Please would you advise me on the following. We have a WB highflow 400 combi boiler. We are noticing that in the colder months the heating works but the hot water is unreliable/not working. We have to switch off the heating to get hot water. Since we switched off the heating for the Summer we have to relight the boiler to get hot water - especially if it's not been used for some time e.g. first thing in the morning. Help!! WB engineers have been out changed 5 parts and the problem still exists - the problem never occurs on their visits and now the parts warranty is expired : (

Miranda, July 2010

All you have described indicates a blockage/partial in the domestic H/E.
The little HW when the CH is on is quite common, they use a thermal store to provide heat to both so if you have the CH on, you deplete the availability of the store to provide HW.
The pilot going out is possibly due to that blockage, the boiler overheats and the interrupter in the thermocouple puts the pilot out.

These boilers were a fine idea, but they literally are a 'washing machine' sized boiler that didn't meet the concept, over-complicated in design (my opinion only).

Ditch the damn thing and fit a Vailliant 937 (also a thermal store) good water flow and more reliable.

Carefree, July 2010