no hot water

[helpitwork]

I have a biasa riva compact boiler model M90E.24S.If i turn hot water tap on the boiler starts for a couple of seconds then shuts down. So i put the C.H. on for about 30 seconds so the boiler fires up. Then i turn it off. i now try the hot water again and the boiler fires up to heat the water.
P.S. if the C.H. is on the hot water will work.

 

[helpitwork]

Only have hot water when the C.H. is on. Biasi riva compact M90E.24S. Anyone any ideas what's worry.