Andy Hall wrote:
On 2008-05-27 20:46:22 +0100, Mark said:

I have just removed a radiator & noted that a small amount of black
water came out (which later settled out into brown sand-like grains).

The majority of the system is fairly new (all radiators and boiler)
but there is some old pipework. The boiler is a W-B combi (i.e.
sealed). There are no leaks in the system.

I see that there are a huge number of chemical cleaner products and
inhibitors. How can I tell which would be most suitable for my
system?

The first thing is that you need to thorughly flush out the sludge and
solids.

There are two reasonable ways to do that:

- Drain system, remove radiators, take them outside and flush them with
a mains hose or pressure washer. Flush water out at each radiator
position.

- Power flush the system. This can be done by paying someone N x
hundred ££s or by renting the equipment and DIYing it.

Not disagreeing that either of the above are the ideal solutions, but
both are reasonably drastic in terms of hassle factor and/or cost.
Given that the majority of the OP's system is pretty new I'd go for
something like Sentinel X400 cleanser, and repeated fill/flush/drain cycles
http://www.screwfix.com/prods/89458/Plumbing/Central-Heating-Treatment/Sentinel-X400-Central-Heating-Sludge-Remover

followed by say Sentinel X100 inhibitor:
http://www.screwfix.com/prods/79683/Plumbing/Central-Heating-Treatment/Sentinel-X100-Central-Heating-Scale-Inhibitor-1Ltr

David