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Discussion Starter · #1 ·
I tried to explain to a homeowner the reason for a vent. It is to equalize pressure's, ventalaite the system and protect the trap from siphonange.

I gave an example of a 2 liter pop bottle. I said,"If you fill the pop bottle up with water and then tip it upside down to drain the bottle the water will just chug out and not drain properly, if you take that same bottle and fill it up again and let it drain and as it drain's poke a hole in the opposite side of opening watch the difference in how it drains." This would be an example of a drain creating negative and the vent equalizing that pressure. Would I be wrong in that particular explanation to a homeowner? Other's seem to think so. I don't know why.
 

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Ok we all know a drain and work without a vent, it drains do to atmospheric pressure bearing down on the water surface, take a kitchen sink that has no vent, fill both sides up and release to plugs in it, what do you think will happen, does it clug out of the sink. :no: it will drain like it's suppose to, :yes: thats do to the rotation of the earth and our moon, now do the same thing only this this time seal the top of the sink air tight, know the mathematics have been altered, it will chug for air. the vent is there to prevent the siphon of the trap seal. The vent is only there to get sewer gases out and to keep the trap seal. Know my logic could be why out in space, but this makes sense to me. So the vent is only there to protect the seal of the traps, and does not make the drain, drain any better/faster.
 

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You are rigth Ron that the main purpose of vent is to maintain the trap seal, and vent out gases. BUT, they also help drains drain better. I have seen plugged up vents cause drains to slow down, they have to chug for air as you said, and this is what slows them down.
This is only the case if the drain is not of full bore like new, take a vent clogged up and knowing it is, now clean out the line to full bore, it should drain to daylight with out slow downs..
 

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Discussion Starter · #7 ·
A fixture will drain with out a vent, but do you notice the difference in draining. If a drain is installed correctly with a vent when it drains it make's a loud sucking noise. If there was no vent would it do the same. I never tried so I'm asking.

Next qustion, How does the drain get enough air on a double compartment KS if both sides are trapped?Unless you install a end outlet waste. But even at that something seems off.:blink:
 

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The pressure is the same on both side of the sink, they will drain down at a steady rate, both will keep it's same level all the way till it is gone, that sucking sound tells me the bore inside the drain is greater then the flow of water passing through the pipe, no restrictions, nothing to slow it down.
 

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Yeah, another reason I prefer to slightly oversize the drains when possible.

A modern 2" kitchen drain performs so much better than an old 1.5" drain.
2" is code for the IPC and the UPC I believe.

Below the slab the only 1 1/2" I run is for the tub arm and the tub vent. Everything else is 2" and up.
 

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Old drains where 1-1/2 here, but code requires 2" on all kitchen sink drains after the trap arm.
 

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I explain venting to HO like this. Lets say the tub is down stream from the toliet. When the water is flushed out of the toliet its like a piston driving down the pipe. Without and seperate vent off the tub drain it could suck the water out of the trap when the force of the water goes by the trap. The VTR helps break the vacuum therefore saving the trap from being sucked dry and causing sewer gas to come up from the main.
 

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In any DWV systems the conditions within the piping at any time can go from positive to negative pressure depending on the fixture draining, atmospheric pressure or even wind blowing across the vent pipe. All of these conditions are not necessaraly "equalized" by the vents. In fact these various conditions can exist in different areas of the piping at the same time. The purpose of the vent is to protect the trap seal form siphonic action which can occur for a number of reasons. Figure 906.2(2) is a good illustration of the most common siphon action that occurs. The soda pop bottle and the straw are not really good examples because that condition does not occur within a DWV system. Fixtures will drain just fine with no additional vent. They get plenty of vent from the open drain, or the atmosphere pushing down on the water in the fixture. What the vent does is relieve the compressed air column that is between the full fixture and the trap seal. When the plug is pulled this column of air compresses, forcing the water through the trap with greater velocity and causing a siphon.
I have to stpo now cause my fingers hurt.:)
 

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I would agree fully with NH Master...

The bottle is a very poor example... Although if you cut the bottom of the bottle off you could use it as a sink in your demonstration...

An unvented drain such as an s-trap can really drain quite well. infact faster than many properly vented drains because of the siphon assist...
the problem is getting the drainage to stop while there is still water in the trap!

A toilet is actually the same as an unvented drain... It sucks the water right out of the bowl and takes all kinds of stuff with it!

The only time an unvented drain will skow down or stop is when it is clogged!
 

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Discussion Starter · #16 ·
I would agree fully with NH Master...

The bottle is a very poor example... Although if you cut the bottom of the bottle off you could use it as a sink in your demonstration...

An unvented drain such as an s-trap can really drain quite well. infact faster than many properly vented drains because of the siphon assist...
the problem is getting the drainage to stop while there is still water in the trap!

A toilet is actually the same as an unvented drain... It sucks the water right out of the bowl and takes all kinds of stuff with it!

The only time an unvented drain will skow down or stop is when it is clogged!
We are not talking about s-traps on fixtures for draining. I refer to a basic sink drain wiyh a normal p trap.:yes: If toilet's did not have vent's don't you think the same thing would occer. That is the neg pressure causing the trap to siphon.
 

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An unvented p-trap becomes an s-trap when the drain turns down, or, the horizontal length allowed to a vent for the dia. of pipe used is exceeded.

Using an improper fitting such as a combo, or, Wye and street 45 to transition from horizontal to vertical on a vented system can also create an unvented s-trap. See the sketch below and note the passage of air allowed back towards where the trap would be located.

 

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Discussion Starter · #19 ·
The reason why the toilet to vent distance on a wet vent is unlimited is because a toilet does not need a vent to operate.

nhmaster, I don't disagree with your knowledge. In wi we have a trap to vent distance for water closets. It does not matter if it is connected to a dry vent or a wet vent. The way plumbing is installed I now see vary's through out the country, it doesn't make my way the wrong way it is just how it is practiced in my state. Have a blessed day.:yes:
 

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vent

I know ron is correct that a vents main purpose is to keep your trap from siphoning but it does so by replacing the air in the system as the water and or waste moves down the pipe. If your vent was clogged or you had no vent it would have to get its air from another source like siphoning a trap and getting it from the drain, but if that drain was clogged like say with hair or grease then it would not be able to get the air it needs and the drain opening is smaller then the vent opening anyway so it wouldnt drain as good. Now Im just rambling like a lunatic. Ron is right.
 
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