I have removed two radiators, replaced them both, but the heat is staying in the lower part of the radiators. I have tried bleeding them but only a small amount of air comes out.The model is a vailliant combi boiler. The water pressure seems low but am unsure which valve is the one to increase the pressure.

Mike, December 2005

You need to top up the system via the filling loop usually located at the bottom of the boiler. Open the two "washing machine" type valves on the hose until the boiler reaches a pressure of 2 bar. When it reaches this pressure, turn the valves off.

Joe O'Connell, December 2005