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I am … very slowly … thinking about replacing the probably circa 1950's galvanized steel drain pipes in my basement with PVC. In particular, I would like to replace the drain line under the bathtub since there is a drip there.

What I am not sure about is how to route things to correctly vent the new trap when I replace the current setup. I believe my easiest (and thus "best") option is to run a vent pipe straight up through the floor to an air admittance valve (AAV) placed (a lot) more than 6" above flood level for the tub. The most natural place to run this vent pipe would be the area just in front of the tub. The plumbing access for the tub is in this area. It is at the back of a closet and still mostly unfinished and thus accessible.

However, the fellow who self-built this house ran the existing tub drain in the exact opposite direction, directly under the back of the tub since this is the shortest path to the outlet to the septic.

From the look of things, venting was never a priority for him. I am guessing he assumed the tub trap was "close enough" to the vented septic outlet that a vent wasn't needed for the tub's trap. I am less trusting in the benevolence of Murphy.

My understanding is that the vent should be placed "behind" the trap and at least 2 drain pipe diameters distant. The vent access to get above the tub's flood level is in front of the tub, the opposite direction of where it seems the trap should go. I have attached the pics below to better illustrate what I'm talking about.

So … how to vent this? Do I try to route the p-trap in front of the tub, vent it, and then turn it back around to run under the tub to the septic outlet drain? Or can I point the p-trap back under the tube, but angle the vent so that it comes back in front of the tub where I can then take it up through the floor?

It's a mystery to me.

view of drain under bathtub

view of trap directly underneath tub

Just for the heck of it …

While the picture below doesn't directly apply to my question, I added it just for context. This pic shows the main outlet to the septic. Directly above it is the WC. The PVC angling up is the 2" vent stack through the roof. The combined bathtub and kitchen drains enter on the opposite side (behind in the pic) of the cast iron. The pipe with the black tape coming from the right is for the bathroom sink drain. The guy who put this together certainly did love him some right angles. <sigh />

view of septic outlet where the only vent is connected

  • If you have an over flow port on the tub it really is vented already. – Ed Beal Mar 27 at 23:17
  • I may have this wrong, but my understanding is that the vent goes on the other side of the trap than the side the drain/overflow port is connected to. The intent being to prevent sucking out the water in the trap. No? (Apologies for the late response. My attention wandered.) – irrational John yesterday
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Why do you think the bathtub is not currently vented?

Not all traps need to be individually vented so you're not always going to see a vent if you don't know what to look for. It's a little hard to tell from your photos, but I think your bathtub is wet vented through the kitchen sink:

enter image description here

Please correct me if I've got the arraignment of your pipes wrong.

  • Your diagram is correct EXCEPT that there are no direct vents on either the KS, BT, or BS. There is only one vent for everything in the house and that is the one connected to the WC drain which is shown in my 2'nd pic in my question. Or to put it another way, every other drain in the house is "wet vented" via that one and only vent. This actually does not work. Hence my speculations about how to improve it. – irrational John yesterday
  • If there is no vent on the KS or the BS, then yes, the system is not vented correctly. The remedy would be to add the vents as shown in the sketch. As long as the pipes are routed as shown, there is no need for a direct vent to the BT. – pdd 15 hours ago
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Is there no overflow on the tub? This picture shows the drain to p-trap plumbing but if there is an overflow then there is likely more pipe under the tub that you can't see and possibly this pipe is vented.

One way to tell might to take a look at the roof over your bathroom. If there is a vent penetration inline with the wall behind the shower drain then you can be pretty sure it is dry vented out that roof vent.

  • Hello, and welcome to Home Improvement. Thanks for your answers (and your question), but note that this answer was already essentially given by @pdd. You should probably take our tour to see how better to contribute here. – Daniel Griscom Mar 27 at 21:22
  • Answers are quite different. @pdd said vented through kitchen sink. I am saying it is vented by the overflow from the tub. Mine also suggests a dry vent versus kitchen sink is a wet vent. If you are saying both answers are that it is possibly already vented, I can agree with that. – Fresh Codemonger Mar 27 at 21:33
  • This is exactly what I said in a comment after reading the first post with the schematic of the plumbing, looking again at the age of the plumbing I would bet there is a overflow vent and that vents the tub. + – Ed Beal Mar 27 at 23:20
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Both of your suggestions work however I think I would just run a T on the PVC coming down to the P trap and run my vent there it would work just fine. Believe me I'm not a plumbing guru but I do service calls for a living as a handyman and it run across a lot of plumbing experience now in situations like that you just have to do the best you can.

  • I am not sure I understand what you're suggesting. The problems I am trying to avoid is having a horizontal run of vent pipe below the flood level of tub. I wonder what would be the "proper" way to do that according to "code". FWIW, I don't much care which code it would satisfy at this point. I'm trying to get it straight in my head how to move forward with this. – irrational John Sep 4 '15 at 3:47
  • I am not a plumber but my understanding of the primary purpose of venting an individual fixture is to prevent the siphoning of another fixture on the same line. It appears your tub individually runs to another line. Why do you need to vent the tub? If the tub is siphoning a sink on that other line I am thinking you would vent the other line not the tub. The tub gets plenty of air when you open the drain. If another fixture is siphoning the tub trap then again you need to vent the other fixture or the common line. But then I am an electrician so I may be way off on this. – ArchonOSX Dec 3 '15 at 7:05
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The guy who plumbed this probably goes out for beer with the guy who plumbed our double-wide. :)

I learned the hard way, by repairing previous work here, that you can't have too many vents. Where to put them though? Your drop from the tub to the P doesn't have a lot of space. I can also appreciate not running horizontal below the "flood level." So, I'd look into what a vent is actually going to do. It's not going to help matters having it above the P (apologies if I'm misunderstanding where you wanted to put it) if the P gets clogged - if anything, water backing up will clog the vent and keep it from doing its job. I'd put the vent somewhere between the P and the main sewer drop, that would give me more flexibility to positioning, and adding whatever bends I need to route around the framing on its way up to the roof. And the T needn't be a horizontal exit - angle it up, that's what I'd do (if possible.)

I learned about the importance of venting when trying to figure out why the toilet in the previous owner's "new" master bedroom toilet would burp and gurgle whenever we ran the vanity sink in the hall bathroom. Apparently they never did. And that vanity sink? The reason I couldn't find where it emptied into the septic drain line, so I could add a vent pipe? is because they T'd it into the vent between the hall toilet and master bedroom toilet. (saving even worse horror stories for other questions.)

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