Third AC

[Andy Smith]

Hello! I'm rolling in a early 2020 3855br with the the AC prep. I am looking at installing the third AC to help with this Texas summer and was wondering if there's an install guide and compatible model list. The unit currently has the two stock dometic brisks and they're doing well, but it's been in the mid 100s and I don't see that changing any time soon. Is this a project a somewhat handy fellow could accomplish without too much cursing, or should I be looking for a mobile professional?

Thanks!

[RMcNeal]

Andy,
I had considered doing that with mine, but I found out that the pre-wire shares power with the bedroom AC, so you would still only be able to have 2 units running at one time. That really doesn't seem like it would help much.
I have read about wiring it with a separate supply and plugging into a 20 amp receptacle on the post, completely separate from the rigs power source, but I haven't done anything yet. I could also run it using the dedicated breaker for the washer or dryer since I do not have those yet, but I'm not sure where the wiring goes from the prewire, or how the power-sharing is accomplished. Hopefully someone who has done it will come along and enlighten us both.

[Andy Smith]

Well hopefully someone has some suggestions, as that doesn't seem like a very useful pre-wire if you can't actually use all three at once.

[RMcNeal]

Yeah, my thoughts exactly.
I know some have said they ran power to their third AC independently from the 50 amp panel. I was thinking of doing the same if I ever add the third AC.

[C&P RV]

Andy, I recommend you look at your electrical panel and see if you find an unused breaker or an empty slot for a breaker. All 2022's were advertised as being factory built pre-wired for a 3rd AC. My 2022 came with the 3rd AC installed by the factory in place of the MaxAir fan and has it's own breaker in the panel. (There's also a loop for that circuit in the basement to connect it to a larger inverter as part of the SolarFlex 400i package, but that won't apply in your case.) I've seen posts in other places where people remove the vent and find the wire terminated there along with the thermostat control wire.

Other things to note - you probably won't be able to connect the 3rd AC to the duct/vents in the ceiling. Mine is a "dump" AC that pumps cold air out one side and sucks in the return on the other.

The other challenge you may have is getting the new unit on the roof and in place. I've had to have an AC replaced under warranty and my dealer used a forklift and two burly guys to move the units around.

Good luck!!

[Ram Montana High Country]

Friends of ours did a 3813 with the third and the load shed prevented running all three. On my 373RD it was wired directly to the outside so I can run another cord to the pedestal. Noisy but functional. More that likely I'd NOT Use Dometic for the third. Think of putting in a transfer switch and wiring into washer/dryers breakers and adding a soft start.

[PhilC]

Here’s a picture of the AC control module in the Montana it’s located behind the wall in the front storage bay under the stairs I’m sure your model has it in a similar location. From the module you can separate the wiring and decide how you want to wire the 3rd AC without the need to fish wires up to the roof. Hope this helps.

[DavidG425]

I have a different model (2020 3813MS) And had the 3rd AC installed by a mobile rech and his helper. They got it on the roof just fine without a forklift. All wiring was already in place. Not sure if they put the breaker in the electrical panel or not. All 3 AC's are Coleman mach 15's, power saver version, one with the heat pump. I can run all 3 at the same time with no power issues.

One thing that helped A LOT when we lived in southern CA was a cover over the roof from www.shaderv.com. With one of those covering the roof, I was able to keep the inside temperature below 80, even when it got to 110-115 outside

[LERD]

Originally Posted by RMcNeal

Andy,
I had considered doing that with mine, but I found out that the pre-wire shares power with the bedroom AC, so you would still only be able to have 2 units running at one time. That really doesn't seem like it would help much.
I have read about wiring it with a separate supply and plugging into a 20 amp receptacle on the post, completely separate from the rigs power source, but I haven't done anything yet. I could also run it using the dedicated breaker for the washer or dryer since I do not have those yet, but I'm not sure where the wiring goes from the prewire, or how the power-sharing is accomplished. Hopefully someone who has done it will come along and enlighten us both.

You're looking at the power of your RV incorrectly. A 50 amp circuit is more than a 30 amp circuit. But it's much more than a 20 amp difference. The main thing to remember is that almost all Montana's have a 50 amp circuit. But unlike a 30 amp rig, you have 50 amps per leg. So ultimately you have 12000 watts of available power to use, not 3600 watts like a 30 amp rv would have. That's a big difference. So you have more than enough juice to run 3 A/Cs all at once. And since it's provisioned for at the factory, you probably only have the 2 A/Cs and small stuff on that leg. i.e.you won't have the microwave or the water heater on that circuit.

[GeologyTek]

One thing to know about the pre wiring on the 3rd air conditioner. If there is a relay in the existing wiring you might not be able to run 3 at the same time. We have a 2018 3811-MS, I had the 3rd air conditioner installed at the shop when I purchased the unit. I added another 30 amp hook up, cord set, and a power selector switch to the unit. Now I can flip a switch and put the camper in 80 amp mode and run all 3 units, or leave it in 50 amp mode and use the relay.

In 80 amp mode I can run all 3 for 45,000btu of cooling power. In 50 amp mode I have 30,000btu of cooling power due to the relay. We live in our unit full time so I have the washer/dryer residential refrigerator and tons of other stuff. The extra 30 amps makes a big difference.

[Mikelff]

I run a 12K portable ac unit and vent out a side slide out window. Sits in a corner and out of the way. Makes a huge difference during cool down and maintaining cool temps. Run it off a 20 amp extension cord and to the pedestal. I have found some 20 amp pedestal outlets do not work so I also have a 30amp to 20 amp dog bone adapter just in case. My 2 A/C units do great below about 97 degrees but after that they struggle. The portable was $300 used but like new. I don't camp much in 100+ weather but when I do, I take the portable and works great. Sure beats the thousands of dollars for a third A/C . If I was full timing or camping often in the high heat I would definitely go with a third roof top unit. Stay cool!!

