John wrote:

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



Thanks for the reply John, I'll flush the other rads, and check for the
arrow on the pump....it's pretty inaccessible, under the floor,
unfortunately.

Incidentally, you mention air being drawn into weak spots in the
pipework if the pump is pulling rather than pushing. If it were pushing,
would I notice water coming *out* of these weak spots?

Many thanks,
Bob