On 10/12/2013 21:41, fred wrote:
In article , Andrew Gabriel
writes
In article ,
Roger Mills writes:
On 10/12/2013 00:37, Bill Wright wrote:
To quote you, "Note that if running the boiler from the genny you will
probably need to make sure that earthing is correctly connected for the
flame sensing to work properly. So basically set the genny up with an
earth spike like a TT system, and tie one leg of its output to the
earth."

Well, that turned out to apply in the case of my boiler, but why? Can
you explain it slowly and simply? Why does the flame sensing need this?

Bill

Dunno exactly - it's something to do with flame ionisation rectifying
the signal or something.

Also, as far as I can see on my 2 boilers, the return path for the spark
is via the casing, so if this isn't connected to one side of the supply
- preferably the neutral(!) - there's no conductivity. Similar, I
suppose, to a car engine - where the engine casting needs to be
connected to the -ve side of the battery for the spark plugs (and other
electrics) to work.

Yes, it effectively rus between live and earth (far too small a current
to trip an RCD though).

Not entirely convinced on this one, my boiler runs on a transformer
isolated supply and I assume the spark generator/flame sense circuitry
does the same so I'm not sure why the earthing or supply polarity is so
important.

You could try temporarily disconnecting the boiler's earth wire, and
then see whether it still works.
--
Cheers,
Roger
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