On Sun, 05 Mar 2006 09:22:08 +0000 (GMT), Tony Williams
wrote:

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?


All of this can be very easily avoided and the risk of pumping over
eliminated by positioning the FE and vent pipes on the same side of
the boiler.

There should be a clear path from the boiler flow side to the vent
with nothing in the way in terms of pumps or valves.

The FE pipe can be connected to a point on the circuit within 150mm of
where the vent pipe connects. The pump is located after that,
followed by the motorised valve(s).

An even better solution is to use an Aerjec air separator which will
do the correct plumbing connections in one place as well as helping to
purge the system of air as it is circulated following refilling and in
general.

The flow through the radiators then becomes irrelevant and if
inhibitor is used, no significant corrosion or sludgng either.

Another issue to consider is that if TRVs are used then the flow
resistance through the CH circuit will increase anyway as they start
to close, so it having an arrangement where avoiding pumping
over/sucking down is on the basis of clear path through the CH circuit
is never going to be clear of potential problems.





--

..andy