are for lo/hi fan on the A/C, so it looks like no part of the left-hand plug goes to the furnace. Still would appreciate verification that the furnace power is run (unswitched) from the fusebox.

A related issue: My furnace is below the fridge and seems to share DC wiring. I have a fuse factory-marked as FURNACE, but no fuse marked for the fridge. Is it reasonable to assume these are the same fuse? I'm thinking if I have a chewed wire, then when I see no power at the furnace, the fridge shouldn't work either. Normally, I wouldn't see that because I'm a shore-power camper, but I could flip the AC breaker and test for it.

If I do have a chewed wire, at least running a new one should be simple, because the fusebox is right next to the furnace.

[ChuckS]

The furnace control board has its 12 vdc separate fed directly from your inside power center DC BUS ..

The control board has power ti it ALL THE TIME.. the “Signal” wired ( blue and blue) simply complete the circuit at the furnace control board to tell the blower motor to spin up..followed by sail switch moving and completing the circuit..followed by LP valve on furnace opening and igniter on furance furing

In your case I would be TS the main 12 vdc feed from the power center BUS to the furnace control board

You can easily see if you have source power at furance control board and then you can jumper the blue and blue wires to imitate furnace operation temporarily for testing thr control board itself and blower motor, etc..

[ChuckS]

Image from a pdf I wrote a while back.have helper turn on fursmce while you measure for 12 vdc at the BLOWER MOTOR connections.

If you don’t see 12 vdc when helper turns thermostat on for furance to run then control board for furnace is bad ..

You are hearing the delay relay click so this is telling me you have 12 vdc at the control board ..

If you measure 12 vdc at the blower terminals when furnace it’s turned on but blower don’t run then blower motor is bad

[LHaven]

If the delay relay is on the furnace board then that's not the click I'm hearing. The click comes from the interface box up over the ceiling inside the air conditioner well.

If I'm supposed to be seeing 12V at the furnace's red wire at all times, then that's where I'll start. If I don't, I'll do the test of running the fridge on 12V and propane to see if it faults, too. Otherwise, I'l run continuity on each blue wire between the interface box and the furnace. I don't think there's anywhere else it can be. I know it's not between the interface box and the thermostat, because I always hear that relay in the box click when I set the thermostat.

[chuckster57]

12V on the furnace red wire should be constant hot and come from the 12V distribution (fuse) panel.

[ChuckS]

Originally Posted by LHaven

If the delay relay is on the furnace board then that's not the click I'm hearing. The click comes from the interface box up over the ceiling inside the air conditioner well.

If I'm supposed to be seeing 12V at the furnace's red wire at all times, then that's where I'll start. If I don't, I'll do the test of running the fridge on 12V and propane to see if it faults, too. Otherwise, I'l run continuity on each blue wire between the interface box and the furnace. I don't think there's anywhere else it can be. I know it's not between the interface box and the thermostat, because I always hear that relay in the box click when I set the thermostat.

The click heard overhead is from either that Zone roof AC control box or gateway box and it is completing the circuit on those two blue wires down on the furance control board ( signal wires )

I would do as you are planning..get to the incoming 12 vdc source leading to the furance control board and verify 12 vdc present at all times

If YES then verify that when you hear that click overhead that the two blue signal wires connected to furnace board read 12 vdc when the overhead click is heard

If YES but blower motor doesn’t run you can easily remove the teo blower motor wires from the board and supply 12 vdc to those leads to eliminate issue with blower motor..

If blower motor runs with jumper wires with 12 vdc and you have 12 vdc at the control board and then control board has failed

[LHaven]

Well, it looks like I finally found my long-term intermittent furnace problem.

One conductor of the "blue wire pair" between the furnace and the HVAC control box is damaged. It's open now, it probably connects intermittently as the fridge shifts on the road or through ambient temperature.

It looks like another Keystone build problem. It's the topmost wire in the photo. Instead of running in the raceway underneath the fridge bracing furring, it's pinched smack between them.

I've asked the RV tech to come out and help me unpinch it. At that point, it will hopefully be loose along its path and can be used to pull a replacement wire in through the ceiling and wall.

[chuckster57]

Glad you found the issue. Keystone isn’t the only ones that put the wiring in “any way it will fit”. Gotta remember a trailer only sits at any one station for a very short time and the crew has a lot to do.

[Bill-2020]

Originally Posted by LHaven

I found a section that explains that the black and red leads are for lo/hi fan on the A/C, so it looks like no part of the left-hand plug goes to the furnace. Still would appreciate verification that the furnace power is run (unswitched) from the fusebox.

A related issue: My furnace is below the fridge and seems to share DC wiring. I have a fuse factory-marked as FURNACE, but no fuse marked for the fridge. Is it reasonable to assume these are the same fuse? I'm thinking if I have a chewed wire, then when I see no power at the furnace, the fridge shouldn't work either. Normally, I wouldn't see that because I'm a shore-power camper, but I could flip the AC breaker and test for it.

If I do have a chewed wire, at least running a new one should be simple, because the fusebox is right next to the furnace.

