venting pipe for toilets

Feb 06, 2012, 10:35 AM

I am remodeling a bathroom and we have a wall that is bumped out about 12-14 inches from the outside wall. Inside that cavity is the PVC vent pipe for a downstairs bathroom that runs straight up thru the second floor and attic and out the roof. I would like to shift this pipe closer to the outside wall to give us some more room for the remodel. Does this pipe have to be 100% straight or can I use some right angle pieces and move it closer to the outside wall? will the vacuum effect for the toilet to operate properly still be ok if I do divert the pipe for this project. Thanks.

Feb 06, 2012, 03:02 PM

Nestor

Ivan:
My understanding is that plumbing codes change from locality to locality in the US so regardless of what answer you might get in here, you'd still have to check with someone locally to see if what you want to do would be allowed where you live.

Feb 06, 2012, 04:03 PM

ivanblank

Nestor, thanks for the reply. However I'm not as concerned about the codes as I am the actual question as to whether altering the configuration of the pipe (from totally straight to a couple of right angles) would affect the performance of the toilet vacuum that the vent creates.

Feb 06, 2012, 04:35 PM

Jaybee

As long as you continue the vent in the same size and keep it pointed in an uphill direction (no traps) then it will work fine as a vent pipe. As to the code issue, while I suppose there could be a really anal township somewhere in the country, it should be code compliant anywhere in the U.S.

Feb 07, 2012, 12:49 AM

Nestor

I agree with Jaybee that putting a dog leg in the vent pipe, or two, or three won't affect the performance of any plumbing fixture vented by that pvc pipe.

But, when you say:

quote:
Originally posted by ivanblank:
...whether altering the configuration of the pipe (from totally straight to a couple of right angles) would affect the performance of the toilet vacuum that the vent creates.

then I'm thinking you don't understand either vent piping or toilets, and I should do some explaining.

A vent pipe doesn't create a vaccuum. If anything, a vent pipe is there to prevent a vaccuum from arising.

You see, when water is running down a sloping drain pipe, it's heavy and flows quickly, and so it can develop quite a bit of momentum. Just in the same way as a subway will draw a wind behind it as it leaves the station and enters the tunnel, water draining down a drain pipe can create a suction behind it. That negative gauge pressure can be strong enough to suck the water out of the p-trap under a sink, shower or tub, thereby allowing sewer gas to waft up into your house through those empty traps. A vent pipe is connected to the drain piping somewhere shortly downstream of the p-trap. That vent pipe's job is to allow air to be sucked into the drain whenever there's a negative gauge pressure in the drain pipe. In that way, air rushes in from the vent piping to fill that partial vaccuum, ensuring that there isn't enough suction behind the draining water to suck the p-trap dry. And that ensures that your bathroom always smells fresh as an Irish meadow in the springtime.

So, vent pipes are there to PREVENT vaccuums from occuring in drain pipes.

A toilet, on the other hand, needs suction in it's drain pipe to work properly. Lemme explain:

A toilet is nothing more than a glorified siphon. With a siphon hose, you suck on the end of the hose to fill it with liquid, and once it's full the laws of physics take over, and the hose becomes a siphon and liquid will flow through that siphon hose as long as it's full of liquid and the outlet of the hose is at a lower elevation that the entrance to the hose.

Obviously sucking on the bottom of a toilet isn't going to fly so we have to find a different way to start a siphon in a toilet bowl. Enter the toilet tank. A toilet tank's job is to pour enough water into the toilet bowl fast enough that the bowl water overflows a "wier" molded into the back of the toilet bowl so fast that the subsequent "discharge channel" (which is the winding pipe you often see molded into porcelain toilet bowls) gets completely filled with water. The sharp bends in that discharge channel are there ONLY to slow down the flow of water through it, thereby increasing the liklihood that it will become flooded with water. If enough water pours over the wier into that discharge channel to fill that discharge channel completely, then the laws of physics again take over and transform that discharge channel into a 2 1/2 inch diameter siphon which sucks the water and everything in the water out of the toilet bowl. As long as that winding discharge channel gets filled completely with water, and nothing comes to interfere with the laws of physics, then you have Sir Isaac Newton's personal guarantee of a successful flush.

So, unlike any other plumbing fixture in your house, you WANT a strong suction behind the water draining out of a toilet to clear the bowl completely, but that kind of suction in a sink or tub would suck the water out of the p-trap. You don't have to worry about that with a toilet because the toilet tank's job is not only to flood the bowl's discharge channel, but to keep adding enough water to the bowl after the flush to refill the bowl again. And so, in a toilet, it's the water in the bowl that performs the same function as the water in a p-trap under the sink. The bowl water prevents sewer gas from wafting into your house's bathrooms through the toilet bowl just as water in the p-traps do.

So, that vent pipe is mostly there for the sink and tub and/or shower in your bathroom. The toilet doesn't need that vent pipe, and lots of homes that have a basement toilet added years after the house was built, DON'T have any sort of venting for the basement toilet at all. The toilet's drain pipe is simply connected to the house's main drain line, and that's it. No vent.

And, it's the toilet bowl's discharge channel filling up completely with water and magically transforming into a powerful siphon that gives rise to a strong flush. The vent didn't do nuthin to help that discharge channel fill with water and doesn't deserve to share in the credit for that.

So, I don't see any problem with the vent pipe not being purely vertical, but when you said the vent pipe creates a vaccuum in the toilet which determines how well the toilet flushes, I saw a real problem there.

I still think you should OK that bend in your vent piping with your plumbing inspector tho. Even if it's against the code in your area, he may have come across this problem before and know a way around it.

