Drain System Vent

[WoodenBoat]

In a site built home, the drain system from the kitchen sink to the bathroom sinks, the showers and the commodes all have vents going up through the roof of the house in order to get rid of noxious odors and allow the drains to empty properly. Is there a way to do this on a bus without having to cut a hole in my roof?

[richard5933]

Here's how Custom Coach vented both of our tanks. If you look carefully, you'll see the little 1/2" vinyl tubing going down from each tank through the floor of the bay. I'd say it's not sufficient venting, but surprisingly it works.

I have also seen people use something called a Stoddard Vent, but can't say how well they work.

The 'standard' method appears to have one vent from the tank going up through the roof of the bus, with a cap on the top to prevent water intrusion and to facilitate venting. I've not seen individual vents from each plumbing fixture being done.

[bevans6]

The vent stack on a bus grey/black tank is different from the air vents on plumbing drains.  Drain vents are there to ensure air after water, so the drain flows properly.  In addition to that the vent on the tank is there to allow not only noxious odours but more importantly methane gas to rise up and out of the tank.  If it gets trapped there is an explosion risk - small but real.  The standard for a main vent is an always rising, no traps path up and out of the tank to atmosphere.  The safest way is up and out the roof.  My tank is vented up and out into the rear axle bay, so I sometimes can smell it but there is no methane trapped issue.

[David Anderson]

I'm with Bevan on this.  If you keep them low you will smell the gas, and if conditions are right, while driving it could create positive pressure in the tank, and if you flush the toilet, oh my, the smell is unbelievable.    I smell mine some days when the wind is just right and they are on the roof. 

The plumbing code allows loop vents, (google sink island loop vent) but there is still a vent going up and away somewhere on the line, because, yes the gas rises. 

I think you will be disappointed in the results if you don't get them up and away. 

[Jim Blackwood]

Never thought too much about the drain vents but I guess that sort of makes sense. Grey tank vent I can't see being much of an issue, but teeing it into the black vent up high should work OK. That black vent is the troublemaker, and one vent line through the roof of the bus shouldn't be any big deal. It can go up through the wall behind the toilet, and there are plenty of very good sealants that will adhere very well to metal, paint, and PVC and hold up to weathering. If the one you use doesn't last there are plenty more to try.

Now I think I'll try this: Get a bilge fan and put it inline just below the roof inside the cabinet over the toilet. Connect it to the bathroom light switch. Then cap the vent line with one of those omnidirectional suction caps. (and of course you always maintain some water in the toilet for a good seal and better flushing) If that doesn't quite do the job, maybe use a bigger bilge fan. Finally, for this to really work well there really should be an air inlet vent to let fresh air into the black water tank. I'm thinking it might not be too hard to run a 1" pvc line into the engine bay and up near the fans.

Jim

[richard5933]

There's not a need to over think this. A simple pvc pipe going to the roof, terminated with the proper cap & flange will provide a weatherproof seal. The right cap will provide an updraft with the slightest breeze. A bilge pump might help, but would draw far more battery than is necessary. A better suggestion is to use something like they use on composting toilets (similar to a computer fan.) Even that is really unnecessary.

The reason I showed what Custom Coach installed wasn't to recommend anyone copy it, it was to show just how minimal a venting system can be made to work.

We rarely have any issues with smell. The only time we have a problem is when someone flushes while we are in motion. For whatever reason, the interior of the bus has a slight negative pressure which will momentarily draw in odors if you take too long to flush.

[chessie4905]

In the early days of rv's and conversions, nobody used a roof vent. Eventually, it was realized that could end odor issues. Our 4104 didn't have one and occasionally there was an odor issue. Leaving the bathroom vent fan lid raised about an inch or two mostly took care of it. Another source of odor can come from the safety from overflow hole in top of toilet bowl. It should have some liquid in it to avoid odors, but many times the liquid in that tube evaporates and provides a direct vent for tank fumes. We put a rubber cork in ours, and always turned off the water pump when not in the coach in case toilet water valve failed or leaked.

[Jim Blackwood]

Please elaborate on this toilet bowl vent Chessie, I'd like to know more about it.

I honestly don't believe I'm overthinking it here. The light in the bathroom only stays on when the room's in use, (could be tied to the vent fan instead) and those bilge blowers don't use much wattage. It'd draw a slight vacuum on the tank and help keep the gases out of the bathroom. Seems to me it's worth a try.

Also, last time we discussed this didn't we find that the smelly gases were heavier than air? Which means they're just going to fill the stack and then run out the toilet if it's open.

Jim

[richard5933]

Couple of thoughts about using a powered vent blower:

• For the blower to do any good it has to run 24/7, not just when the light is on. If you have a problem with stink, it will be worse when flushing but it will likely always be there, especially as the level of waste in the tank rises.
• The bilge blowers I see online draw anywhere from 1.5 amps and up. Doesn't sound like much, but leave it on 24/7 and it adds up.
• Composting toilet blowers only pull about 2.5 watts - much less than the bilge blowers I'm finding.
• For the negative pressure situation we have while on the road, I seriously doubt any of these are going to be overcome the negative pressure pulling sewer gases from the toilet during flushing - it's just too big an opening to overcome with a small fan. (We've just learned to keep the bathroom door closed for a few minutes after use, and we leave the roof vent open in there during that time - it clears out pretty quickly).
• If putting powered vents on waste tanks was all that helpful, I would think that some of the higher end RVs would be using them.

