99damon
Colorado
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There are some conflicting views here on the forum, and I'm looking for the tie-breaker. If you have one vent stack, is it safe to assume the vertical vent pipe splits (inverted Y) into both black and gray tanks?
It's been said by some that the black tank vents to the roof, and the gray tank vents through the vacuum breakers (AKA air admittance valves) under the bath and kitchen sinks. I think maybe not, since the vacuum breakers open when the sinks drain, not necessarily when the gray tank is drained. Or?
Without removing a wall, I can't visualize my vent design. I ASSUME (always risky) that my roof-top vent serves both tanks. Am I wrong?
I see many motorhomes with a single roof-top vent, like mine. How are we venting our gray tanks? What causes this to come up, is I have intermittent gray tank gas-burps into the coach. If I install one of those venturi vents, am I assisting both tanks to vent skyward, or only the black tank?
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Wayne
Fulltimers
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Economical Handyman
S. California
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You could empty both tanks, then run water into the roof vent and see if both tanks gets water in them. Then you would know.
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riggarob
Farmington, NH
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99damon wrote:There are some conflicting views here on the forum, and I'm looking for the tie-breaker. If you have one vent stack, is it safe to assume the vertical vent pipe splits (inverted Y) into both black and gray tanks?
It's been said by some that the black tank vents to the roof, and the gray tank vents through the vacuum breakers (AKA air admittance valves) under the bath and kitchen sinks. I think maybe not, since the vacuum breakers open when the sinks drain, not necessarily when the gray tank is drained. Or?
Without removing a wall, I can't visualize my vent design. I ASSUME (always risky) that my roof-top vent serves both tanks. Am I wrong?
I see many motorhomes with a single roof-top vent, like mine. How are we venting our gray tanks? What causes this to come up, is I have intermittent gray tank gas-burps into the coach. If I install one of those venturi vents, am I assisting both tanks to vent skyward, or only the black tank?
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Well, because of you, I'm gonna have to climb up on the roof to see what I have up there !! Robbie 
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bldrbuck
Boulder, Colorado
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Check in the storage area for vents that don't go to the roof. I forget the name but they work well.
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George B
Northern CA
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I have a black vent directly into the tank and a gray vent on the shower run. All other fixtures are vented with AAV (studor valves) devices. Take DeWayne's suggestion and run water down your vent stack and see where the water goes.
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1995brave
San Antonio, TX
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I got a copy of my Winne's plumbing and my black and gray tanks both vent to the roof with one pipe and a "Y" the shower tank vents directly to the roof.
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dougrainer
Carrolton
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ALL Gray and Black tanks are vented to the roof. Some use a Y and use 1 stack. The vents under the sinks are "emergency" type vents, and are required by code. They are NOT there for venting of a tank during normal use. Usually when you have a Y fitting and 1 stack, it is because of the RV's floorplan and there was not an easy way to run seperate vent pipes. Doug
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yudamann
Outer Banks NC
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Plumbing codes allow the use of a single vent stack through the roof as long as the single vent is sized properly, which is safe to assume that it is since there are not a lot of fixtures inside an RV. But, each tank must be vented through the roof, whether individually or joined to a common stack through the roof. The device mentioned by other posters on the kitchen and lavatory drains is an air admittance valve, AAV. It is not an emergency valve or vent. It is designed to be used where it would be difficult to route a vent pipe from just downstream of the fixture trap that it serves to the vent through the roof. It is a one-way valve, or check valve that admits air into the drain line when a plumbing fixture downstream of the AAV has waste flowing. The admittance of air at this point protects the adjacent fixture trap from being siphoned away due to this flow. The loss of a trap seal often leads to foul odors coming up from the tank through the fixture drain. Plumbing codes do not allow for the AAV to be used at every trap in the system without any through-roof vent; there must be at least one vent that goes through the roof.
yudamann
Outer Banks NC
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Ames
South Central Florida
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We have one vent on the roof and both tanks work so do not worry about it. No odor in the MH and both tanks drain properly. If I ever replace the grey tank valve I will replace the plumbing drain with 3 inch valve to speed up the drain process.
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Polishnurse
Schodack, NY
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An another note to the OP, that valve, some call a cheater vale, quick vent air admittance valves. etc. By your own diff. Air Admittance Valve will let air into the tank when it is being drained. It's like a check valve only lets air flow one way, it's suppose to keep gases from coming back from the tank.
The vents can be connected and it could be with "Y" or a "T Y". But pouring water down the stack may or may not provide you with the which tank question.
You would be better off hooking a vacuum cleaner exhaust to the dump valve (after you have dumped and flushed the tank). Open one dump valve at a time see which or if both tanks are releasing the air thru the roof vents. Simple enough to do. I wish you happy venting now and the years to come. Bill
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