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Old 05-17-2022, 09:26 AM   #1
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Drain Lines and Venting

Is there a general consensus on running sink drain lines (to underbody tanks) inside or outside the chair rail? I know the chair rail is structural and needs to stay.

Specifically, between the chair rail and the exterior skin vs outside the chair rail on the interior of the bus.

Also, can the black and grey water tank vents be combined into one line going to the rear of the bus?

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Old 05-17-2022, 02:21 PM   #2
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If you can run the drain inside of the chair rail it may be easier to repair or maintain later on. There's probably no reason you can't put it in the wall except it will compromise insulating the body.

Black and gray tanks can be combined into a single waste line. It's recommended, actually, so you can dump black first, then flush the line with gray.

It's best located on the driver side, since most dump stations have their dump inlet on the left side of the pad.
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Old 05-17-2022, 04:55 PM   #3
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the vents can be tied together but i would size the line bigger after/at the point where they tie together
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Old 05-17-2022, 05:55 PM   #4
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the vents can be tied together but i would size the line bigger after/at the point where they tie together
I don't disagree however I assumed the drain line would be 3" coming out of both tanks in which case it doesn't need to be larger after they join (only one tank drains at a time).

Side story/cautionary tale: early in my conversion I fell in love with the idea of repurposing materials on hand. So, when my son came by with a hot tub he had ripped out I pulled what I thought was a brand new gate valve before the tub headed over to the dump.

Here's the thing: the gate valves was only 2-1/2", which I thought would be fine 'cuz I only have gray water. Also, I should have checked the o-rings because maybe they are cracked from age, even if the gate valve looks great.

I end up needing to machine the gate valve to get it to fit--hey, no problem, I have a milling machine--and after it all gets put together I fill the tank and the darned thing leaks like a sieve because of the old o-rings.

Three tries to find the right o-rings, and no luck, so I sawed out the gate valve and replaced it with a new one. But now I need to use the same size, which works but I learned it's slower than a 3" gate. And, I have to re-machine the fittings because of the way I sawed out the old valve in my frustration.

So after all that I have a custom waste line that drains slower than I'd like.
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Old 05-17-2022, 07:13 PM   #5
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vent from waste tanks are normally smaller just to let each tank breath in and out in my endeavors.
i dont have a problem with bigger is better because i do commercial plumbing for a living among other things and my old boss said if it calls for 1-1/2" you run nothing less than 2"
yes more expensive fittings by a little to buy a 2" x 1-1/2" tapped tee as needed or a 2 x 1-1/2 that needs a trap adapter that comes with a bagged trap at the right store.
i vent entire barracks of 3 and 4 stories off of 4" with no issues.
the tanks on my big bus though barely used is vented with one inch off of each tank with no problems. it just have to breath to drain.
the drain size is important.
i disagree with the size of the pipe to vent the tank and if we are on the same page why would you put a ballvalve in the vent? which says you are talking drain and i am talking vent.
sorry for your struggles with the drain valve .
re purpose is a score until it aint?
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Old 05-18-2022, 02:36 AM   #6
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my drains are vertical until underneath the bus. when moving sideways, i double the standard drop and that equals about a 2" drop across the width under the bus. my tank may be lower than yours

venting in an RV is different than venting plumbing in a residential situation.

in a house, you have to move a slug of water/waste a distance thru pipes and out of the house.
in an rv, the fixtures are self venting, and the largest slug of water/waste (the toilet) is a straight drop into the waste tank and doesnt need an additional vent to move the waste into the tank.

the venting that is necessary in the RV is relief on the waste tank so that it can receive the waste. that gas escaping stinks, so you launch it high above your bus, but it does not need to be an 1 1/2" like household plumbing.

a good choice for an rv sink drain is the hepvo. its self venting.
an rv toilet is a straight gravity drop toilet and also self venting when you flush.

i used 1/2" pex to vent my waste tank. its been a fine choice for me for 10+ years. i just finished building a van, and did the same, but used 3/4"pex this time. for an exterior vent, i used a marine fuel vent, connected to the pex.

the pex is easy to run anywhere, i try not to have any down loops that may hold moisture.

its worked fine for me for 10+ years. once i thought it was clogged, and put a blast of air thru the vent. it wasnt clogged. but thats why i used the 3/4" this most recent time.

