i live in a bungalow and have a vokera compact 24 combi boiler (not sure how old). the central heating seems to work fine, but when i turn on a hot water tap, the boiler starts making a loud hissing noise. the cold water pressure is fine but the hot water pressure is terrible ie 25 minutes to fill the bath. i've also noticed the radiators cool down when the hot water is running. any help would be appreciated.