Best way to clean and flush a combi central heating system?

[steve]

Can anyone tell me the best way to clean a combi central heating system?

I have loads of blackish bits in the pipes and radiators.

I have drained down and refilled several times, but it is always there.

I often have to remove my secondary heater exchange on the combi to flush these little buggers out or the water temp will flucuate dramatically. For a few weeks after doing this the water is piping hot.

I am assuming that my system needs a really good flush / clean, but how to do it?

Steve..

[Charlie]

I have exactly the same problem. Build up in the secondary heat exchanger causes my boiler to overheat and cut out every few minutes. Remove and clean it (which isn't easy) and all is well for a few months.

Help anyone?