vaillant boiler not staying on?

i have a vaillant vc gb 182eh boiler and it has not been switched on for heating for about 2 years by the last tenants only hot water. Now it only comes on for a minute before going off again. rads are not getting hot at all. could there be air in the system or is it a faulty valve. have been recommended to get power flush but this is an expensive trial and error route.