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Old 02-22-2015, 04:50 PM   #1
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Waste tank Vent

Wa looking things over today on running the waste tank vent & am wondering is there a basic rule of thumb on whats a minable distance from waste tank vent to say AC unit or roof vent????

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Old 02-22-2015, 10:05 PM   #2
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Originally Posted by allwthrrider View Post
Wa looking things over today on running the waste tank vent & am wondering is there a basic rule of thumb on whats a minable distance from waste tank vent to say AC unit or roof vent????
Unless you're doing something odd with your AC, most are recirculating rather than venting types so you don't need to worry about air intake there. For the bathroom vent that's also usually an exhaust, but just for the sake of the argument, I poked around and found that the Uniform Plumbing Code says this:

https://law.resource.org/pub/us/code...o.upc.2009.pdf
906.1 Each vent pipe or stack shall extend through its f1ashing and shall terminate vertically not less than six (6) inches (152 mm) above the roof nor less than one (1) foot (305 mm) from any vertical surface.

906.2 Each vent shall terminate not less than ten (10) feet (3,048 mm) from, or not less than three (3) feet (914 mm) above, any openable window, door,
opening, air intake, or vent shaft, or not less than three (3) feet (914 mm) in every direction from any lot line, alley and street excepted.
My current (pre-Skoolie) Rockwood camper has a 3" high vent cap and is only a foot or so from the bathroom vent. It wouldn't even come close to "code" for residential. I can't imagine the same rules apply. I would separate them as much as you can, IF you can, and not worry about it...
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Old 02-25-2015, 04:04 PM   #3
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Due to the fact of how small our skoolie black tanks are, and the size of your black tank vent line, I would think vary little smell will be coming out.

Here in the country we use what are called septic tanks, and pump outs.

Basically we flush the toilet, it flows through a 4 inch pipe to a tank buried in the ground, not far from the house. There is stews a nice batch of germs, pathogens, ect, and breaks most of the solids down to nothing.
When the level in the tank reaches 3/4 full, a sump pump activates by float valve, pumping the liquid out onto the surface of the ground in a field, swamp, ect.

Now if you live anywhere near one of these, they SMELL LIKE YOU STUCK YOU HEAD INTO THE SEWER.

The vent at the top of the house is a direct line to the septic tank. When I was fixing the roof on one of the houses on that farm, I could not stand to go withing 10 feet of that vent. It was far far worse then fresh poop.

So while I'm at it, I want to say this for the readers.
No one in my county will care about me burning my poop as long as this county continues to use pump outs from septic tanks. The pump out smells 1000 times worse than any smell coming from my chimney.

The alternative to septic tank pump out are whats called "Sewer fields".
In this set up the septic tank is still there, but when full, the sump pump directs it's flow to a underground absorption line. The absorption line is just a bunch of 3 inch perforated pipe, buried in sand or pea gravel filled trenches, then back filled. These trenches are deep enough to keep them from freezing in winter. With this system, no smell ever reaches the out door air.

Unfortunately septic fields don't last forever. They plug off, freeze, ect. On average they have to be dug out, and replaced every 15 years. This is at a cost of $15,000 to $30,000 depending on the size, location, ect.

Nat
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Old 02-25-2015, 04:39 PM   #4
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my black tank vent is out the side at the top of where a window was, its 1/2 pex pipe. been working good for more then 3 years, no smells, no problem dumping.
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Old 02-27-2015, 12:44 PM   #5
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Basically we flush the toilet, it flows through a 4 inch pipe to a tank buried in the ground, not far from the house.

When the level in the tank reaches 3/4 full, a sump pump activates by float valve, pumping the liquid out onto the surface of the ground in a field, swamp, ect.

The alternative to septic tank pump out are whats called "Sewer fields".
In this set up the septic tank is still there, but when full, the sump pump directs it's flow to a underground absorption line.

Unfortunately septic fields don't last forever. They plug off, freeze, ect. On average they have to be dug out, and replaced every 15 years. This is at a cost of $15,000 to $30,000 depending on the size, location, ect.

