Cold air return redux

**DavePhelps** · February 27

I finally got around to purchasing some temperature sensors so I could experiment with different cold air return locations.

The original 6x10 grille (45 cu. in. free air flow) was installed right next to the furnace in my front dinette seat (Elite 1). I never understood this decision and wonder if anyone knows why Oliver put it there. It is within inches of the furnace and two supply outlets. This is not desirable for even heating and does nothing to create air flow below between the hulls, which would help keep that area warmer in colder temps. My experiment showed much better results than I anticipated. First I sealed off the OEM return with tape. Then I cut two 6" holes in some cardboard and replaced those with the rear curb and street side fiberglass seat panels. I then placed sensors on the floor of the basement down with the plumbing on each side under those holes. I placed the third sensor under the sink. When I turned on the furnace, there was very positive intake through the 6" holes, which by the way, provide a bit more free air flow than the OEM grille (~56sq.in. for 2, 6" round versus 45 sq.in. for 6x10 OEM grille, which actually has 5x9" free air flow). I brought the interior temps up from 58* to 68* in about 10-15 minutes. You can see the temperature difference in the pic below; and this is with the return air holes cut in at seat level, not at floor level and as far to the rear as I would want them! I also noticed a big improvement in heat distribution throughout the trailer, although in our small elite1, heat was never a problem in the cabin.

So, before I start cutting holes in my trailer, I ask if there is something I'm missing here, and why didn't Oliver do this to begin with. It seems like a very simple way to markedly improve the performance of an already stellar trailer. Any comments appreciated! Thanks.

Dave

ADKCamper · February 27

Thanks, Dave for posting your experiment results!

It would be interesting to see how the new/improved under-floor airflow performs at some lower outside sub-freezing temperatures, and whether there's any need to attempt to "balance" the under-floor airflow to obtain equal warming in the 4 primary under-floor locations of interest (water pump, outdoor shower, galley sink area and bathroom sink/toilet area).

Your proposed mod is pretty easy to implement if it performs well in all locations in lower temperatures.

Some have expressed concern in earlier posts about too much suction in the under-floor area bringing in cold/damp air through the under-floor drain scuppers. I have no idea how much under-floor suction you might need before this becomes a significant issue...

I have been thinking about pressurizing the under-floor area with a fan in each of the 2 rear corner locations where you would have the new vents, and allowing that air to return via vents from under the bathroom sink area into either the bathroom or the adjacent closet area. Your approach is a little cleaner in that my fans would require routing 12v to the fans and providing an on/off switch mechanism (either manual or via a temp sensor or sensors under the floor)...

**jd1923** · February 28

23 hours ago, DavePhelps said:

The original 6x10 grille (45 cu. in. free air flow) was installed right next to the furnace in my front dinette seat (Elite 1). I never understood this decision and wonder if anyone knows why Oliver put it there...

I also noticed a big improvement in heat distribution throughout the trailer, although in our small elite1, heat was never a problem in the cabin... Dave

Wish you had the Elite II like most of us! And your older hull seems to have a different layout than the late models EIs. I wonder the same, why our return and main ducts are within a few feet of each other (all rear curbside on the EII), not much heat getting to the bathroom. I have seen several other mods regarding this question. Changing the return location is something new here!

Does it make sense to heat between the hulls so much? Isn't the reason for the dual hull construction to be an insulation layer? Otherwise, you'd be blowing a lot of heat to the outer hull, dissipating to the exterior. We will likely never do serious winter camping but would enjoy better heat/airflow in the interior. Thanks

**DavePhelps** · February 29

My interest in this mod is two fold. First, to create more even heat distribution within the cabin area. Second, to keep the basement above freezing as we like to camp in the early Spring and Fall. I think Oliver has overlooked the benefits of proper return air placement and what it can do to increase cabin comfort and durability of the plumbing systems between the hulls.

