KETTLING?

Can anyone tell me why my WORCESTER 240 Combi boiler kettles??

CH demand does not cause kettling

When demand HW - after so long it will kettle and eventually cut out.

Why does this only happen on HW demand and not CH ?? Dosent matter whether CH is on/off etc Just whether HW is demanded. When CH demanded the boiler seems fine and dosent make any out of ordinary noises but when HW demanded you can slowly hear the boiler thinking about kettling !!

I know the theory about scale build up on heat exchangers ( mine has primary and secondry ) but I would have thought if it kettles ON HW it would on CH ??

Does this make sense?

You have a separate calorifier for hot water on that model .
You probably need a Registered gas installer to check your burner pressure in DHW mode

The burner pressure is higher on HW demand as appossed to CH demand, so if there as a scale or sludge problem the boiler will kettle quicker.

What's a calorifier? Only reason I ask is a always feels it's a good idea to get suggestions from other people about issues like this so if I get a proffesional in I can pass on the ideas etc and be able to explain it!

It's the hot water heat exchanger.