I installed a tankless gas water heater about 18 months ago. I used polypro for the exhaust pipe as I understand the MA code to require.

The polypropylene exhaust pipe extends something like 12 inches out from the house, and the exposed portion of the pipe has become extremely weathered looking. It is discolored and has small cracks along the top. The regular PVC pipe for air intake next to it looks perfect condition.

I am wondering if this degradation of the exhaust pipe is something to worry about, and if anyone else has experience with this material and its life expectancy. It seems like 18 months is a ridiculously short time period to see this type of weathering, especially on such a specialty expensive material.

Any input is appreciated!

  • From a fast google search that is one of the worst plastics to be outside in the sun. It probably has some UV blockers in it to last 18 months without shade.
    – crip659
    Aug 14, 2023 at 18:56
  • My whole-house system uses PVC for both intake and exhaust, so you may want to recheck whether that's a genuine requirement or not.
    – keshlam
    Aug 14, 2023 at 19:11
  • 2
    @keshlam All flue gas vent must be accepted by the MA Plumbing Board. MA Plumbing board defines UL1738/ULCS636 listing as criteria for flue gas vent material acceptance.
    – KRW
    Aug 14, 2023 at 19:20
  • 2
    I think MTA's answer is spot on. If in doubt, ask your Local Authority Having Jurisdiction -- your town inspector -- what they would pass as acceptable.
    – keshlam
    Aug 14, 2023 at 19:23
  • Why is it necessary for the pipe to stick 12" out of the house? Mine are all fit with those horizontal termination kits, and they sit fairly flush to the house. Our code only requires that the exhaust is placed a certain distance from doors and windows.
    – AdamO
    Aug 14, 2023 at 20:01

1 Answer 1


248 Mass. Reg. 5.12 says:

Plastic Piping. The only plastic piping which may be used for venting appliances shall be CPVC, polypropylene, and other pipes which have been Product-accepted for that purpose by the Board.

So it's not true that only polypropylene is allowed. Your vent pipe could have been CPVC.

  • My reading of the above ("Product-accepted") is that the actual product sold, not just the material type, needs to be listed with the Plumbing Board. OP may have used polypro not approved for this use.
    – Armand
    Aug 14, 2023 at 23:34
  • @Armand It is a Centrotherm product approved for this use and is the only product sold for this use by the three plumbing supply houses I called in my area
    – KRW
    Aug 15, 2023 at 12:10

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