As for between the hulls, yes, there is reflectix insulation on the floor of the outer hull, but it is also mostly an unconditioned dead air space and it gets cold down there! Especially the rear corners where there is a lot of vulnerable plumbing. The beauty of placing the return air ducts far from the furnace is that it turns this under floor space into a giant plenum where conditioned cabin air flows from the return grilles back to the furnace warming the entire area, not to mention more evenly heating the cabin itself. My test showed a dramatic increase in temperature below by moving the vents further away from the furnace. There was definitely more air flow from the vent on the same side as the furnace. But that's not surprising and could be adjusted for if one was so inclined. Just remember that your furnace has a minimum cold air return volume requirement, it's in your owner's manual. I think if you have properly sized return air vents, then outside air being sucked in through the drainage scuppers wouldn't be a issue, but this needs to be tested. This is also a simple passive setup, no fans or other support needed for it to work. I will say that if I was out in extreme cold for long periods, I might need to employ other measures. Time will tell. Now that I have these sensors, I can keep track better of what's going on between the hulls. I'll also probably add a 4" return vent in the bathroom on the side of the vanity under the TP holder. Not a lot of flat wall real estate in the E1 to place a vent but I think I can just fit one in there. More testing to do, but may have to wait until next winter unless some Arctic blast hits us again. Cheers.

Dave

**jd1923** · February 29

2 hours ago, DavePhelps said:

My interest in this mod is twofold. First, to create more even heat distribution within the cabin area. Second, to keep the basement above freezing as we like to camp in the early Spring and Fall. I think Oliver has overlooked the benefits of proper return air placement and what it can do to increase cabin comfort and durability of the plumbing systems between the hulls.

As for between the hulls, yes, there is reflectix insulation on the floor of the outer hull, but it is also mostly an unconditioned dead air space and it gets cold down there! Especially the rear corners where there is a lot of vulnerable plumbing... Dave

I understand though perhaps 60 degrees is too high a temp for the basement areas. I'd like it to be 45 degrees below overnight when we set the interior to 65.

You wrote "Especially the rear corners where there is a lot of vulnerable plumbing." For sure, check out this picture. These are the Fresh, City and outdoor shower plumbing lines. In the worse possible location along the back outer wall and check out the large brass backflow preventers. The brass as a conductor will allow these lines to freeze more quickly. They should be better insulated, a 12V wire heat wrap added, or just heat the basement as you are doing! I have one of our Ruuvi temp sensors sitting down in this area.

**rich.dev** · February 29

2 hours ago, DavePhelps said:

Especially the rear corners where there is a lot of vulnerable plumbing.

I’m not planning on doing any serious winter camping, but just in case I get caught in a cold front, this is my attempt at improving the insulation behind the garage.

**jd1923** · March 1

@rich.dev this is very nice work! What's your secret on getting back there to do this? Are you just small and thin, or is there another way? My large frame can be a handicap at times, so sometimes i get my son to crawl into the hole! Again, very nice work, even though your avatar has you sitting comfortably in a lawn chair! 🤣

**rich.dev** · March 1

10 hours ago, jd1923 said:

this is very nice work! What's your secret on getting back there to do this? Are you just small and thin, or is there another way?

Haha 6’2” and 220lbs, I was all scratched up from going in like a contortionist! Next project will be to relocate the one furnace duct from under the curb side bed to under the street side bed, and the return vent to the front dinette.

**Rivernerd** · March 1

19 hours ago, DavePhelps said:

I think Oliver has overlooked the benefits of proper return air placement and what it can do to increase cabin comfort and durability of the plumbing systems between the hulls.

Until late 2022, I agree. But, once Oliver started offering the Truma Varioheat option on Elite IIs, a return air vent in the bath was added. Then, Oliver began installing return air vents in the top and bottom of the closet area. I call this attention to "the benefits of proper return air placement."

**jd1923** · March 1

3 hours ago, rich.dev said:

Haha 6’2” and 220lbs, I was all scratched up from going in like a contortionist! Next project will be to relocate the one furnace duct from under the curb side bed to under the street side bed, and the return vent to the front dinette.

I'm 6'2" and closer to 230! Not fun getting in there, but again great job!

