sime combi ?

i have sime ecomefort H.E it is 12 months old i have no central heating connected i only run hot water from it i keep it at 1.5 bar and it works fine my problem is some times when i turn the hot water on it wont fire up and a green light blinks on and of so i turn the boiler of at the wall switch after a few times of doing this it comes back on can any one tell me the reason this happens