glow.worm micron 40FF Fanned Flue Boiler?

Sounds like the thermocouple (the cigarette-shaped bit) has gone in the thermostat control (part number 800849 for your model) which is labelled 9 in diagram 11.1 in the online manual at

http://www.glow-wormorders.co.uk/manuals/pdf/Boilers/Ultimate40FF.pdf

This will render the thermostat dial inoperative, and allow the water to boil.

Mark
November 2008

prob dirty system water or scaled heat exchanger

matt
November 2008