**routlaw** · March 1

2 hours ago, Rivernerd said:

Until late 2022, I agree. But, once Oliver started offering the Truma Varioheat option on Elite IIs, a return air vent in the bath was added. Then, Oliver began installing return air vents in the top and bottom of the closet area. I call this attention to "the benefits of proper return air placement."

I am a bit confused by this placement in the closet area top and bottom. If the closet door is closed which I would assume it to be most of the time those return vents would not be able to draw much air if any. Seems like a curious and odd placement for them to me, but perhaps there is something I don't understand. Thanks

**routlaw** · March 1

@rich.dev looks like you added some additional insulation of some type in addition to the pipe wrap. Don't ever recall running across black fabric faced insulation in the past. Just curious what is this stuff?

**DavePhelps** · March 1

Quote

14 minutes ago, routlaw said:

I am a bit confused by this placement in the closet area top and bottom. If the closet door is closed which I would assume it to be most of the time those return vents would not be able to draw much air if any. Seems like a curious and odd placement for them to me, but perhaps there is something I don't understand. Thanks

I don't get that either, unless they also added a vent in the door. Glad to hear that they're thinking about the benefits of well placed cold air returns in the newer models though.

Dave

**routlaw** · March 1

Dave I would encourage you to check out the virtual tour videos, linked here, to see their placement. However this still doesn't make sense to me because the inside of the closet is mostly sealed. I am at a loss as to why they wouldn't have put a 4" inch vent in the bath, then one under the seats of the EII, and a 3rd at the side of the fridge. Also if you look at each different video the placement of these vents all slightly differ from one to the other.

Maybe someone with the newer EII's or Elites can update and inform us.

**rich.dev** · March 1

21 minutes ago, routlaw said:

@rich.dev looks like you added some additional insulation of some type in addition to the pipe wrap. Don't ever recall running across black fabric faced insulation in the past. Just curious what is this stuff?

Yes 3M thinsulate 600L. The reflectix on the inside of the bottom outside hull is pretty much useless as far as I’m concerned. Reflectix is a radiant barrier and an air gap is required to allow the reflective surface to reflect radiant heat, like between the 2 top hulls, ie. top outside hull, gap, then reflectix on outside of top inside hull. However, with the huge open spaces between the 2 bottom hulls and the reflectix on the inside of the bottom outer hull the R value of the reflectix is approx. 1 maybe 1.5. I have added Thinsulate everywhere within reach on the inside of the bottom outside hull, including the battery and garage doors.

**Rivernerd** · March 1

1 hour ago, routlaw said:

I am a bit confused by this placement in the closet area top and bottom. If the closet door is closed which I would assume it to be most of the time those return vents would not be able to draw much air if any. Seems like a curious and odd placement for them to me, but perhaps there is something I don't understand. Thanks

Our Hull #1291 did not come with return air vents in the closet. I installed them, one top and one bottom.

Our experience has been that before the addition of those vents, the closet stayed about 20 degrees colder than the main cabin when camping in cold weather and running the furnace with the closet door closed. After the vents were added, the closet temps stay just a degree or two below those in the main cabin.

Convection of heated cabin air, between the bottom and top closet vents, explains the difference.

**DavePhelps** · March 1

Rivernerd,

So is warm cabin air leaking in to the closet from around the door as a result of the reduced closet pressure from the returns in there? Or is one of the vents a supply?
Just trying to figure your setup out. Don't see how two returns would warm the space. Glad it's working though!

Dave

**Rivernerd** · March 2

2 hours ago, DavePhelps said:

Don't see how two returns would warm the space.

Trust me, they do.

Think of it this way: the furnace blower creates positive air pressure in the trailer's main cabin. If you left the closet door open all the time, this positive air pressure would push warmed air into the closet. The two vents work in a similar way: the positive air pressure pushes the cold air near the floor level of the main cabin through the lower vent (i.e., a hole with louvers) into the closet. As the warmed air rises, it exits the top vent (hole), creating a convection current in the closet.

**DavePhelps** · March 2

28 minutes ago, Rivernerd said:

Trust me, they do.

