"Ener-Tech 240003 ISS4 board"
Sounds like the etch revision of the basic naked bare board
It's probably one of Potterton's part number 407677 PCB assembly - Bare PCB made by Ener-Tech Electronics.
In which case R15 is 100k ¼W (Brown Black Yellow) and is located next to Relay RL2. It is prone to overheating and you might replace it with 100k ½W.
While you have it apart replace the electrolytic capacitors C4 & C7 both 22uF 63V and C6 4.7uF at 63V. Be sure to observe the polarity of the capacitors and get them the right way round ±.
These capacitors are prone to degradation due to being in an over warm environment all the time. Electrolytic capacitors can be badly affected by heat. Try to get the same or higher voltage rating (but not lower) and 105 ºC temperature rating for longer reliability/life or at least 85 ºC.
An article here tells all about it - with expanding pictures.
http://kaijaks.co.uk/blog/nick/2008/02/10/dodgy-old-boiler
What an answer - quick and complete...
Good luck...
Peccavi, December 2009