in a house you vent the pipes, in an RV you vent the holding tank.
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Old 05-18-2022, 01:59 PM   #7
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my drains are vertical until underneath the bus. when moving sideways, i double the standard drop and that equals about a 2" drop across the width under the bus. my tank may be lower than yours

venting in an RV is different than venting plumbing in a residential situation.

in a house, you have to move a slug of water/waste a distance thru pipes and out of the house.
in an rv, the fixtures are self venting, and the largest slug of water/waste (the toilet) is a straight drop into the waste tank and doesnt need an additional vent to move the waste into the tank.

the venting that is necessary in the RV is relief on the waste tank so that it can receive the waste. that gas escaping stinks, so you launch it high above your bus, but it does not need to be an 1 1/2" like household plumbing.

a good choice for an rv sink drain is the hepvo. its self venting.
an rv toilet is a straight gravity drop toilet and also self venting when you flush.

i used 1/2" pex to vent my waste tank. its been a fine choice for me for 10+ years. i just finished building a van, and did the same, but used 3/4"pex this time. for an exterior vent, i used a marine fuel vent, connected to the pex.

the pex is easy to run anywhere, i try not to have any down loops that may hold moisture.

its worked fine for me for 10+ years. once i thought it was clogged, and put a blast of air thru the vent. it wasnt clogged. but thats why i used the 3/4" this most recent time.

in a house you vent the pipes, in an RV you vent the holding tank.
Good advice all.

I used double the slope (1/2" to the foot) for horizontal drains everywhere, including the shower pan, for the reason you point out-a bus doesn't necessarily sit level when those plumbing fixtures are in use.

I prefer to just plumb the drain lines like I would a residential structure. Easier to troubleshoot in the long run, if something needs work.

While I don't doubt your experience with a smaller tank vent line for the waste tanks during normal operation, do you get any glugging in your traps (assuming you have traps somewhere), or does the hepvo activate when you drain the tank?

For the record I have 1-1/2" ABS drain with traps, all vented per residential practice through a single 1-1/2" roof vent (protected with a cap).

I also vent the composting toilet in the vent line to save having a hole in the side of the bus.
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Old 05-18-2022, 02:27 PM   #8
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the hepvo sucks all the air it needs as it works. it is self venting.

if you didn't vent the tank, you'd get the glug, glug, like a can of gas draining. our flows are small compared to household use. we dont send a 3" slug of waste water very far.
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Old 05-18-2022, 07:57 PM   #9
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I was curious about glugging when you emptied the holding tanks, but maybe you answered it with the hepvo comment?
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Old 05-18-2022, 09:00 PM   #10
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never a glug when dumping waste. the built in tank vent is clear and the hepvo can suck in air if needed.

never once has it glugged.

i was curious if there was a difference with the speed of the drain. at least once, i used a log to force the toilet flush pedal open. that makes a straight 3" vent into the big tank.

it dumps the same. gravity does a good job and the tank drains with force. hepvos and my 1/2"pex have worked fine as tank venting.

as i recall, the time i thought my vent had clogged, flushing the toilet resulted in a bit of spray up from the bowl. i ended up attributing it to the extreme wind near the sand dunes of southern colorado. its never happened since.
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Old 05-19-2022, 12:26 AM   #11
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One thing about waste tanks' vents that's never mentioned is the importance of letting fresh air into the tanks so that the aerobic bacteria can best do their work inside them. I've never understood how a single vent pipe can do this. For this reason, my grey and poo tanks have two 2" vent pipes each: the tanks are inter-connected by a 2" pipe at their front ends, the poo tank has a rear 2" vent pipe running up to a forward-facing louver just below the roof's driprail, and the grey tank has a rear 2" vent pipe that goes down to just ahead of the differential with a venturi on its end. When driving, fresh air is forced into the poo tank through the louver, then it moves into the grey tank, and finally it's sucked out by the venturi effect caused by the air moving under the bus. In other words, there's a positive flow through both tanks, and any smells are left behind at ground level. When parked, the airflow is reversed, with cool air drawn in at ground level and eventually exiting at driprail height through natural convection. This system guarantees that both tanks get plenty of fresh air circulating through them whether while driving or when parked.

Yes, it was slightly more work to do it this way, but I feel that its benefits should justify it. So far, so good.

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