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Thread: Low pressure from PEX manifold

  1. #1

    Default Low pressure from PEX manifold

    I just installed a PEX manifold system and have significantly lower pressure than I did with my galvanized system. The copper pipe from the street is 3/4, which I transitioned to 3/4 PEX. I ran straight to the water heater and have less than three feet of pipe from the water heater to a five port manifold and also about three feet of water from the T to the water heater to a ten port cold water manifold. There were no leaks that I could find, so i don't think I'm losing pressure there.
    What could cause a loss in pressure like this?
    Thanks

  2. #2
    Forum Admin, Expert Plumber Terry's Avatar
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    3/4" PEX is smaller then copper sizing.
    How many bathrooms do you have?

  3. #3

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    Two full baths.
    In the previous system, the copper transitioned to galzanized as it passed the water heater. How is the diameter of 3/4 galvanized compared to 3/4 PEX?
    The faucets that are closer to the manifold have much better pressure than the ones furthest away if that helps ID the problem.

  4. #4
    Forum Admin, Expert Plumber Terry's Avatar
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    Water pipe sizing

    You should have 1" from the meter.

  5. #5
    Retired Defense Industry Engineer jadnashua's Avatar
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    What size is the manifold and the pipes leaving it?

    Galvanized could be quite small ID after a long time. It's hard to compare old stuff with new pipe. Could be clean, could be the opening of a soda straw or smaller.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  6. #6

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    I thought I should have one inch from the meter as well, but it is definitely 3/4. I have enough copper to run it to the water heater and just up to the manifolds. I'll try that and see if it helps.
    Thanks

  7. #7
    Moderator & Master Plumber hj's Avatar
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    Default pipe

    3/4" PEX has only slightly higher capacity than 1/2" copper. What size are the lines from the manifold to the faucets. If they are small, or overly long, that will be the most likely source of your poor pressure.

  8. #8

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    From the manifolds to the faucets it is 1/2 PEX. The shorter lines were OK, but the longer ones were terrible.
    I replaced the 3/4 PEX with 3/4 Copper and that has helped. The shorter lines now have adequate pressure. I ran 3/4 PEX on my longest run, to the kitchen, but the pressure is sill low. It's about a 30 foot span. I guess I'll have to go 3/4 copper there too.....
    I've learned a lot with this if nothing else. I plan on building a house in the next few yeas and this is valuable information.
    Thanks to all those who replied.

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