I wish to do a thorough descaling and de-sludging of my sealed CH
system, and have bought a large tub of Fernox DS40 to accomplish this (I
hope).

I note on the instructions that it says to allow 7 days for it to work
on all-steel systems, but only 4 *hours* for those containing aluminium
rads.

My problem is that I have a combination of both.

Can anyone suggest a work-flow I can use here to ensure a proper job is
done throughout, please?

Note: My system is split into 2 circuits, with flow control valves
operated by the timer programmers. All the aluminium rads are located on
the downstairs circuit and the older steel one's on the upstairs.
Obviously, I *can* isolate the alu rads by screwing down the valves, but
this would mean I would have no heat to downstairs at all for around a
week, and winter is not over yet. And no, the job can't wait until the
Spring. However, if it's a must, then I will do it. :-(

Nigel