Background.
2017 Montana 3710FL
Hi-Cap Dometic Pelica II AC unit (Front)
15k Dometic Brisk 2 AC Unit (Rear)
Both units are ducted

Dad is snowbirding this winter from Montana. He has a very narrow temperature tolerance. It's been nice here in Central California, we needed the rain. Temp started to warm up and the trailer warned up as well. We found the AC units would only keep the coach at 2* below ambient. check the volts, and amps both good. Checked the T-Stats, both working.(individual units)

Started looking at the vents and ducting... And I think I may have found the answer or at least a contributing factor.

This is the way the trailer is ducted.


The orange represents the intake and the grey with output. Blue circles are vents.
Interesting thing the output ducts run the whole length of the trailer. So if one unit is on, it will blow through the other unit.

I took the cover off the intake ducts and was shocked.
One side was cut to 4"x8" and the other at 4"x9".
while the intake opening area is about 5"x12".
The actual duct is about 14" across.
Dometic says 100 sq in area is needed for proper flow.





Nice QC Keystone. Who ever used the router missed the duct wall, then covered it with foil tape.

We moved the vent cover over and lined out the new opening.





Big difference. I re-foil taped up the hole. (This is the other side)



Something I did not get a photo of was the height of the intake duct area. how does .75" sound, that's right less than one inch. We made a spacer 1.75"x 12", covered it with foil tape and placed one on each side of the duct inside to open op the intake area.

I also looked at the vents themselves. Here is the original.



I found these that have a dampner.



I removed the front six vents and cleaned up all the foil tape. I replaced the front four vents with no issues.







Another thing I noticed is the height of the output duct "chimney" was 1.5". The ceiling to duct floor was about 1". So I trimmed the chimneys.




I wanted to separate the front and rear, so I made a "Dam" from foil covered RV styrofoam.



To the rear of the Kitchen vents I put the Dams in place and locked them in with foil tape.



Here is a shot of the final product.



We did a short test, and were about to drop the temperature of the trailer 5* below ambient in 20 minutes. Not too bad, and only running the front AC. (Yes it was about 7* cooler outside than the other test) there is considerably more airflow coming through the vents now.

I may eventually end up doing this.
Note I have added two vents into the kitchen area, moved the Dams back to the rear of them and swapped the front/rear AC units. We'll see once it gets a little warmer.



ZooDad

 

There are many of us that have had trouble with the new "Whisper Quiet" AC system by Dometic. Our unit has no air flow at all and is at the factory as we speak for the broken frame AND the AC problems. It was infuriating camping all last summer in the heat with a brand new camper! I HATE this new system. I removed our vents covers completely and made it a "little" better, but we are very disappointed in the new system. And Montana doesn't really seem to care!

 

No they don't as shown by the chitty workmanship. I would imagine most other mfg's are about the same.......too bad.

 

I would caution sealing off one ac unit to only cool part of the 5th wheel.


When it gets real hot you want them both running and pushing air out all the vents so you can get it cooled down and maintain the temp.

 

I agree with Scott. Main thing is that the plenums are of a proper size and all vents are open and unobstructed.

 

Dometic makes the unit but doesn't design the ductwork or install it in the trailer, if it has bad air flow that's all on Montana not Dometic.

Denny

 

Stay away from Montana..

 

There are two 2016 model Forest River products at my dealer now that have been sitting there since last fall. They both have the Whisper units on them with a very similar ducting. The owners have filed suit against both FR and Dometic as neither unit will cool in the sunlight. They are both there to be sold on consignment IF anyone wants them, but a suit is progressing. FR has offered a fix for some of the units, but will not work in a few. These owners are pi$$ed beyond belief. About the way I was all summer with mine.

 

If they won't cool in sunlight then it's the trailer, all a/c units only put out a rated amount of cooling so if the trailer has to much heat gain because of poor insulation or poor ducting no unit will work.

