Three AC units ? - Montana Owners Club

Half Fast Rusty · 06-04-2021, 11:44 AM

One of my friends just took delivery of their new Montana which had a third AC installed. They were in Redding with 105 degree weather and their unit stayed at the 72 degree setting.

I have a 2018 3810MS that has the normal two AC but is supposedly wired for a third. Has anyone installed the third AC and what were your experiences. What unit is recommended and does the thermostat need to be replaced or updated? We tend to travel in warm areas and having cooling that keeps up sounds inviting.

Thanks for your responses.

**DQDick** · 06-04-2021, 12:20 PM

We added the third AC to our 2017. All the wiring is up there and the phone cord for another thermostat is also up there. The other end of the phone cord should be found behind the control for the fan you are replacing.

Half Fast Rusty · 06-07-2021, 11:10 AM

Thanks Dick, So when you add the third, you are adding a separate thermostat control for that particular unit? Does it interact with the other two in any way or is it a completely separate system?

**DQDick** · 06-07-2021, 06:26 PM

On our 2017 it had a switch that would only allow either the bedroom unit or the third unit to run at a time. The newer rigs may have a different set up since more and more rigs are coming out with all three able to tun at the same time. I set ours up so the third AC was powered separately, but if I had it to do again I'd have left it Keystones way. The bedroom one is off often enough that I don't think it would be a problem.

nellie1289 · 06-07-2021, 11:30 PM

I’ve checked my EPO when both acs are running on my 2021 it’s drawing about 3200 watts with a few other items on like the tv and fridge. Wonder if all three would run if I added third ac it’s pre wired for.

**DQDick** · 06-08-2021, 08:34 AM

I think that's why on our 2017 it's wired to only let two run at a time because you're probably also running the water heater and the frig and getting close to the max on one leg or the other.

Daryles · 07-27-2021, 06:53 AM

[QUOTE=DQDick;1223659]On our 2017 it had a switch that would only allow either the bedroom unit or the third unit to run at a time.

DQDick
The switch you are talking about,
Is that the "Power Sharing Switch" under the stairs behind the basement wall on the right side?

**DQDick** · 07-27-2021, 08:54 AM

Yes, that's it.

Majorpolk · 06-20-2021, 03:59 PM

We have a 2021 3781RL and use the third AC when needed. The new AC units are soft start and we have had no issues.

Jamesb · 06-20-2021, 04:55 PM

On units with 3 AC units, can you run all three at the same time when plugged into 50amp shore power? Or do they cycle only two at a time?

Majorpolk · 06-20-2021, 05:10 PM

We run all three. Each one has its own thermostat.

GeologyTek · 06-20-2021, 10:11 PM

We have a 2018 3811MS that we installed the 3rd AC on. From the factory the unit is pre wired and framed for the third air conditioner in the kitchen. The maxair fan was removed and the new AC thermostat was installed where the fan control was located. Our unit came with a relay so we could only run 2 at a time. I added another 20 amp circuit and transfer switch so we could run all 3.

Now my rig has 2 cords. The standard 50 amp cord for the house and the extra 20 amp cord for the 3rd air conditioner. We it's hot we use all 3 and when it's cool enough we only use 1 or 2.

My page has tons of info on how I hooked mine up.

RMcNeal · 06-23-2021, 03:20 AM

Quote:

Originally Posted by GeologyTek

We have a 2018 3811MS that we installed the 3rd AC on. From the factory the unit is pre wired and framed for the third air conditioner in the kitchen. The maxair fan was removed and the new AC thermostat was installed where the fan control was located. Our unit came with a relay so we could only run 2 at a time. I added another 20 amp circuit and transfer switch so we could run all 3.

Now my rig has 2 cords. The standard 50 amp cord for the house and the extra 20 amp cord for the 3rd air conditioner. We it's hot we use all 3 and when it's cool enough we only use 1 or 2.

My page has tons of info on how I hooked mine up.

I'm considering adding the 3rd AC as you have and connecting it to a separate 30 amp cord or would it be possible to use the circuit for the washer or dryer? I don't have either installed and still haven't decided to add them.
Also, where is your page? Is it on the MOC or somewhere else? I'd be interested in the info you have there.

DavidG425 · 06-21-2021, 08:05 AM

The newer A/C's draw less power, about 13 amps, and we are able to run all 3 at the same time.

GreG L. · 06-22-2021, 12:35 PM

In a normal 2 AC setup one runs on one 50 amp circuit and the second runs on the second 50 amp circuit. Adding a third would mean that it would run on the same circuit as one of the others (drawing around 13+ amps each when running), thus needing a soft start for them both, hopefully the microwave is on the circuit with the single AC.
Plugging into 30 amp would put all three on the same single 30 amp circuit, not going to run all three.

