From: Cynic on
On Sun, 25 Apr 2010 11:21:43 +0100, "Mortimer" <me(a)> wrote:

>I wondered whether it was the "burner lit" sensor (is this still a
>bimetallic strip?)

I'm by no means a boiler expert, but the only two modern gas devices
(a boiler and a room heater) I have had reason to look at both used an
ionic sensor for the pilot light.

Such a sensor uses the fact that the air inside a gas flame contains
ionised carbon atoms. A wire positioned to be inside the pilot light
flame when it is lit has an electrostatic voltage applied to it, and
an electronic circuit senses when that voltage produces a current flow
through the flame. As there are no moving parts or contacts it
*should* be more reliable than a heat sensor as well as being faster

The room heater had the sense wire very carefully positioned so that a
small decrease in the size of the pilot flame would take it out of the
conductive area and shut down the heater. Apparently (I was told) the
flame size and shape changes if there is an excessive amount of CO or
CO2 in the air feeding it, and so this is a safety feature that will
shut down a heater that is operated in a room with inadequate
ventillation. I have no idea whether a boiler is set up in a similar

All the above information was gathered from unverified sources, so I
cannot vouch for its accuracy.


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