God, please let my PVC-to-ABS issue be minor.

April 26, 2010 10:16 PM

ABS, PVC, a six-inch drain and me -- am I as frakked as it appears I am?

I've got a project that I'm working on that requires a 6" drain pipe, which is four feet deep in the ground (at or below the frost point). I've got ABS (black) pipe running for virtually all of it, but because of a local-hardware-store-Sunday-closure/desperation to finish, I've got PVC-DWV joints in several places, glued with ABS glue, to said ABS pipes. Hindsight being 20/20 (since the Menards piping guy said it was "fine" to use PVC/ABS interchangeably for drain pipe), am I in serious trouble with my non-abs-pvc-transition-cement gluing as I (potentially) think I might be? Do I REALLY have to dig it all up, and redo my joints?

posted by liquado to home & garden (12 answers total)

As long as the pieces fit snugly together before you joined them, and you used enough glue, and didn't try to move the joint once the glue had started to set - that is, provided you used the same procedure for making your hybrid joints that would have made a good PVC to PVC or ABS to ABS joint - you're fine.

ABS and PVC cements are both solvent glues based on MEK, and they work by dissolving the surface of the plastic into the glue; once the solvent has evaporated, the plastic surfaces are effectively welded together. So your hybrid joints are now welded together with a plastic "alloy" that's a mixture of ABS and PVC; slightly more ABS than PVC because you used an ABS glue.

If you have any doubt at all, glue an offcut of ABS pipe into a PVC joint, let the glue set for a few hours, then try to crack the joint apart. Betcha you break the plastic before the joint fails.
posted by flabdablet at 10:37 PM on April 26, 2010

This, may be where being in a hurry and listening to advice from someone at Menards/Home D/Lowe's raises up to bite you in your hindquarters. Is this your main line coming out of your house? If it is then I would seriously contemplate going over to your local plumbing supply place, swallowing your pride, and asking a plumber what he/she thinks about what you did, and how. They will perhaps do one of a few things;

1)Blink, shake their head then walk away.

2)Stare at you for a long moment, sigh and then say something like "Well, If I were you, I'd go start digging"

3)Blink, then say something like "well, It may be ok, but I wouldn't do it that way." After which you should run home and start digging.

Seriously, anything around the home that involves 6 inch drain pipe has a lot of work to do, and should probably be done right once. If you can ask 3 plumbers what they think and at least two say you "might" be ok, I would still go back and fix it the right way. Owning a home whose PO apparently was the president of DIY half-assed repair inc., I can say with much conviction that doing stuff half-assed costs time, money and aggravation. Just my 2 pence, mind you.
posted by chosemerveilleux at 11:00 PM on April 26, 2010

Look, let's get realistic here. We're talking about PVC fittings on ABS pipe, yes? Let me show you why you don't need to dig them up again.

The issue with hybrid joints is that ABS and PVC have different thermal expansion coefficients. So let's look those up. For ABS: 74 x 10^-6 per °C; for PVC: 50 x 10^-6 per °C. So in a 6 inch (150mm) joint, over the entire range from freezing to boiling (0 to 100°C) we're looking at a difference of (74 - 50) x 10^-6 x 100 * 150 = 0.0024 = 0.36mm.

But these joints are buried four foot deep in the ground, and they're below the frost line, so they're never going to get as cold as 0°C. And even if you ran this drain 100% full of boiling water for long enough to warm the pipe and its immediate surrounds to a temperature approaching 100°C (which you won't) then you would find that both ABS and unplasticized PVC will soften enough that 0.36mm of movement will flex that joint but not crack it. Bear in mind that it's the ABS pipe inside the PVC fitting that will be trying to expand fastest, so the joint will actually tighten while heated; and that the cooling is going to come from the earth around the pipe, so the PVC will cool before the ABS does, and squeeze the joint again.

Once again, you can test this. Make up a joint using the same pipe, fittings and glue as you used underground, boil it up in a big pot on your stove, then chuck it in the freezer and see what happens to it. My confident prediction: nothing.

And even if you did manage to achieve a small joint failure in an underground joint: so what? The pipes are not going to move; they're buried. And this is a drain, not a pressure line, and a sub-millimetre crack is going to be blocked with assorted gunk and biofilms before it's leaked half a litre of water.

I am not a plumber, but plumbers are in the family and I have observed many of them at close quarters, and I can assure you that many of the nominally code-conformant drainage joints that have been operating successfully under your city for many years now are much, much shoddier than I expect your typical handyman-in-no-particular hurry hybrids are.

