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Posted by jasperdog (My Page) on
Wed, Dec 12, 12 at 23:06

We designed our house plans using a manifold and Pex for the plumbing. Now our contractor is trying to talk us out of using the manifold. Is there any rational reason not to use one?

Follow-Up Postings:

RE: Manifold

ISTM that using a manifold and with each water line having an isolation valve there may cost a bit more on initial construction but make later maintenance tasks a lot easier.

RE: Manifold

If your contractor is trying to talk you out of it, what are the reasons that he's stating?

RE: Manifold

I think his rational is cost - cost for the manifold, cost for the additional Pex. My main objective is to have quiet waterlines without a significant loss of pressure or change in water temperature when multiple water-using tasks are occuring at the same time.

RE: Manifold

Trunk and branch are cheaper to install as they use less pipe, but they also waste more hot water during use. Home-run manifold systems use more pipe so are more expensive to install.

RE: Manifold

So far, all the "pro" arguments that have been presented are wrong.......

LOUD PIPES-Wrong-If the lines are sized correctly regardless of how many fixtures that are in use at the same time, the flow in the pipe is silent. If you can hear the flow in a pipe it is telling you that the line is undersized and the velocity of flow is exceeding the code mandated speeds OF 8" ft/sec in copper & 12 ft/sec in plastic. When the velocity of flow exceeds the code mandated speed it also increases the internal wall friction and dramatically increases the risk of pipe failure due to internal pipe wall errosion.

TEMPERATURE LOSS-Wrong- If the lines are sized correctly there should be no significant loss in temperature, regardless of how many fixtures on the line are in use at the same time.

TRUNK & BRANCH WASTE MORE WATER- Wrong again- with a manifold system regardless of which fixture you use you have to wait for the water to go from the water heater to the manifold then on to the fixture. With a Trunk & Branch system once hot water has arrived at any single fixture the delay to the next fixture is only the distance from that fixture to the tap on the trunk.

With a Trunk & Branch system you can install a recirc loop and have nearly instant hot water at all fixtures, while conserving water.

Home run systems use dozens of times more pipe and in many areas you can get a copper Trunk & Branch system installed for the same price as a PEX home run system.

RE: Manifold

Lazypup, you make a compelling argument for trunk and branch out of copper - BUT -

your whole argument hinges on:

"If the lines are sized correctly regardless of how many fixtures" -and-

"If the lines are sized correctly there SHOULD be no significant loss.."

Those are big "IFs"

and your argument about water waste can't possibly be accurate as EVERY system is different, and you cannot possibly know how far the water heater is from the manifold. So , it is totally dependent on design and execution.

Out in the real world , I frequently see hang and bangers putting in plumbing systems these days, so it is a lot harder for the homeowner to "get a correctly sized or designed system" because these guys can't or don't design a competent system.

PEX is more suitable and forgiving to the less skilled trades we are seeing on jobsites today. Doesn't make it better, just has a better chance of success given market conditions.

I've worked on projects from FL to MI, Los Angeles to Washington D.C. and I have yet to see a copper system that is the same price as a PEX - trunk or home run doesn't matter - even when the plumber is marking up the PEX system because he can. Now I have to preface that with the type of projects we do, big. I cannot accurately say what the average 3/2 or 4/3 ranch house would price out at, but I suspect the financials would be the same.

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