Just moved in a couple weeks ago, and drain in kitchen is slow.....
I have plunged, snaked and Drano-ed...and it is still a slow drain.
I think at least PART of the problem, is that I just noticed that the sink
has no drain vent!
A problem, no?
Now my question is, can I tie into the existing vent, or do I need a
seperate one? If I need to install a new one, does it necessarily NEED to be
vented through the roof? Or is there some alternate method....thanks!
This works just fine. The pipe sizes are chosen large enough that
air can still circulate and neutralize pressure imbalances which might
siphon the trap.
Note that this isn't allowed *everywhere* and the rules changed over
time as well. Allowed in Podunk in the Midwest but not in NYC or LA.
Anyway, that design is not the source of your trouble.
Could be clogged in the drain leading to outside or the branch
to the septic could be clogged or.......
I bought a house that didn't have a vent in the entire system. The plumber I
hired put in a main waste vent and then because the kitchen sink is 20 feet
from the main stack, he installed an air admittance valve under the sink. As
you pointed out, venting requirements are dependent on pipe size and
distance to the nearest vent. The air admittance valve is a good way around
some problems but they are not permitted in some localities.
Where in the US exactly is it permitted to have a sink drain line
going into a septic system with no vent? Regardless of the pipe size,
if sewer gas builds up, the only place for it to go is straight into
the kitchen by pushing the water right out of the trap. And it
certainly sounds to me that the incorrect plumbing is very likely the
cause of his sink problem.
This was accepted practice in many places for a long time (Ohio for
For kitchen drains (as an example), the drain was made 2" for a 1 1/2"
trap. That size trap cannot pass enough water to completely fill the
2" pipe and thus air CAN circulate to balance pressures. The guide
was that a max of 1" (water column) disturbance in a trap was
acceptable. Ref. 1959 Natl Plumbing Code.
For another example, floor drain traps were never vented in the past.
Here again, reliance was placed on the inability of maximum flow to
completely fill the main house drain. Basement showers and toilets were
permitted without vent in the same way.
"Island" kitchen sinks are another example. Here, no vent to the
roof is possible at all. So, a loop of 2" pipe (under the sink)
is permitted to be connected downstream of the trap to act as a
Today, the rules in many places have gotten stricter, requiring vents
where none were used in years past. But the fact remains that these
"ventless" connections worked well and were allowed by code.
How large is the drain pipe?
That can work if the pipe is sized adequately, but for all you know this
was installed by a do-it-yourself plumber with no concern about air
One simple test for whether this is your problem -- slide a length of
1/2" vinyl tube down through the trap so that it provides a small vent up
through the drain. If it flows better despite having the tube taking up
so much of the drain pipe, then venting it ought to help your problem.
firstname.lastname@example.org is Joshua Putnam
Only item in this house that doesn't have a vent connection is the bath tub;
did that because we understood that was OK less than six feet from the main
It has worked finefor some 30+ years, although occasionaly draining a full
tub sometimes appears to be slow. No problems though, possibly because the
bathtub has an overflow that allows some air to gurgle (i.e. vent) down the
pipe with the bathwater.
To avoid many separate vent pipes through the roof we tied all our
individual plumbing vents back to the main stack in the roof space; it
required slightly more pipe but it was glued 1.5 inch ABS; so we have only
one 3 inch pipe projecting through the roof to keep caulked.
It my understanding that you could vent out through say the eave of the
house provided that is competely above the height of the water levels in all
fixtures/sinks in the house. But a more knowledgeable plumbing expert than
I should comment on that.
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