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I am redoing the plumbing for a small studio that only needs (currently) 5 hot and cold lines to go to faucets and fixtures.

I am thinking of a manifold that looks like the following diagram: PEX Manifold Distribution

This question asks about manifold loops, which most answers state is typically for a hot water recirculation loop. The last answer suggests less head loss when connecting both sides of a manifold to a supply. How much of a difference would that make? According to this table, you would lose about 8psi over 100ft of 1/2" PEX at a typical shower temperature and flow.

Also, how would a recirculation pump through the manifold help if the branch plumbing to a fixture doesn't have a return to the manifold? It seems like the manifold temperature might be constant, but if you have a long run to the fixture, you're still waiting on hot water.

What benefit, then, does a manifold offer?

EDIT: Added picture

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    The manifold is larger diameter pipe than the spurs to each fixture. The manifold is short so the total volume in the manifold is small and it fills up with hot water in a short time even if it is not a circulating loop. You can use say 3/4" diameter for the manifold and 3/8" diameter for the hot water spur to each point of use. The water emitted from the fixture is hot in a shorter time than if the spur was 1/2" diameter. The large diameter manifold means less pressure drop at one point of use when another one is turned on.
    – Jim Stewart
    Commented Feb 1, 2017 at 0:51
  • Ok, that makes more sense. I suppose my misunderstanding was in that people make it sound like the manifold, with a recirculation loop provides, instant hot water, whereas you are saying it just means there's less cold water to move until the hot manifold water gets to the fixture.
    – Hari
    Commented Feb 1, 2017 at 6:50
  • I could also see how, if you had daisy-chained fixtures a long distance, the pressure drop on an earlier fixture, when it's on, would disrupt downstream fixtures, and how a manifold can help. It seems more useful for larger systems then.
    – Hari
    Commented Feb 1, 2017 at 6:52
  • The exact diameter of the tubing in the manifold and the diameter in the spurs is important for proper function in a given situation. The values I gave are just examples picked out of the air.
    – Jim Stewart
    Commented Feb 1, 2017 at 9:04
  • One other benefit of a manifold with individual spurs to the points of use is that cut-off valves are usually located just past the manifold. So if there is a leak in a spur (say in a wall) you can cut off the water to that line and the rest of the plumbing can be used. The traditional plumbing system doesn't allow this. Code may or may not require another cut-off at the point of use.
    – Jim Stewart
    Commented Feb 1, 2017 at 9:09

2 Answers 2

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The manifold is larger diameter pipe than the spurs to each fixture. The manifold is short so the total volume in the manifold is small and it fills up with hot water in a short time even if it is not a circulating loop. You can use say 3/4" diameter for the manifold and 3/8" diameter for the hot water spur to each point of use. The water emitted from the fixture is hot in a shorter time than if the spur was 1/2" diameter. The large diameter manifold means less pressure drop at one point of use when another one is turned on. (The exact diameter of the tubing in the manifold and the diameter in the spurs is important for proper function in a given situation. The values I gave are just examples.)

Google: youtube PEX plumbing with manifold

One other benefit of a manifold with individual spurs to the points of use is that cut-off valves are usually located just past the manifold. So if there is a leak in a spur (say in a wall) you can cut off the water to that line and the rest of the plumbing can be used. The traditional plumbing system doesn't allow this. Code may or may not require another cut-off at the point of use.

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implicit is also the fact that using the manifold results in less hidden fittings. That is, it's a straight run ("homerun") from the manifold to the point of use - no hidden tees, etc. There are advantages and disadvantages. The following document is a great overview of options for installing pex:

https://www.huduser.gov/portal/publications/pex_design_guide.pdf

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  • As I read the PEX design guide above the terminology "home run" is what could be called a central manifold, and the terminology "remote manifolds" means multiple manifolds which are remote from the water heater and feed distribution lines to grouped points of use. The traditional design in long use is called "trunk and branch".
    – Jim Stewart
    Commented Feb 3, 2017 at 12:24
  • Yes. Although the remote manifolds tend to be the smaller single piece ones (eg, < 10 outlets), as opposed to the larger ones that might have integrated shutoff valves. For example, I used a remote manifold off a 3/4" trunk to run the multiple 1/2" cold lines to service a single bathroom.
    – aaron
    Commented Feb 3, 2017 at 15:01
  • And from the HUD document (or somewhere) since the remote manifolds do not have integral valves they can be placed in an inaccessible location, such as inside a wall with no access panel. All valves have to be accessible.
    – Jim Stewart
    Commented Feb 3, 2017 at 15:12
  • You used 1/2" for the cold; what about the hot? Could one use 3/8" for hot supply to fixtures with 1/2" cold? Or did you use point of use hot water heaters?
    – Jim Stewart
    Commented Feb 3, 2017 at 15:14
  • ah, sorry, i forgot. I did that work on the hot side. The cold side is still 1/2" copper that I have yet to replace. I don't see why you couldnt mix pipe diameters on the different lines since they are completely independent... but I'm not sure of the advantage of 3/8" PEX... the 1/2" is nearly the same cost and easy to work with (not like 3/4" and above), and the volume of water "lost" in 1/2" vs. 3/8" is really also minimal...
    – aaron
    Commented Feb 3, 2017 at 19:52

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