My hot water is running very slow from all hot water taps.It is a gravity fed system cold tank in attic boiler on 1st floor along with bathroom kitchen downstairs and toilet.I used to be able to run a bath in 5 min now its more like 25 min I read somewhere that I could connect the cold and hot pipes and flush it through.How long do I leave it to flush through and do I turn the cold water on full on .