My CM921 has ben set to come on twice a day 7.00am to 9.00am and 5.00pm to 9.00pm. 17. degrees lowest and 20 degrees highest.
why does the boiler fire up outside these times?
My cm921 is 7 years old does that have any reason for my problem?

Irene, December 2016

It will have nothing to do with the age of your boiler.

It may have something to do with cold weather - some boilers have a frost-stat fitted to fire up the boiler at about 4 degrees C to prevent any pipes from freezing up.

Good Luck..., December 2016