No hot water fault ideal logic heat 15 S plan system 

⚠️no hot water ideal logic heat 15⚠️
Customer called to say they had no hot water, they said the heating worked ok and that sometimes the hot water gets hot with the heating on…
They said there was no fault code or anything on the display of the boiler unless the heating was on…
When I arrived I saw an ideal logic heat 15 and an unvented cylinder with an S plan system so I whipped out the Breakdown Bible to help me diagnose…