..., you may be able to use an air admittance valve here....,
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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.
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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 sink.
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 coming out?
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.
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.
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.
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 pipe.
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 stuff.
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 channel.
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:
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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:
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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 new.
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.
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.
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 an issue.
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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