Set Square wrote:

It seems to me that your system needs a really good flush out, and then
needs to have inhibitor added. By far the best way to flush it out -
particularly with summer coming on - is to remove the rads and take them
outside and give then a good flush out with a hosepipe. While they're off,
flush the flow and return pipes by keeping the header tank full and opening
each radiator valve in turn (with a bowl under it to catch the water!).
Incidentally, take great care not to spill any black sludge from the
radiators on your carpets - it makes a pretty indellible dye!

Hi Set Square,

Right, finally had a go at the first radiator....unfortunately the
results weren't as I was expecting from your description.

I took a rad off and flushed it out, and as expected was a bit mucky,
although not really sludgy. I then opened a valve on the pipes (not sure
which is flow and return, so did each separately.) Anyway, when the pump
was off, I got water flowing, which I let run until it ran clear.
However, when I switched the pump back on (by turning up thermostat) the
water stopped flowing and I could feel the suction at the valve again.
The same symptoms occurred at both valves. By the way, I closed the
valves on all other upstairs rads.

Is this what you would expect or is something else wrong. Additionally,
is there anything else I can try?

Many thanks for your assistance.
Bob