Hi Folks, I have trawled the forum for an answer to my problem, but can't find the exact fit. CH works fine. HW always works when the CH is running. When CH is off (or not demanded) the HW works when it feels like it. Once it is running it stays that way. When it doesn't work and I open a hw tap, the boiler sounds like it is starting (i.e. the fan and pump run) but the burner doesn't fire. The DHW flow switch is OK as it starts the fan and pump and is new. Any idea why the burner won't light before I call out an engineer? One thing I checked was the DHW thermistor and this reads open circuit (should be 3k hot to 12k cold, according to the manual). A month ago I had a completely non functioning boiler and the engineer changed the PCB and a thermistror (don't know which one).