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New Install Under Floor Between Joist Advice Needed

mraupie17 Member Posts: 2
Hi all!

I am a passionate do-it-yourselfer and I am installing radiant heat system. The more I read, the more I learn. Also, the more confused I get at times because so many things have differing opinions. I know I have alot to learn, but in addition to getting everything working, I enjoy the learning and the journey, so I welcome any and all feedback.

Current Issue
I only have 2 loops currently installed. 1/2 pex, 300 ft runs on both. I am waiting to install the rest until I get this working. I pressure tested the system successfully. When I turn on the pump and try to get things working, I am only getting at most .6 gpm through loop 1 and maybe .1 gpm through loop two. The water heater says how much gpm is going through it and is only showing .6 gpm. With so little through the water heater, it doesn't always heat. I tried to adjust the flow on the manifold, but I can't get it any higher.

I did purge air out of the system, but I was reading that I should install an air eliminator, so I plan to install that between the pump and the hot water heater.

I have attached a rough diagram of the setup. Any thoughts or recommendations you have would be appreciated. Also, any thoughts on why I can't seem to get higher .gpm through the loops and the water heater would also be appreciated!


    JUGHNE Member Posts: 11,075
    No attachment shown. Try again.
  • Jolly Bodger
    Jolly Bodger Member Posts: 209
    First thing is get Upnor's radiant design manual. They literally wrote the book on this. Go on their website and they will mail you a copy.

    It is generally recommended to not exceed 250 foot per circuit on 1/2" PEX.

    If you post some pictures of your installation we can help more. My first guess is that bends and fittings in the circuits are not equal and the pump is beyond its curve on head pressure. Especially if it is pushing through an on demand water heater and the PEX Circuits.
    JUGHNE Member Posts: 11,075
    What type of water heater are you using? On demand or a tank type?
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,086
    edited November 2019
    I have found the spring type GPM meter on the radiant loop manifolds are not the most accurate. How are you measuring GPM? What pump are you using?

    Tankless water heaters are not designed for use as heating boilers. There is more resistance thru the heat exchanger than a Boiler designed for heating. This is one of the reasons you may be having low flow issues.

    Tank type water heater may be the answer if you are only doing a small area. If you are doing the whole house, Get a Boiler.

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

  • kenjohnson
    kenjohnson Member Posts: 85
    Hi mraupie17, congrats on getting this far. You'll get a lot of good feedback on this site (I did) if you can provide some good details of your system layout and setup.

    My system is up and running great. My thread is here in case you want to peruse it.

    I have some loops that are 300 feet in 1/2" PEX and getting ~0.5 gpm (per the manifold flow meter) using a Grundfos Alpha pump from a big storage tank. If you are at 0.1 gpm in a loop and 0.6 gpm in another, then I'm guessing you'll find something correctable with the 0.1 gpm loop.