Rads not getting hot

[chrisb2]

I have an open vent CH and hot water system with hot water tank, using a wall mounted Vaillant boiler.
My question is: why do two/three of my upstairs bedroom rads only become lukewarm/stay cold when both the heating and water are on, and yet when the hot water is switched to off at the programmer, and the heating is switched to on these rads become hot.
Annoying when the upstairs is cold and have to keep on switching the water off to heat the upstairs rads.
Thank you. Any advice much appreciated.

 

[jackthom]

First make sure the valves on these radiators themselves are not causing the problem by fully opening both valves on each.

Then you may need to slightly restrict the flow of water from the boiler through the hot water cylinder with a new gate valve. At the moment the path through the cylinder is probably too easy so too much flow is likely being diverted away from the radiators.

The trick will be to close the new valve just enough to keep the upstairs radiators warm without choking the DHW circuit.