In article ,
Dave Baker Dave wrote:

http://www.diydata.com/planning/cent...ing/boiler.htm

Then my pump would be on the blue line immediately adjacent to
the boiler, pumping into it..............

That's where our pump is.... in the return line,
and immediately next to the boiler. It is the
original 1970 plumbing and works ok.

.............and then back out into the line that
feeds the vent and then the rads and cylinder. The vent is
therefore in the wrong place, before the rads rather than after
them and hence seeing all the pump pressure.

ISTM that if the radiator loops are free-flowing, then
the pump will pull a low pressure on the pipe feeding
the loops, lower than the head required by the vent pipe.
Radiator circulation is tickety-boo.

But if the radiator loops get blocked, (by sludge, or
later fitted valves?) then the pump will preferentially
do a local circulation, in just the loop between the
header tank and the vent pipe. Any chance that that
is what you are seeing?

BTW: My old plumbing book has a gruesome tale about
a plumber who blocked a vent pipe with a cork, to try
and reduce the amount of water lost during a drain
down. The resultant vaccuum caused parts of the
plumbing to collapse inwards. Blocking a vent pipe
may not be good news.

--
Tony Williams.