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Old 06-18-2017, 08:02 AM   #1
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Grey water drainage system

Hello everyone!

I am currently planning out my plumbing system, more specifically the drainage into my grey water tank. I know absolutely nothing about plumbing, so could someone please tell me if this drawing that I made would actually work in real life?

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Old 06-18-2017, 09:42 AM   #2
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Sort of, The tank vent looks to be tied into sink/shwr drain? tank vent should be independent. The sink vent on the right does not need to go to the roof unless you already have a penetration for it. Each P trap just downstream should have a "trailer vent" ran as high up as the cabinet or whatever will allow, A trailer vent is a one way type device to allow air into the system when a fixture is being used to keep from syphoning trap dry and seal closed when not. One other thing and this may just be the way it's drawn but use a Y brach instead of a san tee when tieing the sink drain into the shwr drain.
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Old 06-18-2017, 09:56 AM   #3
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This is how you want the vent off the san T while in the rise and not coming off the flat horizontal run. Venting the shwr you will not have a choice though.
The trailer vent is more of a trade used name and think they are really called auto-vent.
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Old 06-18-2017, 09:58 AM   #4
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Tank vents are normally larger and enter the top of the tank.
I think space considerations might make having the drains enter the top difficult. Also, my shower just runs to the sink drain. I think you could simplify the plumbing there. No need to have two separate lines.

I don't see a problem with the drains entering the side near the top. Just my opinion.

BTW, my shower kept breaking the ABS coupling to the drain. I guess there was enough flexing to stress it. I added a rubber coupler from the shower fitting to the drain pipe and the problem went away.
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Old 06-18-2017, 11:27 AM   #5
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AlinCal do you mean that I can use the trailer vents in the cabinets, without breaking through to the outside? I've added a new diagram below with the improvements, I just can't figure out how ill get the shower p trap vented...

MollysDad, I was going to do that originally, but that meant drilling a 2" hole into the side of the chassis, and I don't want a pipe rateling around in there, perhaps breaking the connection to the grey tank. Now I think I'm just going to build up the bathroom floor and run all of this piping under it, then directly down into the tank.

Thanks for the help guys!
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Old 06-18-2017, 11:41 AM   #6
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Shower vent

Actually, if the trailer vent is in fact meant for the interior, I could just stick it up through the floor, stopping right below the shower (in the Frame that I build that the bathroom will be built on top of). Or would that not work... If the vent needs to be ABOVE the drain.

Thoughts?
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Old 06-18-2017, 12:11 PM   #7
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First off..don't enter the side of the tank, you eliminate capacity or end up with part of your piping holding water, as well as that connection having a greater potential to leak.
The trailer/auto vents want to be as high as convenient but in no case below the the shower pan elevation.
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Old 06-18-2017, 12:16 PM   #8
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Not a good pic but kitchen drain with vent fitting up to sub top elevation
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Old 06-18-2017, 12:20 PM   #9
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Shower vent with shower stall to the left, ideally you want access to these as they can fail.
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Old 06-18-2017, 12:56 PM   #10
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Thanks for the photos AlinCal.

I've attached a photo for two possible directions for the shower vent. Would that work if it is running horizontal? I can't think of any other option for running it verticle like I am the two sinks.

Also, does the tank vent need a special cap on it? Or can it just directly connect to the air outside?
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Old 06-18-2017, 12:59 PM   #11
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Those devices at the end of a pipe are called "air admittance valves". Inside the valve there is a flexible diaphragm compressed by a spring that allows air to come into the pipe. That diaphragm blocks any gasses from escaping into the interior. In that sense, the valves are not vents. The purpose of these valves is to prevent liquids that are flowing through the pipe from creating a vacuum.

At the highest end of each drain pipe there must be either a vent or an air admittance valve. There is also an issue when a pipe reaches a certain length after there is a tee, it must have a way for air to enter if that branch/arm exceeds that length. From memory: I think it is after 5' length on a 2" pipe and/or after 3' length on a 1.5" pipe.

