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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.

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  • 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
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    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
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If you increase the diameter of the branch you also increase the time it takes for you to get hot water. In my climate I also like to get ground temperature water so having large branch lines increases the amount of water I have to dump before I get rid of the water in the pipe from the supply entrance to my faucet. This also wastes heat as the branch lines will have hot/cold water and heat or cool the wall spaces - eventually heating or cooling your house.

I like the manifold arrangement but one quick thing to try would be to just plumb the washing machine into the end of the branch line. I this this would allow the other fixtures to have pressure preference over it.

The other thing I'd do is eliminate the crazy 90s if you are redoing a section. The plumbing might look neat but you loose significant flow when you make sharp turns. Really the best way is spaghetti with gradual bends.

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  • On the last part: I live in a (city) jurisdiction where our plumbing code prohibits the "spaghetti" approach, instead requiring all direction changes (in PEX or otherwise) to be made using elbow or tee fittings. (They also require the use of the rigid 20' sticks of PEX instead of the coiled stuff, apparently?) Jul 25 '20 at 16:17
  • I've heard that before from plumbers in my city but then talking to the plumbing inspector he debunked it. Did you hear this from a plumber or from the inspector - I'd give the plumbing inspections manager a call and ask. There is nothing as far as I know in the code, they can fail you on "workmanship" but that is stupid (in this case). The most likely place for pex to fail is at the connectors, throwing extra connectors in so it looks pretty but performs worse doesn't make sense. Jul 28 '20 at 4:48
  • In my case, it's not something I've heard from anyone -- it's written, black letter, plain as day, clearly, on its face, directly into the city plumbing code where I live. (I agree with your conclusion regarding the level of brain-damage involved in the requirement, I'm just far more boxed-in by it than you are thinking.) Jul 28 '20 at 4:54
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PEX is a dimensionally equivalent product. That means the OD outside diameter is equivalent to the OD of copper. The ID inside diameter is less than the ID of copper. The PEX fittings fit inside the PEX tubing acting as flow restriction and thus volume of water. Do not think that the rules of thumb for copper are equivalent for PEX. Normally a 1/2" copper line will feed three shower heads or three faucets with a static line pressure of about 50 psi. You state problems with water pressure due to plumbing. Some people experience extreme variations in shower hot water temperature and equate it to a water pressure problem. It is normal to have hot water temperature variations due to more than one person living in a home. There are several work arounds for this problem. Install a hot water recirculating system and/ or install anti scalding shower valves. Both in good working order should reduce hot water temperature variations.

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

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  • 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 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
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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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