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Thread: Low flow at radiant manifold

  1. #1
    DIY Junior Member JStephens11's Avatar
    Join Date
    Dec 2012
    New Hampshire

    Default Low flow at radiant manifold

    I just installed a pex radiant heating system in between the joists in my subfloor and I'm having trouble getting sufficient flow at the supply manifold. I have 4, 250ft loops connected to manifolds which are centrally located in my house.

    Here is a picture of the 4-port supply manifold assembly I'm using:

    It has 4 ports on the supply and return manifold. The supply manifold has an adjustable flow meter, and the return has a balancer. This is to allow you to adjust the flow so that it is equal in each of the four loops.

    My furnace has a 3/4" copper supply line. I've reduced this to 1/2" PEX and ran that to the hot side of a 1" mixing valve. (There is a 1/2" x 1" reducer bushing installed on the mixing valve to accept the 1/2" pex)

    The outlet of the mixing valve is then reduced from 1" to 1/2" pex, which is then connected to the 1" supply fitting on my supply manifold. (with another 1/2" x 1" reducer bushing)

    My supply manifold has flow meters on each of the 4 loops, and I cannot get flow above 0.2gpm out of each of these lines. I need the flow to be around 2gpm.

    Do you think this is because of the size of my pex lines? I'm thinking I should purchase 3/4" PEX and run that instead of 1/2" pex, but I do not want to spend the money on these fittings if it is not going to fix my problem. I'm also going to need to buy 100ft of 3/4" pex, when I only need to use about 20ft.

    FYI: From everything I've read online, 90% of the time flow problems are because of air trapped in the loops. I spent about an hour and a half yesterday trying to purge the air from the lines, but my flow is so low that I do not think this is possible. What I've been instructed to do is shut the main valve on the return manifold and connect a garden hose to the drain valve on the the return manifold. Then I flow water through 1 loop at a time for 15 minutes out the garden hose to remove the air. When I do this the water is flowing out the hose very slowly...not the steady stream that I'd expect.

    Attached is a basic diagram showing my plumbing layout...minus all the little fittings that I described above.

    If you have any questions let me know and thanks in advance for your help.

  2. #2
    In the Trades Tom Sawyer's Avatar
    Join Date
    Nov 2010


    The circulator should be on the supply.
    The feed piping to the manifolds should be 3/4"
    There should be a circulator to feed the manifold
    The primary/secondary tee's should be no further than 4" from each other.
    In other words, it all needs to be re-piped. Water always follows the path of least resistance
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

  3. #3
    Moderator & Master Plumber hj's Avatar
    Join Date
    Aug 2004
    Cave Creek, Arizona


    Your mixer and manifolds are BOTH 1", so why in the world would you reduce them to 1/2" PEX, which is really about the same a 3/8" i.d. copper? (1" PEX would be just about the same size as your 3/4" copper to the expansion tank connection). Your piping is NOT big enough to support 1000' of radiant tubing. Those valves on the manifold inlets are NOT helping to maintain maximum flow either.
    Licensed residential and commercial plumber

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