Unusual Central Heating Questions - Part II Earthing

[dave]

Andrew Gabriel mentioned in the post "Unusual Central Heating Questions"

"this inverter has no internal link between neutral and earth. You will need to bond these together (and to a reliable earth) for most boiler ignition circuits to work, or they will fail to detect when the flame lights, and continue trying to ignite it."

A useful thing to know. I have just heard of a new CH installation that had troubles working and it turn out to be a bad (main) earth.

Just wondered why this is a problem (afa the igniter circuit is concerned)? I thought it may be similar to an igniter circuit on a gas cooker - which will fire without any earth - as the circuit is "with respect to itself " - just a simple loop and battery. Obviously a combi boiler is more complex and has safety aspect to it etc - but anyone know in detail why this is a problem?

[Bob Minchin]

It is not the igniter per se that needs the earth but the flame detector that will not work with out the earth. In the absence of reliable flame detection, the boiler will attempt to relight including an unburnt gas purge cycle for a minute or so. After s few failed attempts it will lockout on safety grounds.

[Andy Burns]

As far as I understand it (and that's not very far) it isn't to do with the actual ignition, but with the flame detection

Previous discussion here from someone who knows ...

[Brian Gaff]

So, is this a spark igniter circuit. It should be perfectly possible, if the gap ends are both inside the flam for the switch to detect the conductive difference between the gas and the ignited gas, or surely one could use some kind of infra red detector instead. Why does it need the earth? Brian

[dave]

Found this pdf - explains it well. Flame rectification.

[Andrew Gabriel]

One of the electrodes is always the metalwork of the boiler, i.e. earth.

[dave]

Actually on 2nd thoughts I must be missing the point as still cannot see why the Earth is essential. The recification of the flame as per link is clear enough, - but why cannot it be with respect to it's own metalwork - as in the case of eg gas cooker ignition? Only thing I can thik of is that the noise is a reference - hopefully free of noise. I see the currents are very low - < 1uA so maybe that is the reason.

[Geo]

You just beat me to it - can't see that an external mains earth is /required/ for the internal circuitry to function - although the metalwork will obviously be earthed for electrical safety.

[John Rumm]

If the metalwork of the boiler is not earthed at all, then it is "floating" electrically. You won't then get any reliable current flow between mains live and the boiler casework.

[Geo]

Oh .... that is really ... UGLY...

And I re-cabled thermostat to 4 core so the neon did not feed to the old earth wire.

[John Rumm]

Well keep in mind we are talking old earth wire.