I appear to be having a problem with my Wunda UFH manifold, despite water approx 35’ going into the manifold the water going into the top rail (see pic below) is only approx 23k ?, the TRV is set wide open and the pipework is hot both sides of it. I can only presume that the water is being blended with the cool water from the bottom rail. What stops water from the bottom rail being sucked into the pump rather than returning to the heat source???