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PVC drain pipes and hot water

smitch's picture

PVC drain pipes and hot water (post #207176)

 Noticed after installing 1 1/2 PVC drain parts something I really didn't expect. Though I'd well hand-tightened the nuts, I found that when the dishwasher's hot water dumped into the disposer, the PVC pipes must have softened enough to allow the nuts to tighten a little more. I'm curious how stable these parts are over time. My hot water is at about 128 F right now. I've set it back down because I've seen suggestions that it be around 120 F. In any case, I really don't know that this issue wouldn't be seen at 120 degrees. Can anyone comment on the proper way to tighten these PVC parts or the long-term suitablity of them where hot water may be used?

Also - someone suggested using pipe joint compound on the threads and giving the nut a little nudge with a wrench. I have my doubts about both suggestions.

PVC is usually considered to (post #207176, reply #1 of 11)

PVC is usually considered to be stable at 180f or below.

Certainly there is a lot of expansion and contraction from the ~60f to 128f delta your drain was seeing from cold to hot water and that may be what loosened up the nut.

Typically you can start deforming PVC if you put it in boiling water (212f)

Old electricians know you can bend PVC conduit by sticking it in the tail pipe of the truck (idling)


It's being used I think in a (post #207176, reply #2 of 11)

It's being used I think in a very conventional way isn't it? If it's unstable and going to loosen through regular expansion and contraction, then it would seem to be problematic. Question is what is the solution or is there really not a problem. Maybe this never would leak under these circumstances going forward, but I really don't have the experience with these items to know. That's why I'm asking here.

If you get the pipe hot and (post #207176, reply #3 of 11)

If you get the pipe hot and tighten the nut, it will just get tighter when it cools. I usually put a little silicone valve lube (Dow 111 or similar) on these things and they never seem to leak. 


It's not unusual for a (post #207176, reply #4 of 11)

It's not unusual for a plastic compression joint to do this in the first few days/weeks of operation -- thermal expansion/contraction "mushes" (technical term) the plastic a bit, loosening up the joint.  Just tighten it a second time and you should be good.  (Note that this also suggests that you maybe didn't get the joint quite tight enough to begin with.)

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Not to worry--there are (post #207176, reply #5 of 11)

Not to worry--there are literally millions of kitchen sinks draining via plastic drain connections similar to yours. (BTW, the white plastic tubular traps and connector pipes you buy at HD and similar DIY places are actually not pvc in most cases. They are polypropylene.)

As for its tolerance of hot water temperatures, you need not lower from 128; you could have temperatures up to 140 or more, except that it becomes more of a scalding risk the higher you go.

Regards DanH, thanks - I (post #207176, reply #6 of 11)

Regards DanH, thanks - I gather then I have it now. Looks like now I can just leave it alone. Experience makes all the difference. 

Rdesigns: Thanks for your comments. I hope you're not telling me the home store parts are not great (or substandard). Are you saying that I just had the material name wrong, or are you saying that the plumbing supply places would have (the better) PVC materials? I needed to get this fixed in a pinch, so the home store products are immediately accessible. I had just thought that lowering the temp would mean less softening or expansion/contraction range, and hopefully more stability over time. I wonder if many people cover the threads with product or just screw them together.  No one at any home store even suggested anything. Now that I think about it, I don't know why I didn't think of using teflon tape. That to me seems like the better choice and I've never had a problem using it and it allows disassembly without the issues of other product. Anyway, it just may be unneeded.

Polypropylene is the standard (post #207176, reply #7 of 11)

Polypropylene is the standard plastic for tubular drain products. It is not inferior to PVC for that application. It can't be solvent cemented like PVC, but is great for slip-joint connections. (Most pre-packaged trap assemblies come with a PVC adapter ,one end of which can be solvent-cemented to the trap arm, and the other end is threaded to accept a slip-joint connection.)

No thread-sealant or Teflon tape is needed for a slip-joint connection; the threaded nut merely jams the slip-joint washer into its seat, thereby sealing between the male and female parts.

Old-time plumbers often use a wrap or two of "wicking string" that lays between the slip nut and the flat of the washer. It's just cotton string that provides extra friction to help keep the nut from loosening. Not really needed; just "insurance".

That's a neat tip. Thanks (post #207176, reply #8 of 11)

That's a neat tip. Thanks I'll look into that next time for sure. Really excellent knowledge here. Thanks all for lending your substantial experience and expertise.

I had this same problem (post #207176, reply #9 of 11)

My dishwasher also connects to the disposal with PVC. For years, no matter how hard the nut was tightened it would loosen up and eventually leak.  I fixed this by replacing the plastic slip joint washer with a rubber slip joint washer.  It's been 2 years since the repair and the joint no longer loosens and leaks.  In my case, I think that the nut loosened from the disposal vibration not from any hot water issue. 


Note (post #207176, reply #10 of 11)

Regardless of the temperature of your water heater, the dishwasher is probably heating the water independently. Bosch heats the water to 160 before it finishes the cycle.

  Whenever you're working (post #207176, reply #11 of 11)


Whenever you're working on plumbing connections that are mechanical in nature (not soldered or cemented) you should always go back and check the connections a few days later after initial use......things have to "settle in"  after their initial connection and use, It's also a good idea to check all soldered or glued connnections as well, to be sure everything was done correctly and there are no "pin-leaks" .... in other words "always double check your work "