heating wont switch on only after switching on hot water

[cinas]

Hello,
I have a about 15 years old Ideal boiler installed and after replacing a faulty exhaust fan last month I am facing the following problem with the boiler. It only starts when both: central heating and hot water are on. But I cannot start the boiler with only central heating on. That means that every time I want to have the radiators hot I have to warm up the water for the taps as well. Can anyone help me with this case and advise what I might do with it?
Thank you & regards,
Martin

 

[bhm1712]

what type of system? fully pumped Y or S plan? how many motorised valves?

could be the microswitches in the CH valve not working and so the boiler only gets an ignite signal from hot water.