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Is a mixing valve needed for my primary/secondary setup?

I have a mod-con boiler that is piped for primary secondary with DHW on primary and manifold supplying panel radiators on secondary.

I am using the ODR to supply the lowest possible supply water temperature in order to drive efficiency.

I am advancing my knowledge of my system and trying to improve what I have.

One thing I am trying to determine is if a mixing valve should be installed.
The reason I believe it might be needed is once the DHW call is finished (call is 185f supply) and switches back to supplying the manifold (in mild weather 115f), the hot water that is within the boiler is pushed into the manifold causing a sudden spike in water temperature

This sudden temperature creates clicking causing sounds in the pex and radiators. But also brings the temperature in the manifold temporarily close to the max. design temp of the manifold.

Perhaps a mixing valve should have been designed in? Outside of a mixing valve any other ideas to lower the temperature when switching from DHW to heating?


  • hot_rod
    hot_rod Member Posts: 22,120
    your boiler control may have a DHW purge where it runs the pump for a minute or so to purge that temperature into the tank? Most relay boxes do if you wire the indirect through them, look for post purge setting. You could build such a feature also with a time delay relay.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • skiereric
    skiereric Member Posts: 66
    Basic piping. The closely spaced tees are within the boiler.