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sonofcy
11-26-2017, 05:48 PM
2018 Montana 3811MS with heat pump, A/C and gas heat.

I would like to replace the stock thermostat with a Nest thermostat but the 3 wires are not labelled. Does anyone know where I could get reliable info on which wire is which?

chuckster57
11-26-2017, 05:51 PM
Quick check of nest specs say it works with 24V HVAC, RV is 12V.

Wires should be 12V+
Ground
Signal to control panel ( located in AC return plenum)

sonofcy
11-26-2017, 06:37 PM
Quick check of nest specs say it works with 24V HVAC, RV is 12V.

Wires should be 12V+
Ground
Signal to control panel ( located in AC return plenum)
I think it is obvious that the 3 wires 2 of which are + and - 12v are not enough to control a gas furnace, fan, A/C and heat pump. Those wires go to a 'black box' that controls the HVAC equipment and will likely have the standard connections. The trick will be locating the black box but as I am writing this I just remembered I bought one of those inspection cams the other day so I will give that a try.

chuckster57
11-26-2017, 06:45 PM
Control box is at the AC. The wires will be obvious. You have to remove the ceiling plenum (grill) to see it.

sonofcy
11-26-2017, 06:55 PM
Control box is at the AC. The wires will be obvious. You have to remove the ceiling plenum (grill) to see it.
Not that kind of A/C, there are a total of 4 cold or warm air outlets a long way from the thermostat. The wires run down so I will have a look with the inspectoscope tomorrow.

chuckster57
11-26-2017, 06:58 PM
Not the outlets. Unless you have a “whisper quiet” system with returns on the edges of the ceiling, the return will be where the main AC unit is and you will need to remove the grill that has the filter in it.

sonofcy
11-26-2017, 07:01 PM
Not the outlets. Unless you have a “whisper quiet” system with returns on the edges of the ceiling, the return will be where the main AC unit is and you will need to remove the grill that has the filter in it.
Yes, it's a whisper quiet.

brad2388
11-26-2017, 07:30 PM
Nope it wont work.

The rv thermostat is on a communication cicuirt and all talk to each other. Includeing the gas heater.


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sonofcy
11-26-2017, 07:41 PM
Nope it wont work.

The rv thermostat is on a communication cicuirt and all talk to each other. Includeing the gas heater.


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Given that there are only 3 wires on the thermostat I figured it was just a comms circuit, I am going to try to find the black box it talks to and see if it has 'standard' HVAC wiring.

brad2388
11-26-2017, 07:56 PM
Given that there are only 3 wires on the thermostat I figured it was just a comms circuit, I am going to try to find the black box it talks to and see if it has 'standard' HVAC wiring.



Its not.
The boards are relay based and each board has its own logic to it.
If you have the dometic nxt units you can take the top off and if your looking at the front of the ac unit the control boxes are on the left side.

On each control box theres a set of dip switches to select what zone it is and if it has heat strips or a heat pump option.



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sonofcy
11-26-2017, 08:01 PM
Its not.
The boards are relay based and each board has its own logic to it.
If you have the dometic nxt units you can take the top off and if your looking at the front of the ac unit the control boxes are on the left side.

On each control box theres a set of dip switches to select what zone it is and if it has heat strips or a heat pump option.



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That doesn't sound encouraging. The little info I have found includes a photo of the LCD thermostat and it is clearly wired to a control box. I even downloaded a complete schematic but I don't yet know if it is the same model. Will get back to it Tuesday, too tired now and busy out of town tomorrow.

chuckster57
11-26-2017, 08:12 PM
I’m learning something new here too. Haven’t had much exposure to the 10 button thermostats. We just started carrying Heartland and I think they have them.

sonofcy
11-26-2017, 08:16 PM
I’m learning something new here too. Haven’t had much exposure to the 10 button thermostats. We just started carrying Heartland and I think they have them.
Mine is NOT a 10 button, just a mode selector and an up/down pad. It's capacitive and a pita to operate, it changes from C to F on a whim and will continue to increase or decrease after you remove your finger. Typical RV crap.

chuckster57
11-26-2017, 08:29 PM
So you have the 3 button. LCD screen, up and down buttons on right and a mode button on the left?

