In another thread (about ABS pipe), Nestork wrote:
..., you may be able to use an air admittance valve here....,
An air admittance valve has to be down stream of, and at least 4 inches
above the p-trap, but check your local plumbing code. Your local code
may be different.
I have a situation where I may want to use an air admittance valve and I
have a couple of questions about where to place it etc.
In the photo link above, the air admittance valve is below the level of the
If there is ever a blockage somewhere downstream in the sink drain line,
the sink would fill up with water (of course). But, would that mean that
water would back up and overflow out of the air admittance valve?
Also, if the sink clogs, and I use a plunger to try to plunge out the drain
line, would water back up out of the air admittance valve? I know that I
would need to seal up the overflow hole in the sink while plunging, but how
would I be able to seal up the air admittance valve to keep water from
The sink that I have has a significant slow drainage problem. I have snaked
out the drain for at least 15-feet and I don't think the slow drainage is
due to a clogged drain line. (I still need to do more work to completely
verify that). I suspect that the slow drainage is due to the fact that the
sink is too far from a vertical vent stack. So, I am thinking of installing
an air admittance valve.
My sink setup looks similar to the photo -- a sink with a P-trap that then
drops down inside the wall. It's a bathroom with tile on the bathroom side
of the wall under the sink. But, the other side of the wall is in a closet
and I can open that side of the wall up to install the air admittance valve.
Or, if the air admittance valve can be tied into the horizontal part of the
P-trap, maybe I could just put it under the sink and not hidden in a wall if
that is an option.
On Saturday, July 6, 2013 3:56:51 PM UTC-4, TomR wrote:
It's an air admittance valve, not a water out valve, ie it's like
a check valve that only allows flow in one direction, that being
air in. If it malfunctioned for some reason and got stuck, then
water could flow out.
That makes sense. I guess I was thinking that while the AAV can draw air
in, maybe the seal is not watertight so if it was full of water it might
leak inside the wall. But, I think you're probably right and it may not be
If the valve malfunctions, yes - which is why it should be 4 inches
above the maximum water level in the sink.
It is a one way valve. If it is functioning properly it will let air
in, but not out - they are referred to around here as "automatic
vents" They cannot be allowed to vent gasses OUT of the drain - only
allow air in.
It is an option if you can get it between the trap and the wall - not
the preferred location (which is above the overflow level) - but being
accessible may outweigh the hieght issue.
> drain line, the sink would fill up with water (of course). But, would
> that mean that
No, it wouldn't. Lemme explain why you need venting, and then you'll
understand why you can use an air admittance valve instead of a vent
If you've ever been in a subway station when the train leaves, you'll
feel a strong wind. That wind is caused by air being sucked into the
tunnel behind the train. An identical sort of thing happens when water
drains down a drain pipe, and the resulting partial vaccuum that
develops behind the draining water can be strong enough to suck the
water out of your p-trap. You don't want that to happen because that
water forms a physical barrier between the putrid air in the city's
sewer system and the air in your home that you breathe. An air
admittance valve (also commonly called a "Stoddard Valve", after the
company that first marketed them for this purpose) is nothing more than
a spring loaded check valve. Whenever the partial vaccuum behind the
draining water becomes strong enough that it threatens to suck the water
out of the p-trap, the Air Admittance Valve opens, allowing air into the
drain pipe behind the draining water, thereby ensuring that the water in
the p-trap won't gets sucked out.
But, because it's a CHECK valve, it only opens when there's a partial
vaccuum in the drain line, not when there's a positive pressure in the
drain pipe, which is what you'd have if your water was backing up in the
drain pipe. That is, it allows air INTO the drain pipe, but doesn't
allow anything to come OUT OF the drain pipe.
If you plunger that drain like a rabid gorilla, you could possibly
create enough suction to cause the AAV to open and allow some air in,
but any pressure inside the drain pipe will close the AAV.
If this is a bathroom sink, then I may be able to save you some time and
money. Often bathroom sinks drain slowly because the overflow drain
gets all clogged up with hair, soap scum, dead skin, and all that kinda
So, what happens is that when you pull the plug on that sink, air gets
trapped under the clog in the overflow channel. As the water level in
the sink drops, the hydrostatic pressure on that trapped air goes down
and the trapped air expands, pinching off the flow of water down the
drain. That's what causes the slow drainage; the expanding air bubble
under the clog in the overflow drain is pinching off the flow of water
down the drain. If your drain initially flows normally (for the first
few seconds, and then gradually slows down to a crawl, and then you see
a bubble come up out of the drain, or even if you don't and the drain
suddenly flows quickly again, that's typical of a clogged overflow
What you need to do is glue a 1 1/4 inch Hub Trap Adapter fitting to a 1
1/4 inch clean out fitting, like this:
Cement the above (which will tighten around your sink drain's tail
piece) to a 1 1/4 or 1 1/2 inch clean out:
Not the clean out has a threaded plastic cap which allows you to drain
the water without removing the trap adapter from the sink tail piece.
You can tighten that onto your sink's tail piece and screw the clean
out plug into the bottom so that no water comes out the bottom of your
contraption. Set a 5 gallon pail under the sink drain, put a board
across the top of the pail and rest your contraption on the board so
that it's supported from below. Now, fill your sink full of water until
the overflow spout gets filled as well. Now plunger that sink. Since
the water can't flow out the sink drain cuz of your contraption blocking
everything at that end, the water surge from the plunger goes up the
overflow channel and clears all the crap out of there. You'll be
surprised how much crap will accumulate in that overflow channel. Once
you have a whole bunch of crap in your sink that all came out of the
overflow channel, take the board away, and unscrew the plug on the clean
out fitting so that the water in the sink (and all the crap in it)
drains into your pail. Flush all that stuff down your toilet or throw
it in your back yard. When you put your sink drain back together again
(provided it doesn't leak) it'll drain just like it did when it was
I find that I regularily have to clean out the overflow channels on my
bathroom sinks, especially when I have female tenants living in the
apartment because of their long hair.
> wall if
Watch the way the sink drains. If it slows to a crawl, and then you see
a bubble and it speeds up again, it's a clogged overflow drain, not the
lack of an AAV. An AAV won't help a sink drain; it'll just prevent the
water in the p-trap from getting sucked out. But, the sink won't drain
any better. Clearing a clogged bathroom sink overflow drain will
definitely make the sink drain better because of what happens when air
is trapped in the drain as described above.
> ;3089742']Interesting idea.
Do that before you start adding an air admittance valve.
If you're uncomfortable with the idea of taking your bathroom sink's
P-trap apart, cuz of fear it might leak when you go to put it back
together, clear your drain another way:
Go to your local auto wrecker and liberate a car speedometer cable for
$1. Take the cable core out of it, cut to length, flaring the wires on
the end a bit to scrape the inside of the over flow channel, put that in
a cordless drill, and use it as a poor man's motorized snake to clear
your clogged up over flow channel.
Snake with the sink full so that even the overflow channel is submerged
if you can get to that point. Once you clear the blockage, run water in
the sink to keep water flowing down the overflow channel while you
stick a bent wire into the openings in your drain where the water from
the overflow channel flows into.
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