CH engineer explained d is a delay caused when system is returning water that is up to (or slightly above) set temp. The delay is a deliberate safety feature to allow boiler to cool sufficiently before restarting - more efficient than keep stopping and starting which would be very expensive and decrease life of your boiler.
Reason for my shut down = system appeared not to be balanced - hot water coming straight back too quickly without circulating through rads - causing the boiler to shut down.
I have a new property with a two year old CH system and when CH engineer checked, 3 of the MYSON thermostat's pin valves had stuck in the down position so 3 rads could never get enought heat (couldn't draw sufficient hot water so it was returning to the boiler without being cooled by the rad) which caused my boiler to think the system was overheating and my error d shutdowns.
Simple cure = we just removed my thermostat tops (easy, just unscrew knurled ring with large pliers - some may only be finger tight) then tap the centre pin to ensure it springs fully out of base and replace.
CH engineer said anyone can do this.
I'll be checking all of our thermostat pins annually from now on!!
If in doubt, always get a qualified CH engineer to check your system for you - like I did.
garryhq
garryhq
March 2009
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