Drained the CH system last weekend (gravity primary, pumped
heating, original 1964, extended 1995), fliushed and refilled
including bottle of Superfloc. Ran it for six days. Drained again
today - bit grey! - refilled to flush through, but the upstairs
rads. won't fill completely now, all stopped at about halfway full,
even with bleed screw out, no further filling. Never known this
before. Anyone any idea what may be causing this? I'm
puzzled.

4square wrote:
Drained the CH system last weekend (gravity primary, pumped
heating, original 1964, extended 1995), fliushed and refilled
including bottle of Superfloc. Ran it for six days. Drained again
today - bit grey! - refilled to flush through, but the upstairs
rads. won't fill completely now, all stopped at about halfway full,
even with bleed screw out, no further filling. Never known this
before. Anyone any idea what may be causing this? I'm
puzzled.

Airlock somewhere - in my diy experience with old systems you often have to
repeatedly run the pump and boiler then re-bleed. Also, try turning off
most rads so that the full pump pressure is directed towards the area where
there's no flow - it'll eventually blow the bubbles through if you keep
going.

In an earlier contribution to this discussion,
4square wrote:

Drained the CH system last weekend (gravity primary, pumped
heating, original 1964, extended 1995), fliushed and refilled
including bottle of Superfloc. Ran it for six days. Drained again
today - bit grey! - refilled to flush through, but the upstairs
rads. won't fill completely now, all stopped at about halfway full,
even with bleed screw out, no further filling. Never known this
before. Anyone any idea what may be causing this? I'm
puzzled.

Sounds like some crud is blocking the feed pipe from the F&E tank into the
system. This could be some scale which has been released in the flushing
exercise, and has settled there. Someone reported something similar in this
NG only a few days ago. Try back-flushing it by feeding mains in through a
drain point. You'll probably have to block up the vent pipe temporarily.

[I'm assuming that there *is* water in the F&E tank, and the ball valve
isn't stuck shut? That *shouldn't* be a problem since you've obviously
exercised it recently.]
--
Cheers,
Roger
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