If you have more than one unit on the roof, I’m not seeing how it can choose a “zone”. It can be a bit temperamental, but once you understand how to “touch” it, it should be good.

sonofcy
11-26-2017, 08:33 PM
So you have the 3 button. LCD screen, up and down buttons on right and a mode button on the left?

If you have more than one unit on the roof, I’m not seeing how it can choose a “zone”. It can be a bit temperamental, but once you understand how to “touch” it, it should be good.
2 units. Main is A/C and heat pump. The bedroom is just A/C although we were under the impression it was also a heat pump. It has its own LCD thermostat. It has fan, A/C and furnace but as far as I can tell the furnace setting does nothing. The main unit is fan, A/C, furnace and heat pump.
Love the heat pump, no condensation issues.

chuckster57
11-26-2017, 08:37 PM
Ok. That’s what I see a lot of. The main thermostat will have:
Fan
Cool
Heat pump
Furnace
On the left side.
Working on a new SunSeeker class C with this. I’ll see what I can dig up.

MikeD3534
11-27-2017, 05:36 AM
I'm wanting to do the same thing (install a wifi thermostat). Please keep us posted. I have a feeling that the RV industry will have to come up with a "nest-like" product.

ctbruce
12-03-2017, 07:46 AM
2 units. Main is A/C and heat pump. The bedroom is just A/C although we were under the impression it was also a heat pump. It has its own LCD thermostat. It has fan, A/C and furnace but as far as I can tell the furnace setting does nothing. The main unit is fan, A/C, furnace and heat pump.
Love the heat pump, no condensation issues.Open the bedroom unit up and see if the heat strip is connected.

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sonofcy
12-03-2017, 09:43 AM
Open the bedroom unit up and see if the heat strip is connected.

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There is nothing to see inside of these thermostats, they do not have conventional wiring

ctbruce
12-03-2017, 09:51 AM
There is nothing to see inside of these thermostats, they do not have conventional wiringThe air conditioner not the thermostat. The heat strip is in the AC unit.

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sonofcy
12-03-2017, 10:17 AM
The air conditioner not the thermostat. The heat strip is in the AC unit.

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Since I wasn't positive what a heat strip was I googled it and on Camping World the description says it is NOT for ducted units which ours is. That strip goes in the distribution box in the ceiling that I do not have.

The only other time I have heard the phrase 'heat strip' is in connection with a heat pump. It is a simple resistive heater that kicks in around 40F roughly. It is the AUX circuit/light.

I will go up on the roof and see if I can see anything and take pictures if I do. I am also going to try out my new inspection cam to see if I can determine a make/model of the HVAC controller the 'thermostat' talks to.

bobbecky
12-03-2017, 03:05 PM
Another thing to note about the 4 wire comm cable that looks like a conventional phone cable, it is not the same, but is special for the RV system. Our unit has the Dometic single thermostat that controls both AC units as well as the furnace. The comm cable goes from the thermostat to the rear AC, which is dip switch identified as unit 1. The unit 1 box also has a cable that goes to the furnace. Then from that control box under the unit, another comm cable goes to the front AC, which is dip switch identified as unit 2. Because there is only one thermostat in this system, there is a remote temperature sensor on the ceiling of the front bedroom, and the thermostat senses the temperature in the rest of the trailer. The thermostat and the control boxes are together called a CCC II system by Dometic. They got away from using a single thermostat due to problems operating these units and went to two thermostats. It sounds like the new systems are still using the same comm cables which are not the same as a residential AC system wiring.

