Loud explosion while backwashing black tank

[fun2rv]

Today I was doing my regular duty of emptying/backwashing the black water tank. I had emptied both the grey and black tanks and was in the process of backwashing the black tank. Now I know it says not to run the tank sprayer with the black tank shut, but I have always done this and watched the level of the water and shut it off to empty. Of course there has been the couple of occasions that I forgot the time and of course the water has always just gone up thru the toilet and made a mess inside. Thankfully it has always been after I have emptied and rinsed the tank out. But today, the tank wasn't quite full yet and all at once there was a loud boom (like explosion) and water started pouring out of the underbelly of the trailer (2002 3280 Montana 5th wheel). No water came out thru the toilet at all. It was getting late, so I wasn't able to take the underbelly covering off to inspect any of the tanks - will try to do that first of the week. I went on and made sure all the water quit running and made sure the water supply was off and left it alone. Does anyone have any ideas what happened and where or what I should start looking for first? Appreciate any and all help.

[Lynnmor]

As mentioned above the vent pipe may have been installed, or dropped down, against the tank bottom. I have heard of this from several owners. With the vent working properly, there will be no pressure buildup in the tank.

[badboy368]

I would assume there shouldnt be any pressure in the tank if the vent was working properly ...right? :h :?

[Old-Biscuit]

wagonmaster2 wrote:
I have been using the flush connection for years to fill the black tank with fresh clean water for a final wash-out after dumping without any problems. But last fall while the tank was filling, and probably had gotten 1/2 full, I went inside, opened the toilet valve to watch the water level and see if it was clean, and the rush of air blew what little water was setting in the stool out into the bathroom. I have no idea what happened this time that didn't happen many many times over the last 10 year of owning this motorhome but believe me I'll not be filling the black tank this way again, especially after reading about OP's experience.

Could this be an indication the roof vent for the black tank might be stopped up or clogged from inside the tank? Doens't seem like there should be any pressure build up in the tank, which might also explain what is happening here. Didn't intend to highjack this thread but thought my experience might add to the query.

Yes that 'air rushing out of toilet means vent line is blocked or clogged.
Blocked......could be vent line dropped down and water level blocked it off.
Clogged......could be from birds nest/mud dabbers etc.

Tanks do not do well with build up of pressure.

OP:
Tank could have ruptured, flush connection blew off of tank, vent/toilet to tank line could have blown out of top of tank

But from the sound of all the water running out of underbelly......ruptured tank would be my guess.
Especially seeing as you have overfilled it on prior occasions and stressed the tank

[wagonmaster2]

I have been using the flush connection for years to fill the black tank with fresh clean water for a final wash-out after dumping without any problems. But last fall while the tank was filling, and probably had gotten 1/2 full, I went inside, opened the toilet valve to watch the water level and see if it was clean, and the rush of air blew what little water was setting in the stool out into the bathroom. I have no idea what happened this time that didn't happen many many times over the last 10 year of owning this motorhome but believe me I'll not be filling the black tank this way again, especially after reading about OP's experience.

Could this be an indication the roof vent for the black tank might be stopped up or clogged from inside the tank? Doens't seem like there should be any pressure build up in the tank, which might also explain what is happening here. Didn't intend to highjack this thread but thought my experience might add to the query.

[SDcampowneroper]

Very sorry to read of your as yet unknown diagnosis.
Flushing of the black tank is unneccessary if proper use and treatment is given to it when using it.
It is much easier to 'potty train' ourselves, then never needing to power flush, break down the poo pyramid,& the risk inherant in overfilling that tank.
I wish the best for you.

[lj2654]

i know when I was using the flush feature on my coach i heard a loud bang and water was running under the mh on the ground. When I pulled the side panel off I found the fitting broken off where the flushing line enters the black tank.

[GaryWT]

Thus the reason we leave the valve open as directed.

[BB_TX]

A commonly reported failure on the black tank flush systems is the failure (rupture) of the black tank flush vacuum breaker/anti siphon valve. The valve is typically plastic and located either inside an interior wall or under the bathroom sink as it has to be some level higher than the black tank itself. Mine fractured and water poured out the bottom of the trailer, but some water came out inside the trailer also.

But I don't remember any one saying there was a loud boom when it failed.

[Oasisbob]

Does sound like a ruptured tank. There was a large volume of water? You might see if your insurance will cover repairs? Just curious what the capacity of the tank is? Are they really that fragile?

[azjeffh]

fun2rv wrote:
..... there has been the couple of occasions that I forgot the time and of course the water has always just gone up thru the toilet and made a mess inside.

Just an FYI - In a properly sealed system, the water will flow out the vent pipe, not the toilet. I have plenty of experience with that using my Flush King.

Can't tell you what caused your current failure, but I will say I wouldn't rely on my inside indicators when judging tank levels.

[Lovedays]

The same thing happened to me, unfortunately your black tank will probably need to be replaced, mine did. It wasn't cheap and it took about two months to get it shipped, it ruined my summer.
Mine broke jest below where it attaches to the toilet.

I wish I had better news. Good luck.

[ken56]

It sounds like your tank ruptured. If your lucky maybe it was only the water line to the flusher. You still gotta open it up to see what happened. good luck.