J F
New Member
Thinking of a custom RV reno, with an "emergency overflow" on the GreyWater tank (to protect against an overfilled tank from backing-up through the shower drain (the lowest drain) and making a catastrophic mess).
My idea:
Shower drain drops straight down (about 15cm) into P-trap.
Trap arm is as short as possible (a few cm max?) whereupon it feeds directly into straight-inlet of 45-degree wye; 45-degree branch is oriented above the straight path. (The few cm of trap arm is so the wye is not classified as a "crown vent", and so that water exiting the trap keeps flowing horizontally, instead of splashing up the angled branch of the wye.)
Straight-outlet of wye feeds a relatively short horizontal run (about 30cm, to clear the shower area) at a gentle downslope, into a sanitary-tee; this sanitary-tee drops straight down into a second trap.
Second trap's arm feeds into a second sanitary-tee, which drains into the GreyWater tank, as normal.
Back at the 45-degree wye: 45-degree branch is immediately connected to a 90-degree elbow oriented towards the side; this goes to a relatively short horizontal run (about 30cm, to clear the shower area) at a gentle downslope, into another sanitary-tee; this third sanitary-tee dumps "emergency overflows" to the ground below the RV. (See theory, below.)
Theory of operation:
Normal shower use keeps both traps primed; first sanitary-tee (above inlet to second trap) prevents air locks between the traps. Second sanitary-tee prevents trap siphoning.
In case of an overfilled GreyWater tank:
When backflow rushes towards shower trap: 45-degree wye now acts as a "splitter" (instead of the usual combiner purpose for which they're made) -- First, it acts as a sort-of "wet vent", preventing an air lock between the two traps. (This ensures shower trap water is not "blown upwards" through the shower drain if/when the tank is overfilled!) Next, when the backflow water reaches the wye, the water overflows through the 45-degree branch (and out the side of the 90-degree elbow) before the backflow pressure can exceed the 15cm head height of the shower drain. (approx 1500 Pa (1.5 kPa; 15 hPa; 150 DPa; 0.0015 MPa, 0.015 bar) pressure head)
The overflow immediately goes out the side of the 90-degree elbow, to the "overflow sanitary-tee" (which was ultimately providing the inter-trap venting for this backflow), and dumps to the ground.
The vent on this "overflow sanitary-tee" of course also assists the venting provided by the inter-trap sanitary-tee, to ensure neither of the two traps are subject to siphoning.
Essentially: First trap (below shower drain) seals RV interior from the overflow & drain plumbing. Second trap prevents GreyWater tank vapours from wafting out the overflow. 45-degree wye provides the overflow path, between the two traps. Three vent stacks prevent air locks or trap siphons. 15cm drop from shower drain to first trap is for sufficient pressure head (15 hPa), to ensure backflows dump out the overflow instead of coming up through shower drain.
Rationale:
With this arrangement, the two traps stay primed with normal shower use. (If the shower drained normally into the GreyWater tank, and a completely separate emergency-overflow was fitted to the tank with its own trap, nothing would keep the overflow-trap primed!) An overflow without a trap (or a dried-out trap) seems undesirable, as GreyWater tank vapours could then waft out the bottom of the RV, closer to the nose level of humans! (compared to rooftop vents)
The big question: Will this work?
The next question: Is this compliant with all applicable codes?
The final question: If the answer is "no" to either of the previous two questions... is there a better alternative?
My idea:
Shower drain drops straight down (about 15cm) into P-trap.
Trap arm is as short as possible (a few cm max?) whereupon it feeds directly into straight-inlet of 45-degree wye; 45-degree branch is oriented above the straight path. (The few cm of trap arm is so the wye is not classified as a "crown vent", and so that water exiting the trap keeps flowing horizontally, instead of splashing up the angled branch of the wye.)
Straight-outlet of wye feeds a relatively short horizontal run (about 30cm, to clear the shower area) at a gentle downslope, into a sanitary-tee; this sanitary-tee drops straight down into a second trap.
Second trap's arm feeds into a second sanitary-tee, which drains into the GreyWater tank, as normal.
Back at the 45-degree wye: 45-degree branch is immediately connected to a 90-degree elbow oriented towards the side; this goes to a relatively short horizontal run (about 30cm, to clear the shower area) at a gentle downslope, into another sanitary-tee; this third sanitary-tee dumps "emergency overflows" to the ground below the RV. (See theory, below.)
Theory of operation:
Normal shower use keeps both traps primed; first sanitary-tee (above inlet to second trap) prevents air locks between the traps. Second sanitary-tee prevents trap siphoning.
In case of an overfilled GreyWater tank:
When backflow rushes towards shower trap: 45-degree wye now acts as a "splitter" (instead of the usual combiner purpose for which they're made) -- First, it acts as a sort-of "wet vent", preventing an air lock between the two traps. (This ensures shower trap water is not "blown upwards" through the shower drain if/when the tank is overfilled!) Next, when the backflow water reaches the wye, the water overflows through the 45-degree branch (and out the side of the 90-degree elbow) before the backflow pressure can exceed the 15cm head height of the shower drain. (approx 1500 Pa (1.5 kPa; 15 hPa; 150 DPa; 0.0015 MPa, 0.015 bar) pressure head)
The overflow immediately goes out the side of the 90-degree elbow, to the "overflow sanitary-tee" (which was ultimately providing the inter-trap venting for this backflow), and dumps to the ground.
The vent on this "overflow sanitary-tee" of course also assists the venting provided by the inter-trap sanitary-tee, to ensure neither of the two traps are subject to siphoning.
Essentially: First trap (below shower drain) seals RV interior from the overflow & drain plumbing. Second trap prevents GreyWater tank vapours from wafting out the overflow. 45-degree wye provides the overflow path, between the two traps. Three vent stacks prevent air locks or trap siphons. 15cm drop from shower drain to first trap is for sufficient pressure head (15 hPa), to ensure backflows dump out the overflow instead of coming up through shower drain.
Rationale:
With this arrangement, the two traps stay primed with normal shower use. (If the shower drained normally into the GreyWater tank, and a completely separate emergency-overflow was fitted to the tank with its own trap, nothing would keep the overflow-trap primed!) An overflow without a trap (or a dried-out trap) seems undesirable, as GreyWater tank vapours could then waft out the bottom of the RV, closer to the nose level of humans! (compared to rooftop vents)
The big question: Will this work?
The next question: Is this compliant with all applicable codes?
The final question: If the answer is "no" to either of the previous two questions... is there a better alternative?