Plumbing vents and negative pressure

[Ed Hackenbruch]

You do know that your toilet has an overflow tube don't you?  It has to have a little water in it just like any of the other traps.  Also tell everyone to put the lid down before they flush and to open the valve slowly instead of just mashing down on it. Made a difference for us.

[kyle4501]

Would seem that if foul odors are escaping when flushed, then there is no problem with air getting in the tank. The problem is there is already too much air in the tank. 

Need to determine the reason for the tank being at a higher pressure than inside the bus.

Brian, can you extend the vent to be, say 12" taller than it is now? There may be some odd turbulence on the roof providing a high pressure area. The extension can be temporary, say one of the rubber adapter couplings with screw clamps. . . . Take it out for a highway spin & see what the results are.

While the front face of the bus is a high pressure area, the sides near the front are a low pressure area.

Things that stick out cause turbulence that can cause strange pressure areas around them.

These pressure areas are difficult at best to calculate & is why wind tunnels were built.


Good luck with your stinking problem. 

[niles500]

Not to drag this OT - but as I was going down the road in the rain this week - wipers off, and not being a necessity on my bus - I remembered a thread on the wind pressure loads of a bus (directly related to high and low pressure zones affecting drag, etc.) - and I was watching the rain drop pattern as it dissipated on the windshield, noting the high pressure areas, when I began looking in the mirror - It appeared that it was possible to do cursory test of wind load/pressure of your bus by watching the rain drops as they dissipate along the fuselage - worthless maybe in this discussion - but when it comes to things such as ram air affect, cooling mods, etc. - It appears it may be unecessary to tape ribbons all over your fuselage to measure the high/low pressure areas of your coach - merely watch the rain drops in your rear view mirror to ascertain the "pressure areas" - FWIW

[JackConrad]

Need to determine the reason for the tank being at a higher pressure than inside the bus.

While the front face of the bus is a high pressure area, the sides near the front are a low pressure area.

This is what causes problems with our coach. If I open the driver's side window, there is a significant air flow OUT the window, reducing the air pressure inside the coach. When the toilet is flushed, as soon as the toilet valve is opened, the air in the (higher pressure) tank enters the coach in an attempt to equalize the pressures. If we keep the coach windows closed, the pressure remain more equal and no air exchange when the toilet is flushed.  Jack

[gumpy]

I had that same problem but found a rather strange cure. We ran out of cheese. Once we quit eating so much cheese the smell went away.   Now when we eat a lot of cheese we use rest areas, truck stops and neighbor's lawns for our more strenuous daily duties!    Actually on a more constructive note isn't there a 2 way type sewer vent that is made for such problems. Basically it is for basements with the laundry and or bathrooms downstairs. I'll check with a plumber buddy I have if I see him tomorrow. I know he has used them a lot for situations when the traps keep getting sucked dry. Later

Oh, yeah. Forgot about that. I put one of these in my vent system before I reconnected it to the roof. The thought was that the engine would suck air from inside the coach through that vent instead of sucking the traps dry. Didn't work.  It actually leaked sewer gas back into the coach when we'd stop. Wasn't sealing tight. And the engine still sucked the shower trap dry.

As for the back pressure sucking gas from the black tank when the toilet is flushed, we've encountered this, also. It's caused by the suction created by the air rushing around the front corners of the bus. Our solution has been to make sure the toilet lid is closed before flushing, and partially opening a rear window or the bathroom window while driving.

[bevans6]

Maybe install a shroud in front of the vent on the roof to force a negative pressure zone where the vent is? 

[belfert]

Try this experiment, open all the windows in the coach, flush the toilet, smell or no smell. If no smell, then close a window at a time until you notice the smell, if at all.
Some "engineering" questions also for you. What size vent pipe did you use? How many elbows are in the run from the tank to the roof? Reason, a short radius 90 degree elbow decreases flow by 25%. Four of these in a vent stack causes the air flow to be zero acting as if there were no vent stack at all. Long sweeps are not as bad at about 15% each. Virtually everyone has one elbow coming out of the tank from horizontal to vertical, then varies from there.
Baffled tank or not? Was the tank ever over full such that the "TP" floated up the vent stack and stopped it up or have you tried making sure the vent stack is open to the tank?

The vent is 1 /2" and has two elbows.  One is a "vent" elbow and the other is a normal elbow.  Tank has no baffles.

The tank has never been full enough to get stuff all the way up to the vent.

[Blacksheep]

That might be your problem. Looks like your vent pipe diameter is not large enough. My vent (that goes downward) is 2 inches and has been said many times, no problems with windows open or closed while moving or sitting still!

Maybe you don't have ENOUGH of a vent!

Ace

[kyle4501]

. . . .a short radius 90 degree elbow decreases flow by 25%. Four of these in a vent stack causes the air flow to be zero acting as if there were no vent stack at all. . . . .

The vent is 1 /2" and has two elbows.  One is a "vent" elbow and the other is a normal elbow.  Tank has no baffles.

The tank has never been full enough to get stuff all the way up to the vent.

Belfert, is that 1/2 " a typo? If not, that may be your problem, not enough air flow out the existing vent.
Also, if the vent is 1-1/2", is there an another opening to the tank that is in a high pressure area while on the road? That would cause problems . . . .