There must be one true vent from the tank that is open to the atmosphere, to allow the gasses (methane gas is what smells bad) created by decomposing waste to escape freely. This vent to atmosphere also is to let air enter freely when draining the tank.

add edit:
You do not need to add a second air admittance valve for the tub, IF the pipe is less than 3' (including the trap) after it leaves the tee or wye. The air admittance valve at the end of the run will be sufficient.
Explanation: Water leaving the sink would run past the tub/shower trap. If there was enough volume going through to fill the pipe, without a vent a vacuum would be created, possibly pulling water out of the lower trap and/or pushing sewer gas past the trap into the tub area. Since air can come in at the high end of the run the vacuum is broken, both of the traps stay full.
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Old 06-18-2017, 01:18 PM   #12
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Home Depot calls them...1-1/2 in. ABS PTC In-Line Cheater Vent
Vent thru roof is left open.
Typically you would not want the vent running horizontal below the flood rim of the fixture it's serving but you may have no choice?

A W Good memory, 1-1/2" is 30" though. Those numbers are UPC(universal plumbing code standards) and can be fudged without worry.
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Old 06-18-2017, 01:31 PM   #13
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Quote:
Originally Posted by AlinCal View Post
Home Depot calls them...1-1/2 in. ABS PTC In-Line Cheater Vent
Vent thru roof is left open.
Typically you would not want the vent running horizontal below the flood rim of the fixture it's serving but you may have no choice?

A W Good memory, 1-1/2" is 30" though. Those numbers are UPC(universal plumbing code standards) and can be fudged without worry.
Most tradesmen in my area still call them Studor Vents. I think that was the original brand name, during the time they were protected by a patent law.
add edit:
In my Safari the factory installed 1.5" drain pipe leaves the tank then runs on top of the floor beneath the shower where it wyes to the drain. Then it is flat on the floor without pitch beneath the bed through the storage area. Then it's beneath a closet into the kitchen cabinet. There is an air admittance valve under the bed between the shower and kitchen sink and another at the high end inside the kitchen cabinet. It's more than 15' without pitch. 17 years so far, no problem.
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Old 06-19-2017, 07:26 AM   #14
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Thank you AW Warn and Alin Cal!

I feel much more confident now about setting up my plumbing system. I'm now in the process of ordering my custom grey tank.
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Old 06-19-2017, 08:08 AM   #15
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Cool, Good luck with it!
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Old 06-19-2017, 09:50 AM   #16
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I had another thought about your sketch:
If the tank vent and the sink drain are really close and parallel as you have drawn them, you might be able to eliminate one of the tank openings.
There is no reason that the vent to outside cannot be combined with the drain. This is called a wet vent. It is done this way every day.
If there is an obstruction, or some reason for not combining the vent/drain, still the vent can be combined to eliminate one of the air admittance valves. This is a photo of how it is done in my Safari.
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The pipe to the left side is the tank vent that goes through the roof. The pipe on the right is the vent from the sink that is just on the other side of the wall.
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Old 06-20-2017, 08:33 AM   #17
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AW Warn, do you mean that I can just shoot the tank vent to the outside from where the air admittance valve is in the bathroom sink? And therefore completely remove the main tank vent pipe?
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Old 06-20-2017, 08:58 AM   #18
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Quote:
Originally Posted by nico9q8743 View Post
AW Warn, do you mean that I can just shoot the tank vent to the outside from where the air admittance valve is in the bathroom sink? And therefore completely remove the main tank vent pipe?
Your sketch does not show which is the bathroom. But, yes the dedicated vent is not necessary.

There must be a minimum of one vent from the tank that is open to the atmosphere (through the roof). It does not have to be a dedicated vent with nothing else attached to it. If not a dedicated vent, it should be at the high end of either of the drains you show in your sketch.

You can attach a sink drain and/or shower drain to the vent so that if serves as both drain and vent (wet vent). But, be aware that when the tank is drained it creates a lot of suction/vacuum, air coming into the tank through the vent, that could possibly pull liquids out of the traps. That is why some people say a separate vent is better. I have not had that problem.

You can attach/combine a second vent pipe to the main tank vent, as shown in the photo I posted. The photo shows the way Airstream did it at the factory.
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Old 06-20-2017, 09:59 AM   #19
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In summary, if I may....You still need a vent out the roof, but it can also serve as a drain AND vent for any fixture in it's vicinity.
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Old 06-21-2017, 08:23 AM   #20
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Thanks again Alin Cal and AW Warn, the pvc piping in my airstream just got simpler again!
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