[Dratlief]

I chose the FURRION CHILL 15.5OO BTU unit for my 3RD AC unit due to the Texas heat and the SOFT START option. The soft start allows the AC unit to run on the existing breaker circuit without tripping breakers.

Note, if you don’t have shade to block the direct sun, inside temp will hover around 80 deg. even with all 3 AC unit running, BUT, the inside will cool down quickly once the sun is off the RV.

FYI - I used the existing 3RD Prewired AC Romex / breaker circuit, but had to pull an independent thermostat cable. Also added a lot of insulation around the turbo fan opening and then sealed the opening with AC Aluminum duct tape. OFF subject, at the same time, I dropped the inside shower skylight, added insulation ans sealed the opening with AC aluminum duct tape. This was to keep the heat out of the ceiling Aires.

An aerodynamic and powerful 15.5K BTU Air Conditioning system for your RV, the rooftop unit has been engineered with dual fans to increase cooling efficiency and lower operating noise while the wall thermostat gives you effortless control of the temperature and airflow.

Parts List Includes:
- 15.5K Rooftop Unit (FACR15SA-BL)
- Single Zone controller (FACC12SA-BL)
- Air Distribution Box with LED (FACT12LA-PS)
- Premium Wall Thermostat (FACW12PA-BL)
Thermostat wire
Roll of aluminum AC duct tape
Roll of 6” fiberglass insulation

[Dratlief]

Join Date: Jun 2021
Location: Spring Hill
Posts: 35
M.O.C. #29012
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3RD AC Unit DYI Ref 2021 3781RL
I chose the FURRION CHILL 15.5OO BTU unit for my 3RD AC unit due to the Texas heat and the SOFT START option. The soft start allows the AC unit to run on the existing breaker circuit without tripping breakers.

Note, if you don’t have shade to block the direct sun, inside temp will hover around 80 deg. even with all 3 AC unit running, BUT, the inside will cool down quickly once the sun is off the RV.

FYI - I used the existing 3RD Prewired AC Romex / breaker circuit, but had to pull an independent thermostat cable. Also added a lot of insulation around the turbo fan opening and then sealed the opening with AC Aluminum duct tape. OFF subject, at the same time, I dropped the inside shower skylight, added insulation ans sealed the opening with AC aluminum duct tape. This was to keep the heat out of the ceiling Aires.

An aerodynamic and powerful 15.5K BTU Air Conditioning system for your RV, the rooftop unit has been engineered with dual fans to increase cooling efficiency and lower operating noise while the wall thermostat gives you effortless control of the temperature and airflow.

Parts List Includes:
- 15.5K Rooftop Unit (FACR15SA-BL)
- Single Zone controller (FACC12SA-BL)
- Air Distribution Box with LED (FACT12LA-PS)
- Premium Wall Thermostat (FACW12PA-BL)
Thermostat wire
Roll of aluminum AC duct tape
Roll of 6” fiberglass insulation

[Mikendebbie]

In post #11 above - Mikelf mentioned a portable AC unit could be used and vented out the window. THANKYOU For that suggestion. I grabbed the one in DW’s she-shed (she prefers “playhouse”) and brought on this trip. Holy smokes…she is asleep right now in her theater chair with a blanket! Makes a huge difference. Fortunately we kept all the pieces to fit a window. Duct tape covers the holes at the edges.

If you decide to try this - dont forget a short piece of garden hose and a bucket to collect the condensate. The white garden hose is visible in the pic below and a I used a little plastic storage bucket to catch condensate water. The plastic bucket is used to sit the Keurig coffee maker in on the floor under the table for travel. The AC has been running about 5 hours today and there is 2” of water in the bucket already. Our den and bedroom ACs are running full speed. It is 73* inside - 96* and climbing in direct sunlight in the pm. We get morning shade in this site.

[RMcNeal]

That portable looks a lot like the one we have here at home.
Ours has a small hole in the plastic panel for the window so that we can run the drain tubing outside and not need a collection bucket.
You may be able to modify yours to do the same.

[Mikelff]

Originally Posted by Mikendebbie

In post #11 above - Mikelf mentioned a portable AC unit could be used and vented out the window. THANKYOU For that suggestion. I grabbed the one in DW’s she-shed (she prefers “playhouse”) and brought on this trip. Holy smokes…she is asleep right now in her theater chair with a blanket! Makes a huge difference. Fortunately we kept all the pieces to fit a window. Duct tape covers the holes at the edges.

If you decide to try this - dont forget a short piece of garden hose and a bucket to collect the condensate. The white garden hose is visible in the pic below and a I used a little plastic storage bucket to catch condensate water. The plastic bucket is used to sit the Keurig coffee maker in on the floor under the table for travel. The AC has been running about 5 hours today and there is 2” of water in the bucket already. Our den and bedroom ACs are running full speed. It is 73* inside - 96* and climbing in direct sunlight in the pm. We get morning shade in this site.

That looks exactly like how I do mine. Glad it worked out. Mine vents moisture out the vent so I don't have much condensation to worry about. I use foam strip insulation for the gaps in the window and don’t use duct tape, but I’m sure duct tape works just fine. The portable A/C sure is a less expensive option to a new roof top unit.

[rubio171]

We have a 2019 3811ms. 2 weeks ago I bought a Dometic 15,000btu and added it to the prewired spot over the kitchen. Pretty easy install. I removed the relay box, wired the bedroom ac straight to breaker and wired a heavy duty extension cord to the wiring from the prewired roof setup. Now I use it on a seperate outlet on the pedestal which comes in pretty handy. If there is only 30amp at the site then i can still run 2 units by using the bedroom ac and the added ac on the seperate connection.