hi i have a biasi 24s boiler. not sure how old as it was here when my partner bought the house 8 years ago. its been a really good boiler up until a few weeks ago. when we put the central heating on it works perfectly. it is when we put the hot water tap on that it doesnt work. occasionally the hot water does work but will soon run cold again. i have to keep turning the tap onto full to get any hot water and even then it is not very hot. when the tap is on and no hot water is running i can hear a fan like noise coming from the boiler. sometimes the boiler makes this fan noise and the boiler then ignites for only a few seconds and then goes off again and then repeats this process. when i turn the central heating on it makes it much easier to get hot water. The pressure is correct so have no idea what is wrong. can anyone help??