"Set Square" wrote in message
...
In an earlier contribution to this discussion,
Bob wrote:

Remind me of the original problem - it seems a long time since I
wrote the above!


Aye, just about a month ago now

A description of the system - including the relative position of
boiler, pump, zone valves, etc. would help. Even better if you can
take some photos and upload them to a website and post a reference
to them.

Here's the description I posted first time, aswell some other bits I
posted:
SNIP

Ah, I remember the one. With a Bermuda and gravity hot water there won't
be
any motorised valves - so don't waste time looking for them.

It still sounds as if there is a blockage on the flow side of the CH
circuit. Are the downstairs rads ok? When you flush the pipes out with an
upstairs rad removed by opening each valve in turn (with the pump off, of
course) do you get a good flow from *both* valves?

Are the HW and CH circuits separate, or do they share any pipework? If
it's
like the Bermudas which I used to own, the boiler casting will have 4
connection points - 2 on each side. It works best if the HW circuit is
connected to one side and the CH circuit to the other - but you sometimes
find them both connected to the same side, and then splitting into 2
circuits.

Is this a recent problem, or has it *never* worked properly?

From the "suction" with the pump running I'd be immediately suspicious of a
pump installed in the return from the radiators. The suction will lead to
air being drawn into any weakness point (valve glands, pipe joints etc). To
the OP can you see an arrow on the body of the pump or identify which way
the water flows through it and see if it is pulling water away from the
boiler or pushing it back towards it?
A continuous slow ingress of air (oxygen) will lead to corrosion and the
production of sludge which will settle out at points of low velocity and
eventually cause circulation difficulties.

Flushing out everything and then a dose with Sentinel Ferroquest and a hot
circulating flush will help no end

Dont spill anything on soft furnishing or Set Squares prophesy will come
trueg