sonofcy
12-03-2017, 03:12 PM
Another thing to note about the 4 wire comm cable that looks like a conventional phone cable, it is not the same, but is special for the RV system. Our unit has the Dometic single thermostat that controls both AC units as well as the furnace. The comm cable goes from the thermostat to the rear AC, which is dip switch identified as unit 1. The unit 1 box also has a cable that goes to the furnace. Then from that control box under the unit, another comm cable goes to the front AC, which is dip switch identified as unit 2. Because there is only one thermostat in this system, there is a remote temperature sensor on the ceiling of the front bedroom, and the thermostat senses the temperature in the rest of the trailer. The thermostat and the control boxes are together called a CCC II system by Dometic. They got away from using a single thermostat due to problems operating these units and went to two thermostats. It sounds like the new systems are still using the same comm cables which are not the same as a residential AC system wiring.
Ours is a 2 thermostat system each thermostat having 3 wires. Once I can get a schematic of the control box I expect to see 'conventional' HVAC wiring. The one difference may be 12V vs 24V but that is easily dealt with.

fatcatzzz
12-03-2017, 04:34 PM
Since I wasn't positive what a heat strip was I googled it and on Camping World the description says it is NOT for ducted units which ours is. That strip goes in the distribution box in the ceiling that I do not have.

The only other time I have heard the phrase 'heat strip' is in connection with a heat pump. It is a simple resistive heater that kicks in around 40F roughly. It is the AUX circuit/light.

I will go up on the roof and see if I can see anything and take pictures if I do. I am also going to try out my new inspection cam to see if I can determine a make/model of the HVAC controller the 'thermostat' talks to.

There is not a heat strip in a heat pump. Try google for heat pump reversing valve, this is what you should see if it has a heat pump. Basically makes the ac run in reverse. Cold air out, warm air in.

sonofcy
12-03-2017, 04:41 PM
There is not a heat strip in a heat pump. Try google for heat pump reversing valve, this is what you should see if it has a heat pump. Basically makes the ac run in reverse. Cold air out, warm air in.
I am not sure what you meant but to say that a heat pump does not have a heat strip disagrees with the following which is one of many google results

Because heat pumps work by pulling heat from the air outside your home, the colder (specifically, temperatures below 40 degrees) that air is, the more difficult it is for your heat pump to operate this way. So, in addition to this heating method, most heat pumps use strips of electric heating coils as an auxiliary heating source if the unit is not able to convert the outside air quickly or efficiently enough. Heat strips are wire elements in your unit that are heated by electricity, which in turn heat the air that flows over them.

Heating air using heat strips requires much more energy, but your heat pump relies on these strips to perform certain tasks, such as defrosting your unit. But, even though it’s unavoidable (and sometimes necessary) for your heat pump to use the heat strips occasionally, there are things you can do to prevent your heat pump from using them frequently.

chuckster57
12-03-2017, 06:41 PM
Heat pumps in RV’s are an RV A/C “running in reverse”. There isn’t a heat strip in them that I’m aware of. They do have a temp limit and in the applications I see them used in, if the set temp is more than 5 degrees from ambient, the furnace will also operate.

ctbruce
12-03-2017, 07:37 PM
Sonofcy only has 1 heat pump amd one regular AC unit. From above "2 units. Main is A/C and heat pump. The bedroom is just A/C although we were under the impression it was also a heat pump. It has its own LCD thermostat. It has fan, A/C and furnace but as far as I can tell the furnace setting does nothing."

If he has a heat strip, it would be in the regular AC unit. There are several threads on the factory not connecting the plug to the heat strip.

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sonofcy
12-03-2017, 07:54 PM
Sonofcy only has 1 heat pump amd one regular AC unit. From above "2 units. Main is A/C and heat pump. The bedroom is just A/C although we were under the impression it was also a heat pump. It has its own LCD thermostat. It has fan, A/C and furnace but as far as I can tell the furnace setting does nothing."

If he has a heat strip, it would be in the regular AC unit. There are several threads on the factory not connecting the plug to the heat strip.

