76/Plumbing Engineer September 2020

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loop serving the north and south sides of the buildings. For his part, Gary looked at an alternative design basis presented by the UPC in its Appendix M guidance (Water Demand Calculator version 1.4). This approach indicated peak hot water demands of 12 gpm, about the flow suggested by the WSFU. Together, we agreed that a triple trunk loop with much smaller pipe would work just fine and reduce the number of feet of piping (not including the apartment distribution twigs) by a large amount; in large part because we eliminated almost all the hot water risers. Having solved the trunk line issue, we turned attention to the apartment specific distribution design. Here, we chose 3/8" tubing home run twigs from a valved mani- fold for both hot and cold water. The client liked having individual valves for each twig and the seven stub hot/ cold "minibloc" manifold was surprisingly affordable and a truly elegant piece of engineered plastic. Gary and I refined the design and arranged a ZOOM presentation to the clients and the plumbing contractor. Our meeting was very productive and the plumber very supportive - he even suggested an even simpler design - two single loops with short branches to manifolds for each apartment. While these 6 foot branches would be " PEX and add about 5 seconds to the arrival time for each fix- ture, we were satisfied that tenants would not be unhappy. (Figure 2) We consulted closely with the client's favorite architectural engineer to prepare the specs for the design and spec changes and we were in gear - or so we thought. The PEX challenge A couple of days before installation was to start, the plumber had some second thoughts and delivered a whop- ping change order demand to the GC, arguing that the extra pipe length associated with multiple home run twigs would bust his original budget. We argued that 3/8" twigs were easy peasy to pull and would not need insulation for most of their length, unlike the pipe associated with the trunk and branch rectilinear style he preferred. Off-line discussions ensued and three days later, the plumber suggested that we congregate at the construction site to witness and document a "PEX Challenge" - an

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Figure 2