Black tank flushing ...... leak?

[TheLarsons]

Flushed black tank this morning. First tank dump of the season after storing all winter. Noticed dripping from underside of trailer by drain area. Checked inside Airstream and saw small amount water running from underneath bed frame. Do we have a problem?

[ghaynes755]

Only place the black tank flush is on the inside of the trailer is where there is a back flow preventer, usually small plastic device, on the line from outside to the tank. Most often located under the bathroom sink or close to the water heater.

[Wconley]

I agree most likely a failed or contaminated anti-siphon valve. Location varies by model but I’ve always found it near and above the black tank flush connection behind whatever cabinet or sink is there. Lots of threads (including mine) about replacing the plastic valve with a brass one. I’m happy to see on our 2018 trailer Airstream is now using the brass valve in conjunction with a back flow preventer which keeps black tank orders from getting into the trailer.

[pcskier]

Yes check the backflow preventer as mentioned, the older plastic ones can freeze and/or crack. I had the same problem but in my case it was just a loose fitting in the connections, just hand tightened and problem solved.


BTW the backflow preventer has nothing to do with odors getting in the trailer, it is to prevent backflow from the flush spray wand/black tank getting into the hose and water supply.

[Wconley]

Originally Posted by pcskier

BTW the backflow preventer has nothing to do with odors getting in the trailer, it is to prevent backflow from the flush spray wand/black tank getting into the hose and water supply.

That may be true but the anti-siphon valve will do that alone, just like your dishwasher anti-siphon in your home. Without the backflow preventer black tank gasses have a direct path through the air gap in the anti-siphon valve into the trailer. Ask me how I know...

In the picture the brass circled in red is the anti-siphon valve, the gray plastic part circled in yellow is the backflow preventer. Older trailers had a clear and white (I think?) plastic anti-siphon valve similar in shape that was prone to failure. Unfortunately, when they fail, flush water (before it goes to the black tank so it’s clean water) pours out behind the cabinet where its installed onto the floor of the trailer and/or leaking out the trailer below the valve. Even the brass ones are prone to contamination in the valve allowing water to flow into the trailer.

[pcskier]

Originally Posted by Wconley

That may be true but the anti-siphon valve will do that alone, just like your dishwasher anti-siphon in your home. Without the backflow preventer black tank gasses have a direct path through the air gap in the anti-siphon valve into the trailer. Ask me how I know...

In the picture the brass circled in red is the anti-siphon valve, the gray plastic part circled in yellow is the backflow preventer. Older trailers had a clear and white (I think?) plastic anti-siphon valve similar in shape that was prone to failure. Unfortunately, when they fail, flush water (before it goes to the black tank so it’s clean water) pours out behind the cabinet where its installed onto the floor of the trailer and/or leaking out the trailer below the valve. Even the brass ones are prone to contamination in the valve allowing water to flow into the trailer.

Walt, thanks for the education. I always have viewed the whole assembly as just the 'anti-siphon valve' or 'backflow preventer' interchangeably. is the backflow piece stanard, I don't think I've seen it on any of my three 2013-2014 trailers I've had. Unless I didn't realize it

[Wconley]

Originally Posted by pcskier

is the backflow piece stanard, I don't think I've seen it on any of my three 2013-2014 trailers I've had. Unless I didn't realize it

I never saw the gray plastic part on our 2009 or 2013 trailers either. I’m not sure if the old plastic anti-siphon part had the backflow preventer integrated or not. I do know when I replaced the plastic part with only the brass part in our 27FB we started to get funky sewer smells from the closet so I suspect it did have something in there to prevent odors.

I had a Camco filter fail which put charcoal granules into the flush system of our 2018 28’ jamming my anti-siphon valve and causing an O-ring to fail on the backflow preventer (hence the pictures of the pieces). My wife was not happy that we had flush water flowing across the floor from under the kitchen cabinet and exiting the trailer from the wheel well on the opposite side! I ended up clearing the debris from the valve and replacing the preventer.

[pcskier]

Originally Posted by Wconley

I never saw the gray plastic part on our 2009 or 2013 trailers either. I’m not sure if the old plastic anti-siphon part had the backflow preventer integrated or not. I do know when I replaced the plastic part with only the brass part in our 27FB we started to get funky sewer smells from the closet so I suspect it did have something in there to prevent odors.

I had a Camco filter fail which put charcoal granules into the flush system of our 2018 28’ jamming my anti-siphon valve and causing an O-ring to fail on the backflow preventer (hence the pictures of the pieces). My wife was not happy that we had flush water flowing across the floor from under the kitchen cabinet and exiting the trailer from the wheel well on the opposite side! I ended up clearing the debris from the valve and replacing the preventer.

Good to know!!! I have a brass anti-siphon that's been sitting in my medicine cabinet until I get motivated to replace the plastic one, probably just waiting for it to crack or fail :-) I will add backflow preventer when I do!


This forum is so incredibly educational.

[CruizinDux]

Just a FWIW, a few other vac breaker/check valve threads. My parts are in and will likely jump in in a week or two.


post 29

http://www.airforums.com/forums/f163...ml#post2249662


post #3 Peter ~ OTRA15 (a recap with a few links from rmkrum & smithcreek)
http://www.airforums.com/forums/f443...de-175784.html


As I mentioned somewhere in these threads, I talked to the folks at Lakeview RV supply but they indicated they only offered the OEM plastic version. I wanted brass for both the vac breaker (Cash Acme) and the check valve (Valterra) so ordered from Amazon


[ edit: here is some discussion re the function of the parts)


http://www.airforums.com/forums/f444...ml#post2232537 ]

[BDowse]

I experienced the same problem with the backflow preventer freezing and cracking over the winter. Mine on the 27FB was located behind the wall in the closet. I replaced with another from an RV shop which was white so replaced it again last fall with an original equipment from Airstream dealer, '
Lesson learned: When winterizing, blow this out with air pressure along with the other lines and hoses so no water remains in the valve .

[BandLAir]

This thread hasn't been active since June 2019, but it looks like the best one to join.

We have a 2020 FC 27FBT. We took delivery in Nov 2019 and headed south a week later. The first time I applied water to the black tank flush fitting was at a campground in southern Alabama. When I did, water poured out the belly of the TT. After talking with the dealer I located the anti-siphon/backflow-preventer assembly behind the panel at the lower rear of the hall/wardrobe closet. We applied water again, briefly, to the exterior fitting and verified that the leak was coming from around the underside of the cap on top of the brass piece (the anti-siphon valve).

Since we were unable to use the blank tank flush I went ahead and removed the cap on top of the brass A-SV by removing the two small screws. The top of the brass housing is open except for a "bridge" that has the two threaded holes for the screws that hold the cap and a third threaded hole in the center. The moveable portion of the A-SV is inside the brass housing and is supposed to be threaded up into the center hole. It was not; it was just lying inside the housing where it was unable to slide up and seal off the opening when water was applied.

I took this to clearly be a manufacturing defect and called our dealer to get a replacement part. That turned out to be a major problem, but that's a story for another day and, in the end, the dealer was able to make the (correct customer service) decision to locate one and ship it to me. It took me a couple of hours, total, to remove the old one and install the new one. It's worked great every time since then.