When heating comes on water pressure switch goes over 4 bar then starts dropping as quick as it went up any explanations

danzon, April 2010

Check filling loop has not been left open. Pressure drops due to a safety valve know as the Pressure Relife valve letting go, this is design to let go at 3.5bar of pressure.
Fault may be with a blockage in heating, do radiators get hot? if not CH blocked
If rads do heat up then your looking at the expansion vessel.

Chris, May 2010