Apologies if this has already been covered elsewhere, but I'm new to the forum.
I've just moved into a 100yr old house, that had a Vaillant Combi boiler fitted in the attic about 10yrs ago. The first couple of weeks were fine, but I now get intermittent hot water. Sometimes I will turn on a tap/shower and hot water flows through, but randomly I will get only cold.
On examining the boiler I can see the litle plastic 'cam' that moves down (when the tap is on) and pushes on the little metal plate that in turn pushes the small green plunger down that ignites the boiler. However, the little plastic cam doesn't always move down with sufficient pressure to push the metal plate onto the green plunger, but it's only a matter of less than a millimeter. I get round it at the moment by sliding a sliver of bacofoil between the plate and plunger.
Could the problem be caused by insufficient mains water pressure?