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Radiant pex temperature

bunsy86 Member Posts: 1
We have a Weil McClain WTG4 boiler with a zone for hydronic radiant. The pex is staple up Onix, no plates, under ceramic or stone tiles. Insulated with foil & faceless fiberglass, then drywall. Open system.

The one zone radiant heat has a plumber made manifold, with a thermostat and 10 pex runs controlled with a ball valve. The main manifold trunk is 1 1/4" and the pex is Watts 1/2" Onix, 185' runs.  The mixing valve is set at 140 degrees.

We have 2 problems.

1. Though Watts said the 12 branch copper manifold should be able to handle 12  lines of heat controlled by the ball valves, the heat is fine at the start of the branch, but by the end of the trunk, the heat is greatly reduced. Are the 12 branch manifold too much for the heat? Should the manifold have been built in a T?

2. Temperature control -- though the thermostat is set at 140 degrees in a staple up, insulated situation, the pex lines themselves do not seem to be 140 degrees. I haven't actually read them with a meter, but by hand, they are not that hot.

Does the thermostat for the manifold mean this should be the temperature for the pex?

Do I need to modify or replace the manifold so the water temperature is the same all the way though the pex lines?


  • kcopp
    kcopp Member Posts: 3,820
    For starters.....

    If its Onix, its not pex tubing... its rubber. What is the circulator on there? You may have a flow issue... You may want to turn up the temp to 145.
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