remove fan and clean impeller blades of dust build up. Clean fan venturis (part that the air pipes from the air pressure switch connect onto fan)
The master, January 2012
checked that pipe and it is clear
martin, January 2012
By removing the heatshield you are allowing the boiler to breath.There is a large bore pipe,only about 4ins long.This is known as a ventura.Air is drawn into the boiler through this pipe.Sometimes this ventura needs cleaning out.
