Glub-Glub sink drain "physics" question?

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I have often thought about this question, and I may have even participated in discussions about it before. It is about something that I just can't get myself to fully understand, even though I try to think of what is going on and how I think it "should" be working. So, I am looking for people who ca "splain-it" to me in a way that I can understand.
Here's the scenario:
I have a 2nd floor bathroom sink drain. The sink has a typical 1 11/4 inch drain in it and has an overflow preventer port near the top of the sink. The drain line under the sink is a 1 - 1/4 inch chrome drain line P-trap that goes back into the wall. From there it makes a 90 degree turn down and goes down about 18-inches to just below the floor line. Then it makes another 90 degree turn to be virtually horizontal along the underside of the bathroom floor and goes about 8 feet and ties into the side of the 4-inch cast iron toilet drain under the toilet. Everything from the point inside the wall behind the sink and down and across under the floor to the toilet drain is approx 1 - 1/4 inch lead pipe with a horizontal curve in it to make the trip to the 4-inch cast iron toilet drain.
The sink is often very slow to drain. I have used a 50 foot 1/4-in snake to clean it out, which always helps -- at least for a while, But doing so is not easy due to all of the turns it has to make to get through the P-trap, down into the wall, and across to the toilet 4-inch cast iron drain. To make things easier, a few days ago, I cut into the horizontal piece of the P-trap and installed a coupling that I can easily disconnect. When I disconnect that coupling, the 15 fool 1/4-snake goes in easily, makes the 90 degree turn down easily, and makes the final 90-degree to horizontal and easily goes across to the end of the line where it drains into the 4-inch cast iron toilet drain line.
So, I was able to easily clean that line out that way. And, with the snake all the way in, I even slide the coupling and P-trap parts over the beginning (back) end of the snake and run the snake up through the sink drain. Then, I reconnected everything, ran hot water in the sink, and continued cleaning the drain out by pulling the 15-foot snake out and up through the drain with the hot water flowing to clean out the crud.
That worked (again), and the drain now drains quickly.
Meanwhile, here is my question:
Even though I did this, when the sink drains, I still hear a glub-glub sound. It's the same type of sound that one hears when pouring gas out of a gas can that does not have an open air vent in the back of the can. When pouring gas out of the gas can with the air vent open, there is no glub-glub sound -- meaning the gas goes out and air goes in behind it; therefore no glub-glub.
But, why wouldn't my sink draining operate the same as pouring gas out of a gas can with the air vent open -- with no glub-glub sound? When the sink is draining, the drain is open at the top and air can get in (just like having the gas can air vent open). So, why doesn't; that eliminate the glub-glub sound when the sink is draining into an open 4-inch cast iron toilet stack?
P.S. I have the ceiling completely open below the bathroom sink and tub, so I have an easy view and easy access to all of the drain line systems that are there. And, my plan is to re-do those drain lines in a better way that I know will completely resolve the problem. But, I am still curious why I am getting the glub-glub sound now when the sink drains into this open vessel (cast iron stack) with an open air inlet at the top (meaning the sink drain itself).
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On 02/18/2016 08:28 AM, TomR wrote:

<snip< ?

But is your roof air vent blocked in anyway? leaves, dead squirrel etc
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In typed:

Thanks, but I did check that and there is no blockage in the stack that goes up through the roof. Plus, I think that if the stack had a blockage, the same glub-glub symptoms would occur when the tub drains, the toilet flushes, or the first floor kitchen sink drains out. But, none of the other fixtures have that symptom -- just the bathroom sink. And, even if the 4-inch the stack was blocked, I would think that the 1 - 1/4 inch sink drain into the 4-inch cast iron still would not cause the glub-glub because the stack would not be full of drain water -- it would just be running down the side inside the 4-inch stack.
Here is a photo of the open ceiling on the first floor looking up to the base of the bathroom toilet.
http://i67.tinypic.com/9sb3o1.jpg
The toilet sits on the floor about near the area of the center of the photo. Then the toilet drain curves down toward the bottom of the photo where it ties into the main stack that goes down through the wall.
The curved lead pipe near the center of the photo is the bathroom sink where it ties into the toilet drain. The less curved lead pipe that ties into the toilet sewer drain closer to the bottom of the picture is the tub drain. The tub drain does not do the glub-glub routine when the tub is draining.
Again, this is more of a curiosity thing, and a trying to understand the physics of the water flow thing, for me than anything else. My plan in any case is to replace the existing lead drain lines with PVC, and I will be routing them down a different wall and tying them into the sewer drain line in the basement.
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TomR wrote:

Either their is no roof vent or it is blocked. The drain opening in the sink isn't a vent...the drain fills with water, air tries to enter through the sink opening; result, glub-glub.
BTW, there was no need to modify the P-trap for your snake. It is easy to just take off the P-trap by unscrewing the two nuts that hold it to the drain and pipe into the wall.
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In typed:

