I have a dry vent in a basement. Is it true that no revent is needed
if the appliance is within 8 feet of the vent? I have a (28 gallon
capacity) sink with a 1.5" PVC drain that I thought would need a PVC
revent, but since this sink is less than 6 feet from the main (dry)
vent it could save me a little work/expense to do without. TIA
I'm pretty sure the IRC allows 5' for vent to trap length. It's
probably a good idea to have a vent on a laundry tub or slop sink
anyway. Try one of the Studor air admittance valves. The IRC allows
individual vents to terminate with a connection to an air admittance
I generally think it would be better to revent if it is only a little
As to the plumbing codes, the length of the trap arm (the drain
between the trap and the vent take off) is governed by the rule that
the vent opening shall not be below the weir of the trap. The trap
arm is sloped at the usual 2%, so this limits the trap arm length,
depending on the diameter of the trap arm. Basically, if you draw a
level line from the point in the trap at which water would first spill
over into the trap arm (the weir of the trap), it needs to hit the
vent opening rather than hit the top of the trap arm. The purpose of
this rule is to prevent the trap arm from being siphoned dry.
Having said that, apparently the two plumbing codes, the IRC
(International Residential Code) and the UPC (Uniform Plumbing Code)
come up with different maximum trap arm lengths. For a 1.5" trap and
drain, the maximum IRC length is 6', and the maximum UPC length is
3.5'. So you'll need to find out which plumbing code is operative in
One option for greater length is to upsize the trap and trap arm to 2"
(they have to be the same size). Then the IRC allows 8' for the trap
arm, and the UPC allows 5'. Of course, this would require a 2" drain.
Under the UPC, you'll have to increase the drain to 2" anyway before
it turns horizontal: a 1.5" horizontal drain (other than the trap
arm) is only good for 1 fixture unit under the UPC, and your
sink/laundry tub is a 2 fixture unit sink.
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