DavidG425 · 06-22-2021, 10:49 PM

Quote:

Originally Posted by GreG L.

In a normal 2 AC setup one runs on one 50 amp circuit and the second runs on the second 50 amp circuit. Adding a third would mean that it would run on the same circuit as one of the others (drawing around 13+ amps each when running), thus needing a soft start for them both, hopefully the microwave is on the circuit with the single AC.
Plugging into 30 amp would put all three on the same single 30 amp circuit, not going to run all three.

GreG L,
As previously stated, on our new 2020 3813, we can run all 3 A/C's on 50 amp service. No Soft Starts required. The new Power Saver models draw less than 13 amps. I can't speak about running on 30 amp, but the math says not possible to run all 3. Running 2 on 30A would require Soft Starts and not much else using electricity.

Mikelff · 06-28-2021, 12:59 PM

I have a portable stand up 12,000 btu air conditioner vented out a side slide out window. Hooks up to a 30 /120 pig tail at the pedestal. The two roof top a/c’s are on the standard 50 amp plug. I think I am wired for a third rooftop A/C but want to keep my fan, so the portable works great for me. Whole system was under $300(used). Really cools down the main living area quickly and helps to cool the entire rig faster. When below 85-90 degrees my two units work well and in the cooler months the stand up unit is not needed and taken out and stored. The portable stand up fits in a corner of the slide and takes minimal space. Here in Texas with 100+ degree heat that extra 12k of btu’s cooling makes a huge difference. This might be an option for some folks that are looking for some options to adding a third rooftop A/C.

Half Fast Rusty · 07-24-2021, 09:55 PM

OK, finished the installation of the third unit. I stuck with the basic set up using the circuit provided by Keystone. The main issues and frustrations I had was the amount of information available to accomplish this project DIY. Keystone couldn't or wouldn't tell me what "Pre-wired" involved. When the AC unit arrived, it had no documentation with it and I had to find it online. When the Air Distribution box showed up, it had no documentation and I had to find it online. Same thing with the controller and thermostat. Figuring out out how to mount the controller so all the wires were neat and orderly took a couple of attempts. It is pretty much plug and play once you figure out how the parts fit together. You pull the Fan unit out and the wiring is there and hot. be careful. I left the wiring as provided so I can't run all three at the same time at this time. I can always change that if I choose later. I did turn them on and actually I don't think you need them all to run all the time. The new unit is slaved to the bedroom unit. I turned the two original units on to initially cool the coach. The bedroom cools faster and turned off when the set temp was reached. The new unit then turned on and helped cool the main living area. We were about 94 the day I was testing it. I waited until late afternoon when the interior was the warmest. Pretty close to 91 inside. When the new 3rd unit turned on after the bedroom was down to what I had set the room at, 76 the living room had cooled to about 88. Within about 30 minutes, the living area had cooled to 78 and continued to cool to the 72 I had set that room to not too long after. The bedroom and the third unit alternated running with the bedroom having the priority. I think is going to work just fine. The only thing left I need to figure out is related to the thermostat. It is giving me an E4 error which apparently means that it is looking for an outside temp sensor for the heat pump (which I didn't purchase). There should be and probably is a way to disable that code.

If I had to do it again, I would order the lower profile roof unit. Probably added a few inches to the total height of the trailer. Shouldn't be an issue, I hope.

Hope this helps some of you who are looking to add the third unit. The most difficult part of the project was getting the actual AC unit on the roof.

DavidG425 · 07-25-2021, 09:52 AM

Hi Rusty,
Glad to hear that you figured it all out.
Not sure why your new unit is slaved to the bedroom. When I had our 3rd AC installed, they replaced the Fantastic Fan controller on the wall with a thermostat.
Do you know if any of your units are the Power Saver models?

Half Fast Rusty · 07-26-2021, 05:51 PM

Well, slave is probably not the right word. It is connected to the same circuit some way at the converter so both units won't run at the same time. There is a thermostat where the old fan controller was. It is kind of odd to see two thermostats next to each other on the wall. The bedroom unit has priority over the new one over the kitchen. When the bedroom one turns on, the kitchen unit turns off. When the bedroom turns off, the kitchen turns on. The living room unit continues to run all the time or until it reaches the thermostat setting. I put easy start systems on the two original units last year. I haven't decided if I'll put one on the new one yet.

I think that those of you who have older trailers that are pre-wired, I wouldn't be afraid of adding the third unit if you think you need one. They won't work the same as the newest trailers (all three running at the same time), but I'm pretty sure it will work well enough. Like I said, I tested in when it was in the mid 90s and it kept my trailer at mid to low 70s which was what I set the thermostats.

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