Think of it this way: the furnace blower creates positive air pressure in the trailer's main cabin. If you left the closet door open all the time, this positive air pressure would push warmed air into the closet. The two vents work in a similar way: the positive air pressure pushes the cold air near the floor level of the main cabin through the lower vent (i.e., a hole with louvers) into the closet. As the warmed air rises, it exits the top vent (hole), creating a convection current in the closet.

OK, so one of your return grilles is under the closet door? That would make sense. I was imagining both return grilles inside the closet somehow which wasn't computing.

Dave

**Rivernerd** · March 2

18 hours ago, DavePhelps said:

OK, so one of your return grilles is under the closet door? That would make sense. I was imagining both return grilles inside the closet somehow which wasn't computing.

See photo below, showing both the bottom and top vents I installed. As you can see, they are cut into the wall above and below the closet door, allowing air from the main cabin to flow into and then out of the closet. I am advised that Oliver is now installing such vents in new trailers being manufactured.

**DavePhelps** · March 2

On 3/1/2024 at 10:51 AM, routlaw said:

Dave I would encourage you to check out the virtual tour videos, linked here, to see their placement.

Finally did this and I see on the newer Elite1, it looks like the return air grille was moved to the rear curbside bench seat, so that is much better! Can't tell if a vent was also placed in the bathroom, but that would also be a good idea. E2 still has it's main return right next to the furnace.... Rivernerd also mentioned that Oliver has also added one in the bathroom and that is great. Personally, I would still think about closing the main return vent next to the furnace and add other returns strategically placed (like where Routlaw mentioned earlier) to create better below deck ventilation/heating. Remember to maintain the proper cu.in. return air requirement for your particular furnace if you do this.

Finally, and just to be clear, those vents at the top and bottom of the closet door are not return air vents. They probably have good use keeping the closet ventilated and warmer to keep clothes from getting damp or whatever, but they are not part of the return air system and would do nothing to keep the below deck area warmer. If there was, or is a third vent between the closet wall and the bathroom sink vanity, then that would connect the closet to the return air system. Carry on.

Dave

**routlaw** · March 3

Dave I am certainly no expert at these things and honestly haven't given it much thought until you brought this up but everything you state above makes total sense. Have always thought the heating system in the EII was sub par especially noticeable when really cold but it seems like you have worked through this and improved the situation considerably. This is going to be my next mod for sure once it thaws out here in Montana and I can get our Ollie out of storage. Are these 4 inch vents commonly available? If not no big deal I can use something else in lieu of those. Is it possible to have too much return air venting thus lowering its efficiency? I also want to purchase some of the 3M insulation and pipe wrap @rich.dev mentioned too.

My thoughts for now are one in the bath under the toilet paper roll, one below seats probably closest to the rear if there is room and another one close to the fridge at the entry. The existing one will get closed off completely.

It's not a bad idea to have a vent for the closet if for no other reason to vent off heat during summer camp trips. That space if left closed can get pretty darn warm.

Thanks

**rich.dev** · March 3

12 minutes ago, routlaw said:

I also want to purchase some of the 3M insulation and pipe wrap

Here are the links for the 3M Thinsulate 600L and 3M 90 Spray adhesive, you might even find it cheaper, just google it. You can buy the 1/2" pipe insulation in 3' or 6' lengths from Home Depot

**topgun2** · March 3

12 hours ago, rich.dev said:

ou can buy the 1/2" pipe insulation in 3' or 6' lengths from Home Depot

As I have pointed out in other threads, an added benefit of the pipe insulation is that it will help quiet the water pump. Put it on both the hot and cold water lines (like rich.dev shows). For those areas that you can't see and/or get to easily, simply start the in an area that you can get to and shove it along the pipe until it stops. You'll be able to get most - if not all - the pipe covered.

I'd bet too that the Thinsulate will also help decrease noise levels.

Bill

p.s. Ace, WalMart, Lowes etc. all have this foam pipe insulation.

**rich.dev** · March 3

1 hour ago, topgun2 said:

For those areas that you can't see and/or get to easily, simply start the in an area that you can get to and shove it along the pipe until it stops.

👆🏻 exactly what I did.

Cold air return redux

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