On Tue, 26 Oct 2004 05:32:40 -0700, Tim wrote:

snip

The type of temperature sensors you have in the boiler are apt to drift
way out of calibration over the years. So it is possible that either the
high or low setting are not around 65C and 80C as they should be.

There is an over heat sensor (95C) which also ought to have responded to the
boiling? However if there were no flow at all it is _just_ possible that the
steam is being raised in the heat exchanger and never gets to the o/h
stat.

The pump over-run stat is meant to keep the pump running at all times when
the boiler is on and at any time the boiler is above a certain temp until
the boiler cools down. It will only work if the boiler is correctly wired
with both switched and permanent lives and the pump is connected to the
boiler and not in parallel with the switched live of the boiler.

Now any or all of the above 4 'cans' can and do cause problmes with this
model.

I don't know a geat deal about Grundfos Alphas. I do know that the idea of
reducing the circulation effort as the system warms up has got to be bad
news, unless the burner is turned down by the same control (as is the case
in many modern boilers). In particular it sounds like the Alpha also tries
periods of not pumping - that might work on a cast iron lump boiler but
it's really bad news on a low water content boiler like yours.

I'd first replace the Alpha with a bog standard £30 pump.
Then you will be able to diagnose the problems with the temperture sensors
if any.

--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
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