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Old 03-02-2015, 11:04 AM   #11
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Quote:
Originally Posted by lornaschinske View Post
You need a vent in the holding tanks to allow air to escape as the tank fills. Black & grey tanks can share a stack that usually is either thru the roof or very high up (waste water smells you don't want that venting under your bus). You can put an AAV (Air Admittance Valve also called several other names) under a sink stubbed up off a drain. This allows air into the drains so the water flows better (curious as to why? Fill an empty 2L soda bottle with water to the top. Turn bottle upside down quickly to allow all the water to drain out. Now do it again but pop a tiny hole in the bottom and see the difference in the water flow).
Thanks for the 2L bottle comparison Lorna, that's interesting. It helps to illustrate how the pipe beyond the trap is like the neck of the bottle, and if a person doesn't provide for air to enter then interesting things can happen:
  • sink may not drain well
  • sink may drain fine, but the water leaving the sink may behave like a siphon and also suck the water out of the trap, thus allowing system odors back up through the sink.
Air admission prevents these from happening. It can be achieved by:
  • use a wye in the wall where the sink ties in, and extend a vent pipe upward from there
  • same as above, but the vent pipe doesn't have to go strictly upward -- it can go up a little bit, then double back down into the floor. This trick is commonly used when there's a sink in a kitchen island where there's no wall to go up. Proceed with caution, though: if that vent gets water trapped in it.. then it becomes a P or S trap and doesn't vent anymore
  • air admittance valve.
That takes care of the source end of the pipe. Venting is also needed at the other end: think of a cylinder of water moving through a pipe. There must be an opening at the supply side to let air in and prevent creating a vacuum; there must also be an opening at the exit end to allow air to escape and prevent creating a pressurized area. That's where the tank ventilation comes in. Thoughtful design can allow sink vent(s) and tank vent(s) to be combined ans in Lorna's "wet vent.". It's called "wet" because the pipe that vents the tank also carries water into the tank. Diameter of the vents matters. I've figured it out enough to understand that it must depend on the rate of water being introduced and the length of the vent, but haven't bothered engineering it out.. basically there's a volume of air that must be displaced through the vent with minimal restriction, and if it can't, the plumbing system gets revenge. The 1-1/4 or 1-1/2 are sure bets, though I've also read of people apparently doing fine with even 1/2 PEX on the forum.
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Old 03-02-2015, 11:46 AM   #12
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[QUOTE=

RV toilets are generally designed for and recommended to have a straight shot down into the black tank. They have a sliding gate valve in the bottom of the bowl and a rubber seal to keep the odors down under. It's similar to an aircraft toilet.[/QUOTE]

Just to throw a curve ,aircraft designed & built in the last 25 yrs have vacuume
toilets, depending on size of acft 1 or 2 tanks at the rear & one big sucking sound when you flush,
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Old 03-02-2015, 12:51 PM   #13
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Originally Posted by family wagon View Post
I've figured it out enough to understand that it must depend on the rate of water being introduced and the length of the vent, but haven't bothered engineering it out.. basically there's a volume of air that must be displaced through the vent with minimal restriction, and if it can't, the plumbing system gets revenge. The 1-1/4 or 1-1/2 are sure bets, though I've also read of people apparently doing fine with even 1/2 PEX on the forum.
As Lorna mentioned, a pinhole can be a vent. Bigger vents just allow for higher airflow. The vent's main job is to stop the water being drained from creating a siphon and sucking the water out of the S/P-trap under the appliance. Any vent that can provide enough air to equalize the pressure quickly enough will "do".

As for 1/2 PEX that's a little small IMO. "Lucky" doesn't always mean "right" and I suspect they're more the former. Your typical RV is going to have a LOT less water flow (deliberately) than a house, and you're rarely going to use more than one appliance at a time. Still, dump a pot of water from cooking pasta or corn down your sink and you're going to need a lot more airflow in a hurry to equalize it. There's actually a table, in case you care:

Chapter 9 - Vents

although this table is for houses so the length limits are in numbers of feet we don't even have...

I'd definitely go with the biggest vent I can, and it's common to use the same size as the piping itself. I mean, you have all the rest of that pipe there - why not just run a length of the same stuff up to the roof and call it a day?

One comment about RV toilets. Personally, I hate RV faucets and never use them - it's the first thing I replace. But I think the toilets have a lot of value. They're designed to use very minimal water when flushing, their mounting flanges are designed to handle the kind of rough handling you get in a vehicle (without leaking - gah!), and they're super easy to winterize if you aren't full-timing. You can dump a few cups of antifreeze into the tank to protect the dump valve, and about a cup into the toilet for the same reason (or just leave that dry) and you're good to go.
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Old 03-06-2015, 03:05 PM   #14
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Ok here's a silly question, This is my air intake for the motor my waste tank will be mounted just a few feet forward of this, except for the obvious problem of using ridged line why couldn't a person add a coupling to the tube & run the tank vent inside & to the top of the intake tube??????? It's about a 7 ft straight up inside the tube
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Old 03-06-2015, 03:07 PM   #15
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Pic of tube inside
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Old 03-06-2015, 03:08 PM   #16
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Ok here's a silly question, This is my air intake for the motor my waste tank will be mounted just a few feet forward of this, except for the obvious problem of using ridged line why couldn't a person add a coupling to the tube & run the tank vent inside & to the top of the intake tube??????? It's about a 7 ft straight up inside the tube
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Old 03-06-2015, 03:23 PM   #17
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That isn't what the EPA had in mind when they required "exhaust gas recirculation" to improve vehicle emissions!

Nevertheless, it is an interesting idea to help economize on the number of vertical stacks and roof penetrations. If the vent tube ran all the way to the end of that intake it would probably result in restriction to engine air flow. If you connect the vent in with a tee, I wonder how much vacuum develops in the pipe and whether it might have any tendency to make the holding tank walls collapse inward a bit or even suck the water out of the P traps. It might make routine air filter changes less pleasant, too!
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Old 03-06-2015, 06:04 PM   #18
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Would a 3/4 in tube of pex clamped to a 5in intake pipe really cause enough restriction? Plus I'm planning on Hepvo valves instead of p traps
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Old 03-09-2015, 01:05 AM   #19
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Quote:
Originally Posted by allwthrrider View Post
Would a 3/4 in tube of pex clamped to a 5in intake pipe really cause enough restriction? Plus I'm planning on Hepvo valves instead of p traps
I'm not really clear whether you're proposing putting the PEX inside or outside the intake.. Also not any kind of fluids engineer, but daring enough to speculate that air flow resistance depends upon cross-sectional area of the duct. The 3/4 PEX has an OD of 7/8 inch and so its area is 0.60 sq in; if the intake has ID of 5 then its area is 19.6 sq in. A piece of PEX running through there reduces the intake pipe area by 3%, so I agree it's probably negligible. (in my defense, in the picture it looked to be more on the order of 3" pipe, and a piece of PEX in a pipe that size would make a 9% reduction, which probably would be something to think twice about)
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Old 03-09-2015, 10:23 AM   #20
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Have you driven this bus down the highway since that intake stack was installed? I am curious of two things. 1. Cross flow at the top may keep air from being pulled down the tube. 2. What keeps rain out?
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