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tdub4 · Aug 1, 2009, 09:02 PM

Today I replaced a portion of the condensate drain pipe on my AC in the attic. It had been clogged and was dripping water into the overflow pan and needed to be unclogged. I removed a portion of the drain that included the p-trap to ensure I got a clear clean line and I added a piece that will allow me to pour bleach in the pipe annually to avoid this in the future (it didn't have this before). It did solve the clogging problem. However I have noticed the PVC is now sweating on the outside.

A couple of points for clarity:
1) I don't know if it had done this before because I never had looked at it until now, but there is no visible water marks or damage on the floor (we've been here 10 years)
2) It isn't sweating a too much but enough to occasionally drip on to the floor in the attic - which over time...
3) We have 2 units in the attic (one for up and one for down) and the other one seemed to have a very slight (and I mean very slight) amount of condensation on the outside but not like the one I had replaced (you can visibly see the beads of condensation on the pipe)
4) We are in Texas so the attic is very hot and it has been humid lately (although the other one isn't doing it)
5) I used 3/4" Schedule 40 PVC

So - any thoughts on what it could be?
Is it common for PVC to sweat or is there something wrong?
Could I use some kind of insulation around the PVC to keep it from sweating?

Any ideas would be appreciated. Thank you.

hvac1000 · Aug 2, 2009, 03:13 AM

First make sure it is sweating not leaking from a joint not glued properly.

The condensate is rather cold water so when it hits the pipe the pipe will be come cool and it will be possible to create the situation you are talking about. Now I will say that PVC pipe acts as a insulator but it is possible for it to sweat as you call it.
When warm moist air hits a cold pipe the pipe will collect moisture that condenses out of the moist air so you could insulate the pipe with pipe insulation covers or Rubatex/Armaflex or other types of insulation. The pipe you have is probably 3/4 PVC so you will need to use 7/8 OD refrigeration insulation/Armaflex/Rubatex or 3/4 inch copper pipe size pipe insulation. These items are available at Lowes/Home Depot in my area.

tdub4 · Aug 2, 2009, 06:32 AM

Thanks hvac1000.

I am 99% sure it is sweating because it is "beaded" water as opposed to just being wet along the run of pipe. I have looked for a leak between joints and don't seem to see anything... as a matter of fact most of the joints seem dry.

That being said, is there some tye of sealant I could put around the joints to ensure that isnt the problem (just for 100% certainty)? Or would I need to cut those pieces out and do it again? (although I would need to cut out the whole replacement as I am not sure which one it would be)

I will do the insulation as well - I am just thrown off by the fact the other ac unit isn't having this issue as well and they are located in the same attic. I did run the "fixed" one colder yesterday (and longer) to generate enough water to make sure my clog was gone as well - so I am sure that played a factor as well.

hvac1000 · Aug 2, 2009, 10:30 AM

No extra sealant needed. The glue used for the PVC pipe actually melts the pieces together and the only way you get a leak is by not cleaning the pipe or using enough glue. You are also supposed to twist the pipe in the fitting a bit but many never do that. Use the insulation and your problem should go away.