My house is plumbed using PEX trunk-and-branch, with the main 3/4 line coming in, splitting off to 3/4" to the hot water tank, and then a 1/2" trunk that all the other branch supplies tee off from. Hot water is the same, with a 3/4" line coming from the tank and splitting to two 1/2" lines.

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The pressure changes when you are using a faucet or in the shower are very noticeable as other fixtures are used, particularly with the washing machine.

I need to do a couple of changes anyway (including adding an outside tap), am both experienced with and have all the tools for PEX, and don't mind spending a couple hundred dollars on this. How much of a difference will I notice if I:

  1. Replace the 1/2 "trunk" lines with 3/4"

  2. Replace the 1/2" trunk lines with a PEX manifold

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I'm just looking for someone's experience/anecdotal evidence that it would not be a complete waste of time/money to do this.

  • Do you want to do this because of the pressure problems you're having? I'd first check your static water pressure (get a hose bib meter for this) to see what that is, then run a faucet and check again. Ideally, you know the flow rate for faucets and showers, if they are new enough, but regardless, it'll tell you if the problem can even be fixed by moving to a manifold. – Hari Ganti Apr 10 '18 at 0:13
  • Oh, and 1/2" is rather anemic for trunk and branch. 1/2" and 3/8" are commonly used for home-run systems because they have large pressure drops over their length compared to 3/4". – Hari Ganti Apr 10 '18 at 0:15
  • @HariGanti It's mostly due to poor balancing. Pressure is fine with any single fixture, but there are noticeable changes when other taps in the house are opened, with some fixtures being affected more than others. I'm sure the 1/2" trunk is a contributing factor to this -- I am just not sure by how much, and if I'll see a difference of using a manifold vs replacing the trunk line with 3/4". I can't go fully "home run" since I'm not going to replumb the house, but I can at least do it for the 1/2" lines (hot+cold) that serve all fixtures in each bathroom. – gregmac Apr 10 '18 at 14:31
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    I'd think upping the main trunk to 3/4" would be the best use of your resources then. Home run is great because it minimizes fittings, which can leak and add developed pipe length. Check out this link about head loss (pressure loss) in PEX (uponorpro.com/~/media/extranet/files/cdam/…) – Hari Ganti Apr 10 '18 at 18:00
  • Please do check the pressures, though, if you are able. The pressure loss table can give you relative info, but to determine efficacy, you need to know your static pressure. – Hari Ganti Apr 10 '18 at 18:01

Personally I would just live with the trunk and branch. It will be very expensive to replumb the house.

  • I was not asking about replumbing the whole house, just this trunk line (which is 25' or so). It's the difference between ten's of dollars for some 3/4 PEX and fittings, or a few tens more for a couple manifolds and some 1/2 PEX and couplers. – gregmac Mar 20 '18 at 0:53
  • I guess I don't understand how to change PEX trunk and branch to manifold geometry – Jim Stewart Mar 20 '18 at 2:11
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    @JimStewart It's not necessarily very expensive, especially if the OP already has the tools for working with PEX – Hari Ganti Apr 10 '18 at 0:14

If the manifold cost is not exorbitant, I would definitely go with your plan. I can hardly believe that anyone would use a 1/2" line for a trunk. I'd be tempted to use a 1" trunk if you have the crimp tool or can borrow one--especially to feed the manifolds (and your incoming line is 1" or larger).

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