Replaced pump (sticking)and heat exchanger (leaked). Since these changes the boiler started short cycling - wasn't doing it before. Problem exist whether set for HW or CH or both. Manufactures engineers carried out work, also was unable to cure problem. System was power flushed before heat exchanger and pump replaced - any ideas?

Mr Cool, January 2013

It is switching off prematurely when a certain internal temperature is reached. As the internal temperature drops then conditions are reached that enable it to come on again.

I just guess.

A bizarre bad connection issue with internal wiring or the control electronics? Some problem with the fan/air pressure switch combination - being "on-the-edge" - good enough to work at lower temperatures but not when warmer.

What is the fan doing when it shuts off?
What is the Air Pressure Switch doing?

Peccavi, January 2013