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I hope it is obvious since it is the rainy season and I don't really like being on the roof. Does the A/C cover come off easily? If I get it off what am I looking for in terms of a disconnected heat strip?

chuckster57
12-03-2017, 07:58 PM
The cover should be held on by 4 Phillips head screws. You’ll have to lift the back off first.
Model and serial number will be in plain sight and a quick call to manufacturer should verify if a heat strip was option/installed.

sonofcy
12-03-2017, 08:01 PM
The cover should be held on by 4 Phillips head screws. You’ll have to lift the back off first.
Model and serial number will be in plain sight and a quick call to manufacturer should verify if a heat strip was option/installed.
Great. I hear the temp is going up to 9C in a couple of days so will check it out then.

ctbruce
12-03-2017, 08:11 PM
I hope it is obvious since it is the rainy season and I don't really like being on the roof. Does the A/C cover come off easily? If I get it off what am I looking for in terms of a disconnected heat strip?It should be an inside job, no worry about rain unless you have a leaky roof. :-)

Here's a video that illustrates the project. Hope it helps. Sorry, no sound.
https://youtu.be/02iIVGDk_CE

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sonofcy
12-03-2017, 08:26 PM
It should be an inside job, no worry about rain unless you have a leaky roof. :-)

Here's a video that illustrates the project. Hope it helps. Sorry, no sound.
https://youtu.be/02iIVGDk_CE

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Not that kind of A/C, it's an outside job. Curved roof and water plus 75 years old and 14+ feet off ground is definitely worrisome.

chuckster57
12-03-2017, 08:41 PM
I haven’t had to replace a roof unit with the “whisper quiet” configuration, so I’m not positive how it’s held down, but I’m willing to bet the heat strip would be on the “interior” and the plug for it would be there too.

ctbruce
12-03-2017, 08:41 PM
Not that kind of A/C, it's an outside job. Curved roof and water plus 75 years old and 14+ feet off ground is definitely worrisome.Uncle, I give up. Good luck.

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sonofcy
12-03-2017, 08:49 PM
I haven’t had to replace a roof unit with the “whisper quiet” configuration, so I’m not positive how it’s held down, but I’m willing to bet the heat strip would be on the “interior” and the plug for it would be there too.
I don't know where else it would be but in the A/C unit's interior, it would short out otherwise.
It just dawned on me, I can pull the cover on the LR heat pump and see how it is put together then compare that to the MBR unit. It may not be a heat pump but could have a heat strip thus explaining the Furnace setting on the MBR thermostat. I am lookingh for doc'n to see what that says.

brad2388
12-04-2017, 04:57 AM
If you have a ducted ac unit the heating element in in front of the ac coil. Take the top off the unit and remove the black foam case and you should see it.

A heat pump in the rv is setup where the aux is the furnace. Once below 40 degrees or so the heatpump isnt efficent and cuts off and switches to the furnace.

In a house the aux could be heat strips or a gas furnace.




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fatcatzzz
12-04-2017, 06:27 AM
I haven’t had to replace a roof unit with the “whisper quiet” configuration, so I’m not positive how it’s held down, but I’m willing to bet the heat strip would be on the “interior” and the plug for it would be there too.

They are screwed down with 4 lag bolt, from the top. Actually easier to remove than the old style with interior (clamp style) register. Conversations I have had with my dealer and with Keystone who both have stated there is NO heat strip. Here is a service manual for a dometic ac/heatpump. http://bryantrv.com/docs2/docs/acservice.pdf Check out page 23 and it should explain it. Your model may vary, but principle is the same.

sonofcy
12-04-2017, 08:37 AM
If you have a ducted ac unit the heating element in in front of the ac coil. Take the top off the unit and remove the black foam case and you should see it.

A heat pump in the rv is setup where the aux is the furnace. Once below 40 degrees or so the heatpump isnt efficent and cuts off and switches to the furnace.

In a house the aux could be heat strips or a gas furnace.




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It's been below 40 a lot here and at the moment it's 35 and the heat pump is working just fine. I thought when the heat pump wasn't efficient at low temps that a heat strip augmented it. In any case, well below 40 and heating is only heat pump at the moment.

brad2388
12-04-2017, 08:40 AM
It's been below 40 a lot here and at the moment it's 35 and the heat pump is working just fine. I thought when the heat pump wasn't efficient at low temps that a heat strip augmented it. In any case, well below 40 and heating is only heat pump at the moment.



