Can I fix my own boiler?

Hi, can anyone advise on a problem with our boiler? The boiler is only used for heating our water and not any central heating, it was installed when we moved in 15 years ago! the problem is that it fires up and heats water in the bathroom, but whilst it fires up immediately when the downstairs hot tap is turned on(and keeps burning) the water does not get hot and runs just slightly aired. I have tried adjusting the temp on the boiler to maximum without success. thanks