And even if you did have a problem, the solution - digging it up and redoing the joints - is going to be made no harder by a leak then than it is now. So why even contemplate fixing it before it breaks?
posted by flabdablet at 12:03 AM on April 27, 2010 [6 favorites]

Edit: ...a difference of (74 - 50) x 10^-6 per °C x 100°C x 150mm = 0.36mm.
posted by flabdablet at 12:06 AM on April 27, 2010

The cement is basically used to weld two drain pipes together so they don't come apart from shifting. The weld isn't really there to prevent water from escaping - a properly laid drain pipe will not leak. Obviously in a pressure situation you need to provide a water tight or gas tight seal but otherwise water flows downhill. Your drain pipes should interlock bell end up.
posted by JJ86 at 6:20 AM on April 27, 2010

Thanks for the feedback, folks -- this drain is for an outdoor project that is waaaay overengineered (6-inch pipe is insane, but that's what is recommended), will drain only unheated water in the summertime and which will not be running any water in the winter at all. I'm going to do as chosemerveilleux suggested, and check with a plumber for the final word before I consider it a closed matter, however, since I plan on pouring concrete above it, which would make a critical failure down the road worse.
posted by liquado at 7:31 AM on April 27, 2010

Is this going to be inspected by the local jurisdiction? Because regardless of thermodynamics and materials science they can make you dig that sucker up. Just saying.

Wait. I suppose if it's buried, it's not being inspected, is it? How the building dep't would regard it is still a point to ponder though.
posted by werkzeuger at 7:34 AM on April 27, 2010

You don't even need to glue them at all. Clay pipe has been used for thousands of years for sanitary drainage. Once the pipe is in the ground, it takes incredible amounts of soil expansion to pull the pipe apart. That expansion would have to occur over the entire length of the pipe to pull a joint apart. Possible in high clay soils, but not likely.

Plastic pipe can shift horizontally through soil in pressurized water applications where dynamic forces involving the mass of the water inside the pipe will, over time, cause pipe to migrate in incremental amounts. Drain line, at 6", will not carry that great of a mass of water relative to the diameter of the pipe. On rare occasions it might carry 90-95% capacity, but the pipe is not subject to the same hydraulic forces that pressurized line is. Think of how a capped garden hose will shift and twist when you open the valve and pressurize it - drain line won't ever be under that kind of pressure.

All that being said, I would use a rubber connector with stainless steel clamps to connect two pipes of dissimilar material. Don't glue ABS to PVC.
posted by Xoebe at 8:21 AM on April 27, 2010

I will bet chosemerveilleux a dollar that your plumber will say something to the effect of (4) "Well it isn't code and it wouldn't pass inspection, but I sure as hell wouldn't be digging it up again."

Rubber connectors are indeed code for this kind of job, but the plastic pipe actually flows and deforms under the clamp pressure over time, which effectively loosens the clamps. A hybrid glue joint might suffer small cracks under extreme heat stress, but even above ground it will be much stronger ten years down the line than a Fernco.
posted by flabdablet at 9:02 AM on April 27, 2010

Just to close out the thread, we made the decision to pull up the pipework -- and, it was the right thing to do. Apparently someone (I'm not going to say who) actually used PVC glue, which does virtually nothing to hold ABS together. It literally fell apart when we pulled it up.

Thank goodness, our volunteer project manager is back on the case, and will hopefully save me from further stupid mistakes. Thanks for all your advice, folks.
posted by liquado at 12:26 PM on May 4, 2010

Oh, and to summarize what I found out about PVC and ABS pipework:

-ABS is really only used in Canada; PVC is used for virtually all DWV (Drain, Wast, Vent) piping in the U.S.

-ABS glue will maybe hold PVC and ABS together (it's usually yellow), but it's not recommended at all. Clean the pipe with a rag or cleaner before gluing.

-PVC glue only holds PVC pipe together, and you need a primer used in conjunction before you glue your pieces.

-Transition glue can be used when you *have* to glue the two together (PVC to ABS), but it's not normally recommend, except when there isn't an alternative.

-Most importantly, when doing pipe work, TAKE YOUR TIME. Buy more fittings than you need (you can always take them back). Measure twice (at least), dry fit all your parts, and do it right the first time, to avoid all the pitfalls and hazards I ran into on this project. If we had taken more time and not rushed the job, we wouldn't have had the nightmares that we did.
posted by liquado at 12:42 PM on May 4, 2010

chosemerveilleux, I owe you a dollar.
posted by flabdablet at 11:36 PM on May 4, 2010