My radiators used to only get warm on top. A plumber eventually diagnosed the problem as
being the external bypass in my heating system being fully open causing alot of the hot water to go directly back to the boiler. He closed the bypass fully and now the heating is 10x better
My questions are... should he have fully closed this bypass off? Is there no in between Fully open or fully closed? If the boiler has an inbuilt bypass what should the external one be set at? Will it damage my system etc if its fully closed?...
As long as your boiler has an internal bypass then leave it closed - otherwise it can cause problems as you have found, when left open the water could return at the same temperature as the flow, the system will not work effectively or efficiently.