I agree, but I wanted to have a straight shot into the drain line toward the wall and not have to deal with one more turn at the beginning for the remaining piece of the P-trap. That way, I knew that the only two turns that I needed to deal with were the 90 in the wall and the 90 at the bottom making the turn to the horizontal piece.
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In typed:

P.S. This worked out great and I was glad that I put the coupling in so I could do that. It made the whole snaking process a breeze.
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On Thursday, February 18, 2016 at 9:28:24 AM UTC-5, TomR wrote:

I think your sink drain pipe is going too far without a vent near it. Add a studor vent under the sink after the trap and see if that helps.
There are a bunch of rules about pipe sizes and distance without a vent but I just generally assume if I'm going more than a few feet I need a direct vent connection or a studor vent.
If you get it draining better it's also unlikely to accumulate crud and you won't need to snake it so much.
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In typed:

That is sort of what I always assumed would be the problem. But, here's the part that I don't understand:
What if, instead of a sink, I had a big funnel that was attached to a 15 foot garden hose. And, I put the other end of the garden hose into a 4-inch cast iron pipe. If I pour water into the funnel (the substitute sink), I assume that the water will just flow into the funnel, through the hose, and into the 4-inch cast iron pipe. I would not assume that the water going down would go "glub-glub" since the top of the funnel is open. I would assume tha the water just goes down on its own and there would be nothing holding it back because the funnel is open.
If I did a similar experiment, but with a closed gasoline can attached to the hose instead of the open funnel, I would assume that the water leaving the closed gasoline can would go "glub-glub" because air needs to get into the gasoline can to replace the volume of water that flows out of it.
I may not be describing this well, but that's the part of the whole scenario that I don't understand. The water is flowing out of an open sink and into an open container -- so why the need for a vent and why the glub-glub?
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You're not considering the trap. With poor or no venting the down flowing water creates a siphon effect on the trap. That's what causes the gurgling.
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In wrote:

Ahah! I think that may be the answer! At least, it is an explanation that I can picture and I can understand why it makes sense. I didn't think about the effect of the trap as opposed to just a straight hose running down into a sewer line. Thanks.
I guess that one easy way for me to test this theory would be to run a piece of flexible small diameter tubing down through and past the trap, leaving one end out above the water line in the sink, and try draining the sink. My guess is that the tubing will work as a temporary air vent and the glub-glub sound. won't happen. If I can find a piece of tubing laying around, I'll try it and post the results back here.
I also just did a Google search for "under sink air vent" (a.k.a. air admittance valve) and then I clicked on Google Images. That showed a lot of under sink AAV's. Since I already put a coupling in the horizontal part of the P-trap, I could probably change that to a Tee and install an air admittance valve there. I am not sure if they make 1 1/4 inch air admittance valves (since the P-trap pipes are 1 1/4 inch chrome), but I'll check.
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TomR wrote:

You'll need to either...
A. Pinch off the end you are not inserting until the other end is past the trap
B. Blow out the water from the tubing after it is past the trap
Otherwise, the portion within the trap will be filled with water as it passes through the trap.
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TomR posted for all of us...

Just keep sucking on it until the taste changes... 8-)
--
Tekkie

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You assume incorrectly, as explained below. If you don't believe me, try it and see.

The "glub-glub" isn't water going down. It's air coming up.

The issue is not whether air can get *in* to your garden hose through the open funnel, but whether air can get *out* of it. Before you start pouring water into the hose, the hose isn't empty, it's full of air. Then you pour water into the hose, on top of the air. The air, being far less dense than the water, will tend to rise. If there isn't a vent somewhere for it to escape through, it has to come back up through the hose and glub-glub-glub into the funnel.

Again, it's not the liquid that makes that sound, it's the air. And if you try that experiment, you'll have two separate sets of "glub-glub": air entering the can, and air leaving the hose.

So the air that's already in the pipe, below the water, can get out.
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TomR wrote:

There is also no air in the hose to be displaced.
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The p-trap on the sink is quite different from the funnel. The p-trap doesn't allow air through.
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What do you put down the sink drain that can cause clogs? Soap and water normally don't cause a clog. In 50+ years of home ownership, I've never heard of a bathroom sink clog. Grease in a kitchen sink perhaps, but not a bathroom sink.
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On Thu, 18 Feb 2016 21:00:26 -0800 (PST), snipped-for-privacy@att.net wrote:

Hair.
--
Maggie

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wrote:

+1
It usually ends up in the trap at my house. I think it is the combination of hair, toothpaste and the various creams you ladies run down the sink that does it.
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On Fri, 19 Feb 2016 00:39:14 -0500, snipped-for-privacy@aol.com wrote:

I keep a drain strainer in all my sinks to catch the hair. I've also got one of those long thin plastic things that has like serrated edges up both sides... if a clog happens, put that thing down the sink and pull. It catches the hair clog and you can often just pull the hair clog out.
I can't remember what the plastic tool is called, though.
--
Maggie

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wrote:

I have seen them on TV. I just use a piece of brazing rod with a hook on the end.
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