Toilet drain runs 15 feet...

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In theory, you can get away with any value that's fewer pipe-diameters than the inverse of the slope.
Meaning less than 48 pipe-diameters for a 1/4" per foot drain. Which means you can draw a horizontal line from the lip of the trap to the vent-openning.
SInce most codes permit anything from 1/2" to 1/8" per foot, that would work out to 8' from trap to vent, in a 4" pipe, but only 6' for a 3" pipe.
In practice, you want to vent as close to the trap as you can, as long as you're far enough away to keep flushing action from lobbing shit into the the vent and plugging it. (Which I think is supposed to be around 1.5 to 3 pipe diameters.)
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"Trying to drain a toilet without a vent is like trying to pour liquid from a can with only one hole. It'll go "glug-glug" instead of flowing smoothly."
What I find interesting though is the fact that you must vent a toilet *downstream* of the fixture. Totally counter-intuitive if you think that the purpose is to provide air for draining. The slug of water has to run a few feet before it gets to the vent. My plumber told me we can't put the vent behind the toilet (upstream), but he didn't say why. (probably doesn't know the why). I'd love to know why that is.
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I think it's because when you flush the toilet, you get a sudden rush of water that would go upstream and downstream. Since there's more than just water in there, you could get debris pushed upstream, which could potentially block the vent.
By placing the vent downstream, every time you flush, you're washing away any debris left behind from the previous flush, thus keeping the vent line clear.
This is the same reason codes do not allow horizontal venting from the drain line. The vent has to come off the drain by at least 45 degrees so there's less risk of it being plugged up.
Of course, assuming you have space in the joist bay, you could vent off the drain "downstream", then run the vent back towards the wall to go up and out.
Anthony
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