On Wed, 27 Oct 2004 04:50:33 -0700, Tim wrote:

snip
I had not realized the Alpha could emulate dumb modes so that is
worthwhile, certainly any mode which turns the pump off even though power
is applied is going to cause problems.

The over-heat stat if it operates should have shut the boiler down
requiring manual intervention to restart.
On the old permanent pilots it simply killed the thermocouple circuit and
the gas valve dropped out.
However yours maybe an electronic ignition model?

The over-run stat works like this: The pump is connected to the common
pole of the thermostat. When cold the over-run stat connects the switched
lived to the pump when hot the permanent live is connected to the pump.

A failure mode of these devices is that a 'dead zone' can emerge between
the cold and hot states leaving the pump completely unpowered at certain
temperatures.

--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html