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Open Roads Forum  >  Class A Motorhomes  >  Maintenance Issues & Tips

 > white dust coming out of AC vents - any ideas?

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dougrainer

Carrolton, Texas

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Posted: 06/25/21 08:23am Link  |  Quote  |  Print  |  Notify Moderator

DrewE wrote:

The part of the air conditioner on the roof is (or at least should be) separated from the ductwork side by a solid barrier, of course with some wires and tubes going through it. Outside air and inside air do not mix by design. If you think the source might be the air conditioner itself, it would be better to remove the inside cover/louver on the air conditioner unit and examine things from the inside. (There's no ducting as such on the outside side part of the air conditioner, just the shroud over the condenser, its fan, and the other external bits.)

Unless the powder is appearing from that air conditioner unit itself, I'd tend to suspect a source near where the pile is forming instead; but there's certainly no problem with taking a look at everything. If nothing else, it's well worth looking at the outside part of the air conditioner and cleaning out dirt or debris if needed to keep it working efficiently and effectively.


Bingo. There is NOTHING in a RV rooftop AC unit that will produce debris or dust. What DOES produce this dust is, at EACH roof AC you need to make sure there is NO possible way the cavity of the roof between to top and the interior ceiling has open egress areas. If so, the AC unit will "pull" air and debris from this cavity as long as from the inside of the RV. The cavity will be behind the AC filter's, hence the fine debris. This is where AC Foil Tape comes in very handy to seal off those areas. If your AC units have the inside Plenum mount, you remove this mount and you will see the gaps on the 14 inch square roof cut out. SEAL those gaps and your problem goes away. IF top mounted AC you will have to remove the unit from the roof and do the same thing. Doug

willald

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Posted: 06/25/21 09:28am Link  |  Quote  |  Print  |  Notify Moderator

dougrainer wrote:

There is NOTHING in a RV rooftop AC unit that will produce debris or dust. What DOES produce this dust is, at EACH roof AC you need to make sure there is NO possible way the cavity of the roof between to top and the interior ceiling has open egress areas. If so, the AC unit will "pull" air and debris from this cavity as long as from the inside of the RV. The cavity will be behind the AC filter's, hence the fine debris. This is where AC Foil Tape comes in very handy to seal off those areas. If your AC units have the inside Plenum mount, you remove this mount and you will see the gaps on the 14 inch square roof cut out. SEAL those gaps and your problem goes away. IF top mounted AC you will have to remove the unit from the roof and do the same thing.


Thank you, Doug! I was hoping you'd see this thread, and might have seen this issue before and could offer some insight. [emoticon]

I will definitely check the cavity you mention here in the ceiling. I do recall some time ago, a mobile RV mechanic I had out doing some work on the RV, he put quite a bit of foil tape in there to try and make it more efficient.

Doug, if the AC unit is pulling this debris from that cavity....Why, would there be big piles of the dust under a vent, even days or weeks after AC unit has been off? That, is the part that really puzzles me, and has me concerned it might be critter activity of some kind. The 'piles' are only coming out of just one AC vent, also, the one over the sofa. Its one of the ones closest to the front AC unit. The rest of the vents only spew it out when AC is running.


Will and Cheryl
2021 Newmar Baystar 3014 on F53 (7.3 V8) Chassis ("Brook")
2018 Jeep Wrangler JK ("Wilbur")

dougrainer

Carrolton, Texas

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Posted: 06/25/21 12:55pm Link  |  Quote  |  Print  |  Notify Moderator

Your ducts may be filled with the dust. When I have had this, I use compressed air to blow out the ducts to remove any residual dust. Also, You HAVE to remove the Inside plenum to see and fix the gaps. Doug

Rick Jay

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Posted: 06/25/21 01:28pm Link  |  Quote  |  Print  |  Notify Moderator

Did you let Hunter Biden borrow it? LOL

~Rick


2005 Georgie Boy Cruise Master 3625 DS on a Workhorse W-22
Rick, Gail, 1 girl (25-Angel since 2008), 1 girl (20), 2 boys (21 & 18).
2001 Honda Odyssey, Demco Aluminator tow bar & tow plate, SMI Silent Partner brake controller.


willald

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Posted: 06/25/21 02:15pm Link  |  Quote  |  Print  |  Notify Moderator

Rick Jay wrote:

Did you let Hunter Biden borrow it? LOL


Hahahah, I *KNEW* comments like this would come up. [emoticon]

willald

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Posted: 06/25/21 05:36pm Link  |  Quote  |  Print  |  Notify Moderator

Well, after taking the vent piece off and having a look inside the ducting, think I see what the problem is. Just gotta say this right off the bat: this is another example that shows just how shoddy this motorhome was built, haha. I am NOT impressed with how it’s built, has me seriously thinking about trading it for a quality built unit like a Newmar.

Anyway, the roof consists of a thin layer of some kind of plywood, then about 1” of solid styrofoam insulation. There is some kind of paper material glued down to the styrofoam insulation, then the duct cavity. Right there at the duct opening, that paper has come unglued from the styrofoam quite a bit, allowing air to get in between the paper and styrofoam where it ain’t supposed to get. Air blowing up into that styrofoam is blasting little bits of the styrofoam out. The duct opening we saw the most coming out, is where the paper has come up the most, but it looks like its happening at the other duct openings, too.

Sooo, how to fix this is the question now. Thinking about using some foil tape at each duct opening, and tape that paper back down all around it, to keep air from blowing up into the insulation. That, or maybe trying to glue that paper material back down around the duct opening with some kind of glue.

Thoughts?

Rick Jay

Greater Springfield area, MA

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Posted: 06/25/21 05:47pm Link  |  Quote  |  Print  |  Notify Moderator

Well, without seeing just what your access to the areas is like, I think I'd be tempted to say try the foil tape for ducts. I've had good luck with that over the years. If properly applied it seems to stay in place for a long time. Whether it matters or not, though, I wouldn't buy it at a surplus store. I'd try to get a "name brand" or something with good reviews.

Good Luck,

~Rick

willald

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Posted: 06/26/21 12:35pm Link  |  Quote  |  Print  |  Notify Moderator

Update: Well, I took Rick’s advice, picked up some good quality foil tape at Lowe’s this morning, and set to work on the duct openings. Cut the tape in several strips about 1/2” wide, and used them to tape all around the vent openings so none of the 1” styrofoam insulation is exposed. Ended up doing this to every duct where we’d seen white dust coming from, a total of 5 of them.

Cranked up air conditioner, and….no more white dust. Problem solved. That foil tape is good stuff, seems to hold and stick really well. Will definitely keep it in the motorhome in case we need it again.

Thought about doing all the duct openings/vents, and may well on the future. All is good for now, though, so gonna move on to more fun stuff, like getting back out camping. [emoticon]

Rick Jay

Greater Springfield area, MA

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Posted: 06/27/21 02:29pm Link  |  Quote  |  Print  |  Notify Moderator

Great! Now it if doesn't work, you can blame me! LOL

Hopefully that will be a permanent cure to the problem! [emoticon]

~Rick

ocean bound

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Posted: 07/04/21 06:51am Link  |  Quote  |  Print  |  Notify Moderator

It is more likely the glue they use to glue the ceiling together is drying up and falling down they use a lot of glue and usually extra stuff that Spills out that’s not between anything will dry up in fall down I had the same thing happen to me mine was yellow tint to it and eventually just went away


Allen&carrie 2006 georgetown XL359 37ft


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