All that said it won't cost that much to try, can't do any harm, is easily reversible, so if you think it will help give it a try and let us know.

[buswarrior]

Where's the air coming from to feed this vent line blower?

Needs a supply, to have an exhaust.

No fan is going to overcome having the toll window open or bad door seals, when a flushing event is attempted...

For cleaner visuals, there once was a busnut who used the fittings in the bottom of the kitchen sink to get through the roof. Find a flatter place to punch through... and then all stock parts to connect, seals for the purpose...

Built in critter guard?

Happy coaching!
Buswarrior

[chessie4905]

Don't know if porcelin top toilets have them, but the common plastic have a 3/8 to 1/2" hole at top on inside of bowl to handle an overflow. Similar to the one on a sink.

[Jim Blackwood]

Kinda long:

I didn't know about the toilet overflow but it makes sense, wonder how you're supposed to keep water in the trap? Overflow the toilet occasionally? That seems wasteful. A home toilet would probably have a water bleed that fills it, so I wonder if that's a thing?

The air pressure differential causing this problem is very small, something on the order of less than 1psi.  And there are at least two and probably 3 modes of airflow in operation here. That part of it can get a little confusing. So to simplify, let's define conditions. 1st would be static, parked, little or no wind. 2nd would be parked with significant wind, 3rd would be moving. In each case it seems to me it would be helpful if one of two things were happening, and maybe both. Those would be: a) Regular somewhat steady airflow through the tank and; b) a depressed pressure level in the tank compared to air pressure in the bus. Again, each of these can occur during any of the 3 modes so we have essentially 6 conditions to verify. If a system can satisfy all 6 conditions it should be promising, wouldn't you say?

I have yet to do the analysis on the system I'm thinking about, at this point it's an off the cuff strike at a viable remedy given what I know from experience. Depressurizing the bus by opening the ticket window or from a leaky door is sure to complicate things. To overcome that you would have to find a lower pressure area to draw from (larger vacuum) and it is likely that the rear of the bus can satisfy that requirement. So in mode 3b we would need to use a manometer while traveling to confirm that a site at the rear of the bus shows a pressure drop over the interior with the window open, a fairly simple test. But not one I can do until I've got tags on the bus. A simple U-tube liquid manometer would work. Find this location and gases inside while OTR can be a thing of the past, and I encourage you all to experiment. Possibilities might include air inlets for the fans and even the engine as well as locations near the edges of the flat rear surface of the bus.

But how does that work in the other 5 conditions? The most difficult to reconcile with 3b would be 1a, stationary, no breeze, constant flow through the holding tank. Honestly the only passive way I can think to achieve that is with a temperature gradient and unfortunately that brings in a whole host of additional operating conditions. It might be possible but would take a good bit of thought, what with the various conditions of heat/cold, inside/outside, and day/night. I'm not sure I'm up for that challenge just yet.

Also, the size of the vent tube matters. Suppose you were able to hold tank pressure a couple inches of mercury below interior pressure. That relative vacuum would accumulate somewhat in the tank. But the toilet drain is probably what, about 2-1/2 to 2-3/4" in diameter? That's going to bleed off any accumulated vacuum really quick and the size of the suction line is the only thing slowing it down. Now it doesn't seem prudent or practical to run 3" pvc pipe out the rear of the coach and it shouldn't be necessary if someone isn't holding the flush valve open, but at the same time a 1" line won't slow the bleed-off down very much. It might not be enough.

OK, so sticking with 3 for now, how about 3a? Suction is wonderful but where's the fresh air come from? Roof vent maybe? First off, any fresh air vent is going to compromise 3b action so it would have to be a smaller line. One of those omnidirectional suction caps should help reduce this effect some, and a (on when occupied) bilge fan could also help preserve the vacuum. Presumably the draw is still going to be to the rear under all cruising conditions but at low speed the bilge fan could help.

What happens in condition 2b is going to be somewhat dependent on finding a rear vent location that isn't affected by wind gusts. Otherwise the bilge fan should serve nicely. Wouldn't do to pressurize the tank, would it? For 2a we're dependent on the roof suction vent cap.

So there you have it, as far as I've figured it out. Just at a guess I'm inclined to think a 1-1/4" roof vent line and a 2" rear vent line could be about the best compromise but that's just seat-of-the-pants figuring and could be refined with actual numbers if we had them.

Jim

[robertglines1]

stinky  situation-------Scott Crosby   had found good stink adative..   Don't remember name.  By the way when you get ready for tanks--   I have a local guy who makes good ones..   under 200$ each for hundred gallons---fresh and waste..locates fittings where you want.  Bob

[Jim Blackwood]

That's not bad, I was planning to weld up some stainless but it'll likely cost that much and more. Nearing completion of the fresh water tank, just have to get material for a cover and time to work on it.

Jim

[David Anderson]

Kinda long:

I didn't know about the toilet overflow but it makes sense, wonder how you're supposed to keep water in the trap? Overflow the toilet occasionally? That seems wasteful. A home toilet would probably have a water bleed that fills it, so I wonder if that's a thing?

RV toilets have no trap.  It is a straight drop to the tank.  Keep that line short and as near vertical as possible