If I recall correctly (and it's been a while since I dug into the back of a propane fridge), even if you are on shore power, the fridge still requires 12v to it's control board, the 120V is just the heater. I don't recall seeing a transformer on the back of the fridge that would take that 120V and output 12V.

But you've already found your problem - so this doesn't help. I just thought I'd put my 10 cents in (inflation ya know).

[sourdough]

Originally Posted by chuckster57

Glad you found the issue. Keystone isn’t the only ones that put the wiring in “any way it will fit”. Gotta remember a trailer only sits at any one station for a very short time and the crew has a lot to do.

^^^^In the mass produced RVs coming out nowadays, from any brand, there in no telling what got missed as things are thrown together as the workers are literally running back and forth trying to get the unit to the next station. Simple things; an example would be my current trailer. After purchasing I went through THREE AC switches on the side of the water heater (the little upside down one). Could not figure it out; amps right, wiring right, water heater right...after the 3rd one, which was pretty hard to push back into the hole, I finally observed that they were being forced back into the foam insulation on the tank. In an effort to do something different I took a screwdriver and cut out a rectangle in the foam for the switch to freely sit. Two years now and nary a problem. Little things, strange things...but I can see them happening when running to get the job done.

[chuckster57]

Originally Posted by Bill-2020

If I recall correctly (and it's been a while since I dug into the back of a propane fridge), even if you are on shore power, the fridge still requires 12v to it's control board, the 120V is just the heater. I don't recall seeing a transformer on the back of the fridge that would take that 120V and output 12V.

But you've already found your problem - so this doesn't help. I just thought I'd put my 10 cents in (inflation ya know).

There isn’t one as far as I know, at least I’ve never seen one.

[LHaven]

Originally Posted by Bill-2020

If I recall correctly (and it's been a while since I dug into the back of a propane fridge), even if you are on shore power, the fridge still requires 12v to it's control board, the 120V is just the heater. I don't recall seeing a transformer on the back of the fridge that would take that 120V and output 12V.

I pulled the FURNACE fuse when I was working today, and the fridge eyebrow went off... so now I can write FRIDGE under the same fuse to keep track.

I think I may have told my story before about how my TV blew a transfer case in NM 20 years ago, and while the local dealer fixed it, we had to "store" our old rig at a local CG and take a rental car back home to make work responsibilities. We figured we had plenty of propane to ride out the duration, not understanding that we were also draining the battery. When we got back, the battery was dead, it took us a year to get the stank out of the fridge, and not even the coyotes would eat what we dumped at the end of the runway. (Luckily, nothing nauseates vultures.)

[jasin1]

Originally Posted by LHaven

I found a section that explains that the black and red leads are for lo/hi fan on the A/C, so it looks like no part of the left-hand plug goes to the furnace. Still would appreciate verification that the furnace power is run (unswitched) from the fusebox.

A related issue: My furnace is below the fridge and seems to share DC wiring. I have a fuse factory-marked as FURNACE, but no fuse marked for the fridge. Is it reasonable to assume these are the same fuse? I'm thinking if I have a chewed wire, then when I see no power at the furnace, the fridge shouldn't work either. Normally, I wouldn't see that because I'm a shore-power camper, but I could flip the AC breaker and test for it.

If I do have a chewed wire, at least running a new one should be simple, because the fusebox is right next to the furnace.

I suggested it may be on the same circuit last month if you recall..

[LHaven]

Originally Posted by jasin1

I suggested it may be on the same circuit last month if you recall..

Oh, no doubt. I wasn't too surprised at that because they're both wired behind the fridge. I was more surprised that the fridge got zero billing in the fusebox.

The furnace spec says my furnace can grab 11.5A. On a 15A fuse, that would leave 3.5A for the fridge, which seems a little skinny. The fridge manuals are quite tight-lipped about the current draw, save to say that if you have the optional icemaker (I don't) it eats an extra 1.4A. Wiring them on the same circuit seems... adventurous... to me, but I admit I don't have all the details.

[ChuckS]

Nice work finding the issue. Once resolved there should be no further issues ..

[LHaven]

So once again, Mickey was the problem. (Photo)

The tech disconnected the fridge and pushed it about a foot into the cabin to access the wires. He freed the stuck cable, but the cables separated when we tried to use the old one to pull the new one. That's when we found the chewed-up portion above the ceiling, where our Mickey did all his damage (including this) a couple years ago. (We've successfully shut out the mice since then.)

So we had to remove the fridge entirely into the cabin to re-fish the wire. This was a two-man job, and I'm really grateful to have had a guy on hand who knew where all the screws and connections were from experience.

My tech cut an armhole into the ceiling above the fridge (inside the compartment where it's not visible) where he could reach both ends of the cable run above the ceiling, and that worked very effectively. Then he reinstalled the cutout piece with thermal aluminum tape, rewired the furnace connections, we lifted the fridge back into the cabinet, and he finished up all the re-mounting and gas piping. The fridge, furnace, and A/C all work now!

He also installed check valves in the water heater, I blew all the remaining "glitter" in the cold line from my toilet feed into the shower drain, and reconnected the toilet.

I think this is the first moment since I have owned this rig that everything in it works properly! I believe I will sleep in it tonight just to celebrate!