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So per the drawing.
Is it probable that when the floor Loop pump is off , that the other pump can force hot water through the mixing valve and into the floor Loop? That is what I am seeing on site. I guess maybe this heat exchanger is restricted so that it's easier for the water to go the other way?
Or if that would be typical and actually that in-floor Loop may need a zone valve on it?
Or is that a malfunctioning mixing valve?
I verified that with the floor Loop pump off and the other pump still running, I saw temperatures as high as the boiler Supply temperature so at one point it was 156. That would be with my strap-on pipe thermometer at the manifold entering the floor , so a good six feet from the pump and it happens in, oh maybe 10 seconds it'll run up from 110 to whatever boiler Outlet temperature is it that time.