if you mean this one from the
Water, Water Everywhere technical article page at
Mark's RV..... This article is written by someone who lives in a sticks-n-staples RV for folks who have sticks-n-staples RVs to read. So talking about the vent stacks isn't going to impact any of them nor do they realize they have one (or two) unless a bird or a group of ambitious dirt dobbers build a nest in the stack. Other than that, it is a collection of some very good articles with lots of info. If you are going to spend much time boondocking in NF campgrounds (with unthreaded water pipe stands) then make a water thief like he shows (he calls it a water stealer).
You need a vent (stack) to allow air to escape the tank as incoming fluid displaces space. It doesn't need to be big. Don't think you need to vent your tanks? Fill an empty 2 liter soda bottle with water, then turn it upside down to empty the water out. This is what your holding tank will do when you dump it. Now fill it again and dump it out again but this time pop a small hole in the bottom of the bottle (top when upside down) and see what happens. THIS is why you want to have vents both in your tanks and in your drains. You can use an air admittance valve (AAV) for your drains to let the air in but you need to vent the holding tanks to let the fumes & gasses out along with the displaced air. You do want the waste holding tanks to vent above (or as close to the roof line as possible). Both black and grey water stinks big time after sitting around a few days in the summer heat. Our black stack (thru roof) is 2" pipe. It's the size of the vent pipe hole in our RV black tank. The pipe runs up next to the wall then over (about 21" to 24" from the interior side wall) and up thru the roof. We do have a "mobile home" covered roof cap (gooped up with the clear Henry's 212 elastometric sealant then screwed into the roof) that we picked up at a big box hardware store to keep the birds from nesting in it (lousy house wrens will nest in anything). The pipe/cap is right at the same height as the peak of the roof. So we aren't "above" the highest part of the roof. It works. Our sink is plumbed into the black tank. We lived fulltime in a Class C for a few years and decided we wanted and needed the "extra" water added to the black tank. Plus the vanity sink is on the same side as the black. No wet stack for the vanity sink. It's not needed. the 2" stack handles the water flow just fine because we used the 2" air stack as a drain for the vanity sink. there won't be a whole lot of water going down the drain there. The grey tank & drains are another matter. If all we planned on having was the kitchen sink and shower drain into the grey tank we would probably just use a wet vent on tanks. But we will also have a dishwasher and a washing machine, both of which dump a lot of water in a very short time. So the drains get a vent (probably wet as it most likely will use the same pipe as the washing machine drain with the pipe extending up to the roof) as well as the grey tank (we will tie the drain vent & tank vents together near the ceiling so we only have a single hole popped thru the roof almost directly opposite the black vent stack). The way we will set out grey tank up is to be able to bypass the grey tank completely when we are on a sewer hookup. We will need the drain vent for that, Otherwise we will have slow drains that could back up into the shower.
BTW, on the Class C both the black and the grey tanks tied together into a single vent stack which also acted as the wet vent stack for the drains. For clarification... what I call a "wet vent" is a vent pipe that is also a drain for a short distance (in our case, from the "T" to the tank... less than 12"). I wouldn't run a LONG drain in a wet vent and I wouldn't go under 1" for a wet vent. I would let my wall space dictate that and go as large as I could up to 2". More than that is overkill on the size of drains we have in the buses.
But what do I know. I've been up since 3AM and its 1PM. I need a nap.
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