Valiant Combi and hot Water not Working but Heating is Fine? Help and information on all topics relating to your central heating, air conditioning and ventilation issues.

[Pooston]

I'm having problems with my combi boiler. Heating works fine, but when I turn on the hot tap whilst the heating is on the boiler fires down.... and no hot water.
Once the heating is off then I can get plenty of hot water no problem.
Any ideas?