What is MPT PVC and why is it so hard to find parts?
I am trying to solve a plumbing problem, and I’d be greatful for any help.
I am replacing an “S” trap on a bathroom sink for a friend. He installed the drain with home depot mpt pvc – the cheap, thin stuff that comes in kits.
I’ve got three problems:
1. Venting: The drain pipe is in the floor, thus the “S” trap. There is a roof vent in the drain line, about five feet from the sink (and after the WC), which I assume will not prevent siphoning at the sink drain. So I want to install a vent. I can’t put in a proper roof vent. My solution is to install an AAV. First question: Is this a reasonable solution (as long as it is permitted by the code)? Second question: Does the AAV need to be in the drain line prior to or after the trap? Or does it matter?
2. PVC sizes: I am confused by the PVC choices, and their compatibility (or lack of) with each other and the exisitng pipe. The existing sink drain is 1 1/4″, with pipes labelled 1 1/2″ (the first fitting goes from the 1 1/4″ to the 1 1/2″) so I bought a bunch of fittings labelled 1 1/2″ and 1 1/4″, and some short sections of PVC labeled 1 1/2″ and 1 1/4″. Of course, I can’t get them to work with each other. The 1 1/4″ pipe I bought is a differrent size than the existing 1 1/4″ drain pipe. The 1 1/4″ fittings I bought don’t work with the 1 1/4″ exisiting pipe. Some of the fittings don’t work with the pipe I bought, even if labelled the same size. So, I figured I have two or three different standards of pipe, but I can’t find any information about this online, and there is no indication at home depot that the pipes and fittings are incompatible. Some fittings aren’t even labelled with a standard, only with a size. Some have a “G” on one end (if that end is a compression fitting – which won’t work with PVC of the same labelled size). Some of the pipes are labelled “Schedule 40” and “MPT.” I figured “MPT” means Male Pipe Thread, but that makes no sense, because this is pipe, not thread. I also bought an “S” trap kit, labelled 1 1/2″ (Dearborn Brass), but with no standard on the package, and the pipe included has no size label, only “PVC tubular ASTM F409”. Again, I can’t find info on what that means. I did search here, and read FHB’s artcile on PVC – it doesn’t go into standards.
In addition, home depot sells a very limited selection of the cheap, packaged fittings, in a section separate from the “real” PVC. Why? Is the cheap stuff to be avoided?
All I want to do is get from the brass pipe in the floor (male thread 1 1/2″ inside, 1 3/4″ outside) to the sink drain (1 1/4″ outside), with an “S” trap and an AAV in between, using techiques and material which will last.
So: What is “Schedule 40”, “MPT PVC”, and “ASTM F409”?
Should I avoid the packaged, cheap PVC?
3. The “S” trap: The “S” trap kits have a joint in the “S”, so that the trap water will be behind the joint at all times. Isn’t this a bad design? What is the best way to build an “S” trap?
I know this is asking a lot, and some will advise that I call the plumber. But I am only looking to understand this stuff, do it right, and make it work. None of the tecniques is beyond a DIYer. I just need a little info, and am very grateful for any help.
Thanks,
South Side
Replies
There is a specific fitting you need that goes from those 1 1/4" or 1 1/2" thin compression tubes to the thicker PVC or ABS pipe (ABS is the black plastic). Look for that fitting by the PVC/ABS fittings.
Personally I like using the cheap plastic parts in the package for the P trap, then have that feed into the PVC.
In your case, I would do it like this from the floor:
ABS up from the hole in the floor with a sweep T, vertical is capped with the AAV.
Sweep is connected to a short length of ABS, capped off with the ABS to compresion fitting.
From here, you use the bagged P trap kit back up to the sink drain.
Maybe I didn't read closely enough and don't understand the issue, but it sounds like you need a threaded trap adapter. You'll never find that at a box store, so you either go to a plumbing wholesaler and persuade them to sell you one part, or you get a threaded plastic fitting that lets you screw to the brass and glue to the other side, and then glue in a plastic trap adapter. Hell, they may not even have the screw/glue part at box stores.
You can get those at the Big boxes - I did!
There is 1-1/4 or 1-1/2" tube
There is 1-1/4 or 1-1/2" tube (thin wall) and the same size pipes that are thick walled. The cheap stuff is usually thin wall tubes which may not fit the way you want to the thick wall pipes. The thick wall is usually called schedule 40 which means how it may be used: drinking water and its pressures, drain pipe and how it needs to be supported, etc.
Any drain pipe that is installed behind the sheetrock should be schedule 40 thick wall pipe. You can use thin pipe for short sections of drain under the sink, trap, etc. Thin walls will not fit into schedule 40 for glueing. Thin wall tube is connected to the main drain with adopters that use plastic or rubber washers and nuts.
You should search for illustration of how the vent is installed over the trap. Main idea is to get rid of S and create a T on its side. If your town inspector accepts the vent, you should have no problem. There was recent article about it on this magazine, but any library should have book on it, or google images.
Take a photo of your situation and take it to Homedepot. This is not complicated work and you can even use flexible coupling (rubber) for joints. Rubber coupling may even work for unusual pipes because rubber will stretch. Someone at the HD should be able to tell you what pipes you need, instead of you buying whole cart of stuff and trying to see what fits.
The schedule 40 pipes are measured from inside side of the wall. The thin wall tubes may be inside or outside, but usually they are made to fit into the schedule 40 loosely.
Going from S to T may not give you enough room for trap to sit directly under the sink drain, and here you need to be little creative but you can use flexible drain line (looks like accordion) to compensate. The flex tube may clog more easily.
Good luck.
The terminology you need in order sort this all out is to understand the difference between "pipe" and "tubular" sizes. Example: 1-1/2" TUBULAR products (the kind that come in packages with the p-trap and j-bend included) are meant to be able to slip inside a 1-1/2" PIPE. The seal between them is made by use of a SLIP JOINT, which consists of the slip nut and washer that comes in the packaged product. You can also connect 1-1/4 TUBULAR into 1-1/2" PIPE by simply using a 1-1/2 x 1-1/4 washer with a 1-1/2" slip nut.
As others have said, you can do this by adapting to plastic at the existing metal pipe and using the right fittings to connect a p-trap and an AAV.
The adapter fitting you need to start is a 1-1/2 " FPT/PVC slip fitting--which means a Female Pipe Thread x glue-in female PVC adapter. You screw the female threads onto the existing male pipe, and glue PVC pipe into the female slip end.
From there, on up to a sanitary tee, the branch of which will become the trap arm for the p-trap, and the top of which will extend 4 or 5 inches up to recieve the AAV.
At the branch of the sanitary tee, you install a "trap adapter", which goes from a glued connection on one end to a male thread on the other. On the male thread, you will use a slip joint connection to seal in the tubular end of the j-bend that is part of the packaged p-trap assembly. The end of the tubular j-bend inserts an inch or two into the trap adapter.
For the record, tubular products, even though they are white like PVC, are not acually PVC--they are usually polypropelene, and cannot be glued with PVC cement. However, the white trap adapter that is often included in the package is PVC.