Solution to Prevent Grey Tank Backflow?

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Solution to Prevent Grey Tank Backflow?

Postby cwaet » Thu Oct 07, 2021 5:04 pm

Well we now have 3 trips under our belts in the cargo conversion and have run into an issue that I need to dig into before going out again. We have a sink drain which connects under the shower before heading through a hepvo valve, then drops 4" down underneath the camper to a tube to the grey tank mounted underneath the camper. When it is full driving home, we have the unfortunate experience of the grey tank water coming back up and into the low point drain (shower). Luckily it isn't a black tank and it is contained, but still unpleasant. How do others prevent this? Is a large check valve at the tank the solution?

A separate but related issue is for both my fresh water and grey tank air vent hoses are getting water up them and blocking air from being able to vent in. It may just be the way I've routed them and may just need to have get a 90 deg fitting so that the hose goes straight up instead of out then up.
Last edited by cwaet on Thu Oct 07, 2021 11:25 pm, edited 1 time in total.
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Re: Solution to Prevent Grey Tank Backflow?

Postby rjgimp » Thu Oct 07, 2021 11:24 pm

Yes, I absolutely see a check valve in your future!
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Re: Solution to Prevent Grey Tank Backflow?

Postby DrewsBrews » Fri Oct 08, 2021 7:35 am

The hepvo valve seems like a sort of check valve. I'd think that would have prevented this but I don't have experience with them. I suppose they could be more for blocking gasses rather than full force of backflow water.

I have always been leery of check valves in general. I wonder how easy it is to get them gummed up to stop functioning.. especially when they see lots of time sitting in between trips for gunk to dry up inside the valve. I'd put my trust in a ball valve for the long term, but would have to manually close it before hitting the road. Just remembering to do that would be the hardest part for me.


For the breather... assuming there are no droops/low spots in the breather hose for water to gather (using gravity against you).
Is the hose getting a slight kink? That can restrict the air's ability to move the water in the hose out of the way. Getting rid of the restriction will hopefully resolve the issue. larger diameter hose is probably the ultimate fix.

[Nerdout]
Surface tension of water is the enemy here. Any restriction at an air-water transition reduces the total surface area of the water that the air has to push on.
PSI= Pounds per square inch. Reduce the square inches of area to apply the force, and that reduces the total force applied for the same psi. Larger hose increases this area quite a bit for not much diameter change. (2x inner diameter= 4x area for a circle)

12" of head pressure, for example, is only about 0.4psi. And maybe a slight restriction in a small breather hose drops the cross sectional area down to 0.25sq inch in a spot water likes to sit... That leaves 0.1 pound of force that the air has to break the surface tension with.
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Re: Solution to Prevent Grey Tank Backflow?

Postby Socal Tom » Fri Oct 08, 2021 10:13 am

Are you sure you aren't over filling the tank?
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Re: Solution to Prevent Grey Tank Backflow?

Postby flboy » Mon Nov 15, 2021 7:39 am

Sounds like over filling the tank if the tank is lower than shower and it is backing up to the shower. Next time, try draining some gray water off and see how it does.

I know in my experience, with two people , dishes, and a shower, mine fills up fast (25 gallons) and will start filling up the vent hose connection which is slightly lower than shower before it joins the vent stack also connected to BW vent at toilet.

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