[SOLVED] Why would our black tank all of a sudden have pressure ??

[SOLVED] - See my last post
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Scenario:

- Water in the toilet bowl
- Flush head (as in open valve to the tank below)
- Obvious pressure in black tank since water in toilet bowl blurps (blurps is my technical term  ) up when flush ball valve is opened
- Watch out!  Don't have your face too close to the bowl, or you will get a bath of the former bowl water being ejected into your direction

We have 50k miles on our coach and have never observed this behavior before.  Yes, the roof vents are clear and open...

??? ???

I would say your roof vent is blocked and the gas produced as a byproduct of decomposition create pressure.  Should be no pressure if the vent is working.

I think Marc has it right.  Ours only does that when the tank is full above the bottom of the vent tube.  It's not a gas buildup but just the air that has to escape to allow the water to flow down.  If your tank isn't full, then it's possible the vent tube has slipped down.

Mine does it when there is not enough water inthe tank to foat the foaties away and a pile develops under the discharge pipe. like Ned said the preasure is just the air being displaced by the incoming flush. Not sure you configuration but you may have a sewer pipe blockage.

wayne

Mine begins to do that when the tank is full, i.e. in serious need of dumping.  A clogged vent would do it too.

I have a hard time understanding how pressure can build up by adding more liquid/solid, thus squeezing the air.  My reasoning is that anytime you flush, you open the valve, stuff falls in and the air displaced by stuff is released in the toilet before the gate is closed again.

To me, pressure can only come from gas produced by decomposition, or expanding gas from heat.

However, regardless of source, pressure can't build up if the vent is working.

56kz2slow said:

However, regardless of source, pressure can't build up if the vent is working.

Click to expand...

That's what I keep coming up with.

Thanks guys for the ideas - much appreciated.

Notwithstanding that the decomposition of the fecal matter may produce some pressure, the real problem is displacement.  If the Vent Tube is blocked the air in the tank has to go somewhere when the Toilet is flushed to allow the new water and whatever to enter the Tank..  The only place it can escape is thru the open that you are creating when you flush.

It would take several days for decomposition to build up significant pressure.

Fix your Vent Tube it has either dropped or is, in some other way, blocked. 

I have a hard time understanding how pressure can build up by adding more liquid/solid, thus squeezing the air.

Click to expand...

It is not really "pressure", at least not in an ongoing, static, sense. As the water goes in from the bowl, air has to come out to make room for it, so it "burps". If the air cannot easily escape up the vent stack, it pushes back through the toilet flush valve instead (the path of least resistance). The vent can get clogged with some tank debris that washes up into it, or of the water level in the tank is high enough to cover the inlet to the vent pipe on the tank end. That inlet sometimes protrudes down into the tank a bit (the vagaries of the manufacturing process).

John,
Try sticking a water hose in the roof vent and running some water down into the tank. May flush away a clog in the vent pipe. Not a great chance of success, but easy to try.

We had the problem come to a head (pun intended) this morning.  Jane said the water in the bowl was not draining into the tank  ??? - what?!?  I used a trusty coat hangar to poke down the head drain and sure enough, toilet paper, etc. had blocked the drain.  Several pokes later, it dropped down to the near-by bottom of the tank.  I completely filled the tank with water, and then pulled the valve.  Lots of paper, etc. ultimately came out came the drain (we have a clear elbow to watch the action.)

Analysis:  we have been on water and electric sites only for the last 22 days and filled our black tank up quite a bit on two occasions.  It seems like I have not done a good job of rinsing the tank.  We're not new at this - we've used our rig for almost 50K miles but I learned a valuable lesson.  User plenty of water to flush (which we don't like to do when we need to stretch our tank capacity.)  

Thanks everybody for helpful comments.

Everybody has this sort of problem sooner or later, John. Systems with a 45 degree bend in the bowl drain are more likely to have it "sooner" than "later", especially if you need to economize on the flush water. 

Would pre-loading the black tank with water at the same time you fill your fresh water tank help you in the boondocking type scenario?

I was thinking if the tank were 10% to 20% full of plain water, that the mountain wouldn't be able to establish a good foundation.  Yes, you'd lose 10% to 20% of your black holding capacity before needing to dump, but you could cut down on water consumption per flush, conserving your potable water.

Alternatively, it might be time to plumb in a pump from your grey water tank to your head, and use that for your flush source to conserve potable water.

just a few thoughts,
jim

It's always a good idea to add a few gallons of water to the black tank after emptying it.  It keeps solids from accumulating in the drain pipe at the very least.  You don't need a lot, just enough to fill the drain pipe and cover the bottom of the tank.

Many built in tank rinsers are about as effective as standing on the flush pedal and perhaps holding a running garden hose over the bowl.. In short, they just run water into the tank.

Now some of them.. like the Tornado, and another that I don't know the name of, Which consists of a short rubber tube that, when pressurized, waggles and wiggles and basically shoots water in ever possible direction... MIGHT do the job better... But I don't know.  I know the standard units are not much better than a garden hose held over the toilet

Most of the wands I see (A wand is something you hook a hose to the top end and stick the bottom end down the toilet) are... not much better

However there are two wands I've seen that stand a chance.. ONE I made my own version of

First: I think it's called: The RV-Twister.. this is a wand with a brass spinner on the end.. It shoots two powerful streams of water, and spins to spray them in all directions.. You move it up and down in a motion many will understand. and ideally it will shoot every square millimeter of wall, clean the snesors, blast apart any "poop-a-mids" and do a good job..

I say in theory cause I"ve not tried it

The other is the Valterra Master Blaster.. I have seen this wand in exactly ONE store.. about 3 feet behind me as I type  Other than there, and the Valterra catalog, I've not seen it

This is both a description of it and what I did, what I recommend follows

What I did was take a 1/2 inch PVC pipe, it was actually a scrap I had in stock, I put a hose fitting on one end, and an elbow, short piece of pipe and cap on the other end

I then drilled a hole in the end of the cap.. about 1/4 inch (I think  7/32" was the bit I grabbed)

Now you do have to AIM this one at what needs blasting,, IE: The sensor wall, twist and turn a bit as you move it up and down in the classic motion.

The only concern I have with the Master Blaster is that this is basically the design they use, but instead of the very strong 1/2" PVC and cap.. They have a plastic "Fire Hose" type nozzle.. TO my eye it looks like it might easily break off during insertion or removal.

What I suggest is a 1/2 inch PVC pipe. hose fitting on the top (NOTE: Feed it via a backflow preventer or two.. I use two, also a cut off valve)  cap the lower end and drill the side of the cap, instead of using the elbow and cap like I did.  Will go down the old hole easier if there is no elbow.

I may re-build some day.

Good ideas all around - thanks guys.  I have a built-in flusher, so maybe the trick is to fill the black tank full before dumping when we're spend a lot of time without a sewer.

I have the http://www.campingworld.com/shopping/product/tornado-permanent-tank-rinser/4969Tornado, and can attest to it's performance. No false readings from dangling paper, no build up. Cheap, easy to install, and well worth the purchase. It also makes partial filling easy without dragging a hose through the RV or using your freshwater tank.

You will want a black freshwater hose to go with it.

RV Roamer said:

It is not really "pressure", at least not in an ongoing, static, sense. As the water goes in from the bowl, air has to come out to make room for it, so it "burps".

Click to expand...

Ok, I misunderstood the original problem.  I was thinking actual pressure, like when you pop open a cold one with the nice "pish" sound we all like.