What is the procedure for flushing the system. My system hasn't been touched- apart from boiler servicing- for about 20 years, now. I have a boiler system containing seven rads, a mixture of double and single fin. None of the rads have thermostats. There are two tanks in the loft and a copper hot water cylinder as part of the system. There is a room thermostat in the passage and a controller in the kitchen.