As for the flow reduction of 25% . . . . It doesn't work quite like that. A restriction from an elbow or tee etc simply adds to the effective length of the pipe. The vent stack with 4 elbows will still allow air to pass, but at a reduced rate compared to the same length straight pipe.

[belfert]

The pipe is 1 1/2".  I left out one of the 1s.

I didn't realize 2" was in the code when I did the vent, but I doubt the size of my 1 1/2" vent is causing the problem.

[Len Silva]

If you ever go to a rally where they have a honey wagon servicing RV's, you better be sure you have a large enough vent so it doesn't collapse your tank.

[RickB]

Brian,

I didn't have time to read through all the posts so someone may have already suggested these two things.

We have a grey water and a black water tank and there is no question that if you allow food into the grey water tank, through washing dishes, and you don't consistently empty that tank it will stink at least as bad as the toilet, so if you have a grey water tank don't count that out as being the culprit.

Also, we installed a side mount hose rinse port on the side of the black water tank it just hooks up to the rinse hose at most rv dump sites, we also use the blue stuff to keep down odors but the real issue seems to be improper or incomplete flushing of the black water tank before travel, I was told by a guy early in our RV experience "don't drive until you have dumped your blackwater tank because it will form layers of difficult to remove crap. (sorry there was no way to say that any other way)and we also don't use the toilet while we are driving, we either pullover at a rest stop or if it's at might time we pull the bus to an offramp, do our business and then continue on down the road keeping in mind that I will be up bright and early to use the RV dump site.

I personally don't think there's really any way to completely keep a 40' X 8' insulated enclosure smelling like roses with people using the restroom. That is exactly why there is a no #1 policy on tour buses. That's also why I thank God personally often for a good fan on the ceiling of our bathroom.

Rick

[Skykingrob]

Hey Brian
"I didn't realize 2" was in the code when I did the vent, but I doubt the size of my 1 1/2" vent is causing the problem."
Actually, most will be suprised, except for the engineering types, with the actual answer. Flow rate in 11/2" schedule 40 pipe is 0.0141 cuft/min, with 2" schedule 40 is 0.0233 cuft/min. This equates to a 63% extra flow rate for just 1/2" bigger pipe. Is this all your problem, doubtful, but certainly could be contributing. There will also be some effect from the standard elbow, but as Kyle pointed out, the effect is not total.
Some more questions. Are there any other things on the roof (roof airs, fan vents, etc.) near this vent stack that could cause airflow turblence and thus effect the airflow? This relates to the question from before about noticing if the smell is worse with the coach traveling through winds from different directions. Also, how hgih above the coach roof is the vent stack extended. Does it have a cap or is it just straight pipe?
Do you have any answers from the other questions yet?

Rob
91 Prevost LeMirage XL
Missouri

[belfert]

I can't answer the other questions because I can't drive the bus right now since I have yet to have the windshield replaced.  The black tank is also completely empty and pretty clean at the moment.  I haven't even dewinterized yet.

I have an RV360 cap on the vent so it doesn't extend up very far.  The vent isn't really that close to the rooftop A/C or the emergency exit.

[Sean]

... the real issue seems to be improper or incomplete flushing of the black water tank before travel, I was told by a guy early in our RV experience "don't drive until you have dumped your blackwater tank because it will form layers of difficult to remove crap. (sorry there was no way to say that any other way)and we also don't use the toilet while we are driving, we either pullover at a rest stop or if it's at might time we pull the bus to an offramp, do our business and then continue on down the road keeping in mind that I will be up bright and early to use the RV dump site.

Wow, I haven't heard such balderdash (and, please note that I resisted a pun here) in a long time... I think that individual gave you some bad advice.

I can't imagine having a fully self-contained RV, but having to dump the tanks every time you move.  FWIW, we live aboard full time, and we dump about once every two weeks.  We never have problems with sewer odors in the coach (well, OK, sometimes the cat box starts to smell a little, and my wife reminds me it's time to go out and clean it).  Also, logistically speaking, dump stations are few and far between, by comparison to overnight parking spots.

One reason I think this was bad advice is that the best way to ensure that solids are completely flushed out of the tank is to make certain the tank is at least ¼ full before dumping, and preferably ½ full or more.  Moreover, driving while there is a reasonable amount of fluid in the tank is one way to mitigate the buildup of solids on the tank walls.

If you have to dump before driving to keep the odors down in your coach, you likely have some bigger problem, such as improper venting, blocked vents, dry traps, or air current and/or pressure problems.

I would also mention that "the blue stuff" that you are using might be part of the problem.  If it is a biocidal type additive, it is likely killing any bacteria and enzymes that should be in your tank taking care of business.  I would suggest you change to a strictly enzymatic additive, possibly with a non-biocidal deodorant additive if you feel you need it.  FWIW, we seldom add anything to our tank at all -- in five years of full-time living, we've added perhaps two gallons of enzyme, usually after tank maintenance.

JMO, of course, and FWIW and YMMV.

To speak to the OP, might I suggest you get yourself a couple of smoke pencils, and see what they do when the commode is flushed while driving?  I would think that knowing whether or not a big puff of air was coming inside through the flapper valve would provide a starting point for further diagnosis.  Otherwise, we're all just stabbing in the dark here...

-Sean
http://ourodyssey.blogspot.com