This message has been edited. Last edited by: Nestor,

Feb 07, 2012, 07:57 AM

ivanblank

Jaybee and Nestor, thanks so much for all the information on this subject. I wasn't that knowledgeable on the terminology of the workings of the vent pipe and I probably did misuse the term vacuum. Again, appreciate the expertise. Ivan

Feb 22, 2012, 10:33 PM

swschrad

there is a rule of thumb in air handling that four 90-degree bends is all you can use. it cuts the airflow below half.

I'm prone to thinking in terms of sweep ells on vent lines, because not only does air have to flow, but water has to flow downhill into the drain when/if it condenses from vapor to liquid in the vent. living in a freezy place, I like the idea of nothing in the way to stop it, cause a block, and cut off the airflow.

I have been told by folks who know that you never reduce a DWV line roof to street in size, but larger diameter vent penetrations are often required going through the attic and roof to insure no ice buildup.

short version: I would not "double offset" the vent line... if you move it closer to the exterior wall (but in the warm zone, please) then also move the whole pipe with a new roof cut and weatherproof.

Oct 09, 2012, 01:03 AM

Farrah White

quote:
Originally posted by Nestor:
I agree with Jaybee that putting a dog leg in the vent pipe, or two, or three won't affect the performance of any plumbing fixture vented by that pvc pipe.

But, when you say:

quote:
Originally posted by ivanblank:
...whether altering the configuration of the pipe (from totally straight to a couple of right angles) would affect the performance of the toilet vacuum that the vent creates.

then I'm thinking you don't understand either vent piping or toilets, and I should do some explaining.

A vent pipe doesn't create a vaccuum. If anything, a vent pipe is there to prevent a vaccuum from arising.

You see, when water is running down a sloping drain pipe, it's heavy and flows quickly, and so it can develop quite a bit of momentum. Just in the same way as a subway will draw a wind behind it as it leaves the station and enters the tunnel, water draining down a drain pipe can create a suction behind it. That negative gauge pressure can be strong enough to suck the water out of the p-trap under a sink, shower or tub, thereby allowing sewer gas to waft up into your house through those empty traps. A vent pipe is connected to the drain piping somewhere shortly downstream of the p-trap. That vent pipe's job is to allow air to be sucked into the drain whenever there's a negative gauge pressure in the drain pipe. In that way, air rushes in from the vent piping to fill that partial vaccuum, ensuring that there isn't enough suction behind the draining water to suck the p-trap dry. And that ensures that your bathroom always smells fresh as an Irish meadow in the springtime.

So, vent pipes are there to PREVENT vaccuums from occuring in drain pipes.

A toilet, on the other hand, needs suction in it's drain pipe to work properly. Lemme explain:

A toilet is nothing more than a glorified siphon. With a siphon hose, you suck on the end of the hose to fill it with liquid, and once it's full the laws of physics take over, and the hose becomes a siphon and liquid will flow through that siphon hose as long as it's full of liquid and the outlet of the hose is at a lower elevation that the entrance to the hose.

Obviously sucking on the bottom of a toilet isn't going to fly so we have to find a different way to start a siphon in a toilet bowl. Enter the toilet tank. A toilet tank's job is to pour enough water into the toilet bowl fast enough that the bowl water overflows a "wier" molded into the back of the toilet bowl so fast that the subsequent "discharge channel" (which is the winding pipe you often see molded into porcelain toilet bowls) gets completely filled with water. The sharp bends in that discharge channel are there ONLY to slow down the flow of water through it, thereby increasing the liklihood that it will become flooded with water. If enough water pours over the wier into that discharge channel to fill that discharge channel completely, then the laws of physics again take over and transform that discharge channel into a 2 1/2 inch diameter siphon which sucks the water and everything in the water out of the toilet bowl. As long as that winding discharge channel gets filled completely with water, and nothing comes to interfere with the laws of physics, then you have Sir Isaac Newton's personal guarantee of a successful flush.

So, unlike any other plumbing fixture in your house, you WANT a strong suction behind the water draining out of a toilet to clear the bowl completely, but that kind of suction in a sink or tub would suck the water out of the p-trap. You don't have to worry about that with a toilet because the toilet tank's job is not only to flood the bowl's discharge channel, but to keep adding enough water to the bowl after the flush to refill the bowl again. And so, in a toilet, it's the water in the bowl that performs the same function as the water in a p-trap under the sink. The bowl water prevents sewer gas from wafting into your house's bathrooms through the toilet bowl just as water in the p-traps do.

So, that vent pipe is mostly there for the sink and tub and/or shower in your bathroom. The toilet doesn't need that vent pipe, and lots of homes that have a basement toilet added years after the house was built, DON'T have any sort of venting for the basement toilet at all. The toilet's drain pipe is simply connected to the house's main drain line, and that's it. No vent.

And, it's the toilet bowl's discharge channel filling up completely with water and magically transforming into a powerful siphon that gives rise to a strong flush. The vent didn't do nuthin to help that discharge channel fill with water and doesn't deserve to share in the credit for that.

So, I don't see any problem with the vent pipe not being purely vertical, but when you said the vent pipe creates a vaccuum in the toilet which determines how well the toilet flushes, I saw a real problem there.

I still think you should OK that bend in your vent piping with your plumbing inspector tho. Even if it's against the code in your area, he may have come across this problem before and know a way around it.

Hey Nestor your information is awesome, the definition of venture pipes is well explained and I get huge knowledge from your post. Keep it up

Oct 09, 2012, 04:47 PM

CommonwealthSparky

Using 2 45 degree fittings instead of 1 90 degree ell may help with your problem. But I too see no problem with introducing a jog or two if needed for a remodel. As when you think about it air pressure should not be affected adversely with two additional turns.