I just went through this whole fiasco myself...and am now a verified 'semi-expert' in what an atmospheric relief backflow preventer does (and does not) do...
First off, on page 1 of the comments, there is an excellent picture of the $40 Valterra backflow preventer (in case you are wondering who makes the one shown, which was what I had).
On my 40' Cedar Creek 5er, my problems started long ago (apparently) and I was oblivious to the start...only started 'fixing' things as they failed...until all was replaced. To START with, I noticed a POWERFUL stench and bad 'bubbling' issues when I would flush my toilet...TONS of back pressure. I thought it through a bit, and figured 'hey...I'll bet my tank vent pipe is plugged up...might need to run an auger through it to clean it out'...and with this, I got up on the roof, popped the rain cap on my black tank vent pipe, and began to auger. I WAS HORRIFIED...after running a power-auger for a good 3-minutes (and drilling through what had to be 4-6 inches of ****), I finally punched through to the tank cavity itself! IT HAD BEEN COMPLETELY PLUGGED! (and the most god-awful stench you've EVER smelled rose up like demon-hordes coming for me)...seriously...that stench stayed in my nostrils for 2-days afterward.
Well, I figure 'hey, great, now time to turn on the tank flush system and give the old black-tank a good rinse out'...so I turn it on, and I have two PEX joints pop...nice...hello Shark Bites and new pex.
Ok...time to turn on the water (again)...time to wash the old tank. I turn on the water, and within 2-minutes, I have water spraying down from the upper deck's hole-in-the-floor...you know, the one where the plumbing passes up to the bathroom sink basin (where the backflow preventer resides). Ok...city water off, running for the bathroom...to find...NOTHING! Maybe a drop, but no more!
HMMMMMM....
I go back out, got water all over the elbow and PEX at the floor pass-thru...ok, must be a cracked (plastic) PEX elbow...so I cut it out, SHARK BITE the repair, good as new. Turn the city water back on (I had the outdoor shower, which I use to pass into the 'tank wash' fitting port, turned off at the time)...I turn the shower back on, and I have WATER AGAIN! Spraying down!!! WTH?
So, I turn off all water, go back to the bathroom, check up under the sink...and this time (with a bright halogen flashlight) fully scope out under the basin...and then I see it...the backflow preventer valve...water glistening all over it...the top cover cap slightly askew...IT WAS CRACKED!!! The backflow preventer housing had cracked...almost in half!
To make matters worse, the 'tank side' of the valve was HALF FULL OF ****! (yes...real ****...no imitations here).
So, Needless to say, I ran to the nearest RV store...picked up a Camco backflow preventer, and installed the replacement. No more troubles...works like a charm, and all is well (for now).
HOWEVER...lessons from this include some interesting ones (that refute what a few people have said in this long thread):
1. YOU CAN BACKPRESSURE a backflow preventer by way of a closed black tank valve and a plugged vent pipe...resulting in some ungodly pressures in your black tank on a hot day!!! This will PRESSURE FORCE FECES up your black tank wash line, all the way to your backflow preventer! (I have a picture of the nasty thing, but care really not to post it here).
2. Park water pressures are high, but not high enough to burst a new valve...but the same can't be said for a 7-year old piece of plastic...they age, they get brittle due to chlorine, and they fail.
3. When you go down the road, you vibrate and shake...this does NOT do good for any plastic you have banging around (including your backflow preventer).
4. BACKFLOW PREVENTERS serve a purpose...because had I not had the one (that failed), I would have backflowed FECES into (at least) my outdoor shower water manifold! While it would NOT have entered 'city water', it could easily have reached my shower manifold...and I don't really want that to happen. I guess if you are plumbed up differently, then you MIGHT have gotten FECES into your own RV fresh water PEX...can't really address that here, though.
5. Every time I am tempted to 'just take the backflow preventer out of my system and put an elbow in there', I consider what the tank-side of that backflow preventer looked like...and I stop considering the thought further...
Skeeter Saurus
2010 Chevy 2500HD LS9 6.0 Vortec (360HP/370Torque)
5,000-lbs Air Lift suspension (10,000#-GVWR)
2012 36RE Cedar Creek (39'11") Champagne
Frick - Male Siamese (consistent main culprit)
Frack - Female Siamese (partner in crime)
Wife - Boss for 35-years, and general overseer to consistent madness