If youll notice that the inside fan will stop. Thats the defrost. Then once it gets out of defrost it will resume.

Im not sure of the tempature at which it stops. But was thinking ours was in the 30s.


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sonofcy
12-04-2017, 08:45 AM
They are screwed down with 4 lag bolt, from the top. Actually easier to remove than the old style with interior (clamp style) register. Conversations I have had with my dealer and with Keystone who both have stated there is NO heat strip. Here is a service manual for a dometic ac/heatpump. http://bryantrv.com/docs2/docs/acservice.pdf Check out page 23 and it should explain it. Your model may vary, but principle is the same.
Very strange, they say NO heat strip and prove it with a manual that has on page 30 the description of the heat strip and the instructions to diagnose it.

I also notice a reference to the 24V transformer which does mean that it may be possible to hook up the nest via conventional wiring.

Thanks for the info.

sonofcy
12-04-2017, 08:50 AM
If youll notice that the inside fan will stop. Thats the defrost. Then once it gets out of defrost it will resume.

Im not sure of the tempature at which it stops. But was thinking ours was in the 30s.


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What do you mean by 'inside fan'?

bobbecky
12-04-2017, 05:35 PM
Very strange, they say NO heat strip and prove it with a manual that has on page 30 the description of the heat strip and the instructions to diagnose it.

I also notice a reference to the 24V transformer which does mean that it may be possible to hook up the nest via conventional wiring.

Thanks for the info.

Be aware that the manual at the link provided is from 17 years ago, and may not accurately reflect current technology with the newer heat pumps. First page at the bottom shows 10/00, October of 2000 which is the publish date.

sonofcy
12-04-2017, 05:36 PM
Be aware that the manual at the link provided is from 17 years ago, and may not accurately reflect current technology with the newer heat pumps. First page at the bottom shows 10/00, October of 2000 which is the publish date.
Did not notice that. I will get a look at mine in a day or two now that the rain has stopped.

brad2388
12-04-2017, 08:05 PM
What do you mean by 'inside fan'?



The fan that blows air out of the vents in the rv. It will stop while in defrost. Normally it last ~10 minutes


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Dylant
07-15-2018, 01:29 PM
Sonofcy,
Were you ever successful in figuring out the installation. Got a 3811MS a few months ago, with the temp swings here in ABQ, Nest is a must. Any help would be much appreciated.

sonofcy
07-15-2018, 01:35 PM
Not yet. Been busy with a bunch of other stuff. The problem is it is probably inside the wall behind the closet or the control panel above it. I will need to get in there to install my solar (not the pos that Keystone installs) so when I do that it will either be visible or I will likely give up since I have no other clues.

sonofcy
12-27-2019, 02:17 PM
Sonofcy,
Were you ever successful in figuring out the installation. Got a 3811MS a few months ago, with the temp swings here in ABQ, Nest is a must. Any help would be much appreciated.

I found where the elevant wires might be, but I am not in a frame of mind to tackle it. Today I get an email from a kickstarter project launched today. It is an adapter plate to go between the Dometic wiring and the Nest wiring. I am interested but not for the $300 asking price. I am not sure the project will get funded as they are looking for about $325,000 over 35 days. I have another way to tackle it that I will investigate this coming spring. If anyone is interested in implementing whatever solution I come up with if any, send me a private message.

GeekSquadOfUn
12-27-2019, 03:06 PM
I congratulate you on this initiative, but I am not ready to invest close to $300 on this, a tad too much in my mind. Under $100 you would have traction.

I would suspect, if you have indeed figured out the issues with the 24V, the 2 speed AC and the C wire, it may also work with other smart models, like the Ecobee or GLAS.

Good luck - I'll be following this. In my coach I have iN-Command by ASA controlling my HVAC and am quite happy - not as granular as my Ecobee at home, but it's great.