Polybutylene (PB) Plumbing in Drinking Water Systems

Cruising south in 2017, I became aware that my house water pump was cycling on and off at random intervals.   I proceeded to change our water pump head/valve assembly, but that repair action attempt left the symptom unaffected.  After a period of vigorous self-denial, I had no choice but to accept that I must have had a slow leak somewhere in the house potable water system.

Sanctuary is a 1988 Taiwan-built trawler.  Many boats built in the period were fit with polybutylene (PB) plumbing and PB plumbing fittings.  PB water line “pipes” are gray in color, somewhat flexible, and the fitting are gray plastic.  Our PB system was marketed under the trade name of “Qest.”  Aboard Sanctuary, our potable water plumbing is 3/8” diameter tubing, which means 3/8” ID (inside diameter) and 1/2” OD (outside diameter).  The system fittings are, therefore, either  3/8” by 1/2” MPT (Male Pipe Thread) or 3/8” by 1/2” FPT (Female Pipe Thread).

In the 70s through early 90s, PB systems were used in many building, RV and boat applications.  When it became clear that PB fittings failed as they aged, there was a Class Action lawsuit settlement called COX v. Shell Oil et al.  to compensate PB installation failures in installations between January 1, 1978 through July 31, 1995.  The defective PB fittings were discontinued and the product removed from the market.  Today, replacement Qest fittings of “better” materials are available as replacement parts from a variety of sources, including big box stores, ACE Hdwr and many Internet vendors.

My leak was in the cold water feed to our galley and aft cabin shower, in a predictably inaccessible location.   In my search for the leak, I furthermore identified two non-leaking fittings with visible cracks in the body of the compression nut.  The leakeI had planned to replace two nuts and have some spares.  I wound up using five of those six nuts as I worked on the system.



Anyone with PB plumbing aboard should check it at least once a year for these kinds of failure.

DO NOT OVER-TIGHTEN THESE NUTS; no more than one-quarter turn past hand tight.


4 thoughts on “Polybutylene (PB) Plumbing in Drinking Water Systems

  1. Jim Stradling

    Wow! No comments yet? . . . interesting. Hey Gil – what “brand name” is now the recommended supplier for this quickie type of plumbing? I’ve used these quick connect fittings that are “push-in & use” types and that have a somewhat de-pressable collar to release the tubing and have mixed feelings about their quality but more their reliability (due to the ever-lurking Murphy syndrome). No failures to this point in time but, of course, I’ve now incurred the wrath of “those that wait for the unsuspecting”. What do you feel is a suitable time frame to maybe replace those types of fittings?

    1. gilwellbear Post author


      I have seen the fittings you’re asking about, and I have held them in my hand. I have not used them in service. There has recently been a detailed discussion of those fitting on the MTOA’s member list. The consensus there was that they are great for TEMPORARY repairs, but can be problematic in new work INTENDED TO BE PERMANENT. The consensus seems to be that they leak over time if they have any kind of lateral or bending forces on them. I would assume that the vibrations of a boat while underway could have a negative impact on their integrity.


  2. David

    Jim, I am sitting here on Peapod reading this article listening to my fw pump cycle for 1 second at random times. Time to have a look at these connectors. Groan.


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