Nat

Dang Nat, where the heck do you live? I've always lived in the country myself, but we NEVER pump sewage out onto the open ground. We use field lines, which are 8 inch pipes with perforations only on the bottom (keeps roots from growing into the lines). Buried 4 feet at a minimum (the frost line in the southern states) with 1 foot of gravel below and 6 inches above the pipe. Occasionally a new line will have to be run, but usually they last 50 or 60 years. And never put a pump into a septic tank, natural build up and gravity takes care of the job just fine.

But I guess that wasn't what this post was really about, oops.

I originally came to this post to see how the black water tanks need to be plumbed. I still find it hard to believe that there isn't a trap below the toilet, yet the bus doesn't fill up with methane.
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Old 02-27-2015, 01:34 PM   #6
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Around here just about every home has exactly the same setup: house main septic outflow gravity-feeds to a big septic tank, and you have a "leach field" out from there. If the field is too close to the surface or you get water pooling in your backyard you can get a smell sometimes, but it's pretty rare. You're supposed to get your tank pumped every 2-4 years but it's usage-dependent. I suspect a lot of people don't bother until there's a problem.

On topic, here's what I was planning:



(shamelessly stolen from a Google Image Search)

This is a pretty typical DWV vent-stack setup, no surprises. It's super easy to set up - any reason not to do it this way?

The only thing I'm still torn about is that we want to do a lot of traveling to places like Alberta, BC, Alaska... you know, places where the temperature is measured by the alphabet ordering of its name. In our "occasional" camper we just dump some antifreeze into the tanks to protect the valves and wait for a thaw before we dump, not that we use it that much in winter. I can't decide how I want to insulate and protect the tanks, set up tank heaters, etc. Is there a thread where people have talked about this? I don't see anything specifically on the topic.
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Old 02-28-2015, 05:31 PM   #7
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Due to the fact of how small our skoolie black tanks are, and the size of your black tank vent line, I would think vary little smell will be coming out.

Here in the country we use what are called septic tanks, and pump outs.
Nat
Up until midway thru the third grade all we had was a 2 holer outhouse plus a honey bucket in the room where my 4 sisters my brother & myself slept, I refused to use the bucket & would go out in the middle of the night & use the outhouse. Then when I was in the 3rd grade that house caught fire in the middle of the night & was a total loss so we built a house with indoor plumbing & had a septic tank for that. Dad used to make us hoist 5gal buckets of stuff out & dump it on the garden, we would have potatoes as big as your head.. Stuart
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Old 02-28-2015, 09:50 PM   #8
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I originally came to this post to see how the black water tanks need to be plumbed. I still find it hard to believe that there isn't a trap below the toilet, yet the bus doesn't fill up with methane.
Residential toilets (those I'm familiar with in the US) have a trap built into them; there isn't any trap in the building plumbing below. Some people use a residential toilet in their skoolie but I didn't in mine. I don't want the trouble of remembering to drain the bowl nor the hassle of cleaning up what sloshes out if it doesn't get drained before driving! Probably uses more water, too.

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.
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Old 03-02-2015, 08:02 AM   #9
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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.

Ahh, this answers my question, thanks. I'm still quite a noob to this whole building an RV things. Didn't realize that the RV toilets had a gate (thought the thought had crossed my mind). That make much more sense.

On to my next ridiculous question, I love getting educated......
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Old 03-02-2015, 09:24 AM   #10
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Includes a diagram of a typical self-contained RV fresh and waste system. Does not show vent pipe. 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). Depending on how your drains are run, you may need to add more than one. I have one on the washer drain that serves the shower and to some extent the galley sink. When I add the dishwasher to the galley, I will add another AAV under the sink. The bathroom sink drain is plumbed into the vent stack for the black tank (making it a "wet vent"). We did not want any more holes in the roof than absolutely necessary, hence the AAVs.

BTW, I do not have an RV toilet. I put a residential toilet in.

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Old 03-02-2015, 10:04 AM   #11
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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, 10:46 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, 11:51 AM   #13
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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, 02:05 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, 02:07 PM   #15
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Pic of tube inside
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Old 03-06-2015, 02:08 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, 02:23 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, 05:04 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, 12:05 AM   #19
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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
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, 09:23 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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