have been called to a job where rads are cold at bottom and half way up rad.I was going to hire a power flush but was a bit wary as the rads and pipework look quite old.I did not want to risk blowing a rad or pipe work.I have put 2 bottles of sentinal sludge remover in 14 rads 60 btu boiler s plan .how long can this be left in and the best way to flush system thanks