Greg C wrote:

If it is the diaphram, you may find it simpler to just add another
expansion chamber elsewhere, rather than trying to get at the original one.


Could you explain that a little further? How would one do that?

You can buy expansion vessels pretty cheaply on their own (as you would
need to if you converting a regular vented heating system into a sealed
system for example).

Something like the 12L one here, ought to be more than adequate for a
good proportion of systems.

http://www.bes.ltd.uk/products/109.asp

The actual size you need will depend on the volume of water in the
system. The following guideline figures come from Ideal's documentation,
but should apply to any sealed system:

Assuming pre charge pressure between 0.5 and 0.75 bar

System volume Expansion vessel
(litres) volume (litres)
25 1.8
50 3.7
75 5.5
100 7.4
125 9.2
150 11.0
175 12.9
190 14.0
200 14.7
250 18.4
300 22.1
For other system volumes
multiply by the factor across 0.074

If the vessel is too small then you will still get the over pressure
symptom described by the OP, however it does not matter if it is larger
than required.

The expansion vessel can be installed anywhere on the system as long as
it never gets gated off by zone valves etc. Note however that some
boiler makers do specify preferred locations for expansion vessels.

Installation is typically just a case of tee'ing into the heating loop
pipework and fitting the vessel to the tee (having pumped it up to half
to three quaters of a bar pressure).

Lots more on this subject in the sealed system FAQ:

http://www.makewrite.demon.co.uk/SealedCH.html


--
Cheers,

John.

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