Our 37ft trailer has only one Dometic 15000 heat pump and on 90' days it just fine but it also has insulated duct board for ductwork and it's well insulated both in the ceiling and walls. It's the old saying you get what you pay for, the only problem is all the good manufacturers have stopped production because people wouldn't pay for quality so now we have nice looking low quality units to choose from.

Denny

 

So true... except the industry is just giving what the uninformed customer wants. They want big tv's, shiny gizmos and gadgets not thinking about pulling down the road over rough conditions and using them in the heat or cold.

Heck we read it on all sorts of forums about problems that the dealers/ manufacturer tries to wash their hands of once the customer signs the paper and leaves the parking lot.

As long as there is a buying public this issue will not change. It all about number of units out the door and that won't slow down because of QC. They can give lip service and empty promises and a much cheaper price.

 

Years ago when I was very involved in HVAC design I attended a training session held by the RVIA for technicians. What stuck me is the trainers knew next to nothing about ductwork design. No turning vanes, ductwork deadending, just ridiculous systems.

As I said at the time, "if I designed and installed systems like these in residential construction, every one of them would be red flagged by the inspector". Things are the same now as they were then. I frequently add outlets, change ductwork, etc in our own RVs. The way air flow is choked off in factory designs is tragic!

Steve

 

Last year after adding the front and non ducted a/c I made about the same mods as the OP. The ducting actually deadheaded about a foot to 18" on both runs from the original rear a/c and stalling much of the air flow. I added some flow directors, cutting out the deadhead area plus replaced the existing vents with a modulating type, replacing the restricting pieces. Mine came with short tail pieces so didn't have to cut them down. These mods, with the single a/c made about a 10 degree interior temp difference and sped up the interior cooling time by a significant amount. The second a/c is a godsend though we can only use one at a time while at our 30 amp seasonal site but the two while traveling made the 100 plus daytime temps in Nevada, Utah and Texas almost acceptable - that is as long as you didn't go outside.

The cost for doing this worthy, IMO,mod is only about an hour's time, 6-7 bucks a vent, a piece of whatever works for you as a flow director/dam (I used foam pipe insulation from Home Depot, curled in an arc) and an 8 buck roll of aluminum tape also from Home Depot. Luckily, the vents and a/c all matched the ducting just fine.

 

When I was in AC school the biggest point the teacher liked to point out pertaining to duct work is that a 90 degree bend cuts the flow at that point by 50%. Sweeping bends greatly increases efficiency. All ducts should also come out of the plenum at 90 degrees to the dead head. Do you think anyone in the RV industry really takes all of this in consideration?

 

Both of them there have had the ducts completely inspected and different air dams were installed. Nothing! The duct system is very similar to the OPs and have had new units put on them as well. As far as it being the trailer, I agree to an extent, but Dometic is the one that gave them the blueprints to use. At first they were marketed as the "racetrack design" which were oval systems. They didn't work well at all because they were just circulating air. Then they ran straight ducts down each side. Now the problem on all of them is that the off-door side gets most all of the airflow as both AC units sits the same direction on the roof and the ducts coming out of the units both go to the left. Then they thought that they couldn't draft well, so FR actually came out with a vent that goes directly under the unit looking much like the older ones with the 'quick cool' feature. This was done to a few of the Sandpipers at my dealer and it improved the cooling of the camper to some degree, but in direct sunlight, they still can't cool, and on 30A with one unit, you can forget it. Both of the owner's, like me, had Sandpiper units before with the old style AC units that were able to keep the units cool. These won't. Rumors of a class action have begun to circulate and may encompass thousands of units. There are many unhappy owners out there with the new units. Have you noticed that many of the new units are now coming with 3 AC units on top with a load management system?

 

This whole thing sounds like a cop out for poor design and workmanship.

I did HVAC and refrigeration for a living and I can smell a load when I hear it.

If it cools better with a direct dump well that says it all, the ductwork is two small or its leaking into the ceiling.

Again if it can't cool in the sun the trailer has poor poor insulation.

Denny