Interference from gas boiler

Most likely the thermostat contacts are making most of the trouble - replacing the themrostat may likely fix things.

In message , Steve H writes

Relay on it's way out on the pcb? Air pressure switch.

Any switching device where there is not a good ohmic contact between the contacts is liable to spark and cause RF interference

Contact suppressor gone u/s?

What contact supressors ?

You jest sir

Is it a continuous noise or intermittent? If intermittent does it tie in with the boiler starting up or shutting down? That would indicate arcing across the contacts of a thermostat or possibly relay.

If it's continuous (presumably all the time the boiler's on, not when it's off) then maybe it's a funny with the flame rectification circuitry. Is it a mains frequency (50Hz) or double that (100Hz) buzz? (Same pitch as any noisy transformers or similar you might have.)

-- John Stumbles

-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

-+ Question Authority

In message , John Stumbles writes

I wouldn't have thought so

Low voltages in the solo 2 are achieved by C-R droppers, it doesn't contain a mains transformer

I thought I didn't see one :-) (I was working on one earlier in the day of the OP) I asked so the OP could compare the frequency (since not everyone wil identify 50 or 100Hz instinctively!)

How do they get the flame rectification signal? It's enough to make my volt stick light up so it must be many tens, at least, of volts and I'd guess (without having traced the PCB) that it'd be straight off the mains via a capacitor. I'm also guessing that that goes into the cold end of the ignition coil secondary. What if the capacitor was breaking down? There's a nice bit of inductance to knock up some spikes to generate EMI, n'est-ce-pas?

However the OP hasn't replied about whether the interfecence is intermittent or continuous. My reading of his post was the latter, but I guess everyone else assumed the former.

-- John Stumbles

-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

-+ Experiments to demonstrate the existence of Sod's Law by dropping slices of buttered toast all failed. That's Sod's Law.

I had the same problem as you with interferance on my baxi solo II boiler.

Fixed it. Ordered a new thermostat fron MJTcontrols see link below

or you search through appliance type, baxi, thermostat and then listed as

You might have to phone, i couldn't register my new account details online.

cost me £12.50 including VAT and delivery, next day delivery.

I fitted mine yesterday and no interferance, takes about ten minutes to replace. Just need a philips type screw driver and pair of pliers. Two electrical connectors so the power needs to be off at the boiler.

In message , John Stumbles writes

The flame sense is a very small current received back down the HT lead

No, the flame sensing circuitry is basically a sensitive amplifier of the current and is referenced to earth. You must just be seeing mains pickup.

No

The only relevant capacitor is the one which charges up the spark circuit, and is normally a Mylar capacitor. All that happens when it breaks down is that eventually it doesn't charge up sufficiently to create a spark

Are you saying the presence of the flame causes a current to be generated in the electrode, and this current is detected to sense the flame? That's a different system to flame rectification, isn't it?

My understanding is that a flame, being composed of ionised gas, can pass current, which can be used to detect the flame; but that systems which simply use this mechanism can fail to danger if some conductive material bridges the circuit allowing current to pass with no flame present. Also that a flame will pass current better in certain directions (in line with the flow of appropriately charged ions) than others, and that this phenomenon is used for flame rectification systems by applying an alternating voltage to a sense electrode and sensing the net flow of current in one direction; such systems being liable to fail-safe since any muck across the contacts cannot produce the one-directional current flow that the flame does.

So in a flame rectification system I understand you need a source of alternating voltage - which is readily available from the mains - and a small-signal amplifier. Thus I expected to see something like:

AC mains capacitor electrode flame low-pass amplifier __ filter / \ /\ / \ | | / \ | | | ---| |-----+========>| | \ / | | | | | \__/ | \/ |\ | | \ |_______________/\ /\ __+__| \__ flame OK \/ \/ | | / signal --- | / --- |/ | __|__ /////

I've omitted the spark ignition circuitry for clarity (and because there's only so much I can do with ASCII art :-)

So (a) is my undedrstanding of flame rectification correct? (b) has the Solo 2 got a different system?

Possibly: I was originally looking for the ignition signal, but ISTR the signal that lit up my volt stick stopped when the flame was established.

-- John Stumbles

-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

-+ Just because you're paranoid doen't mean They're not out to get you.

In message , John Stumbles writes

There is a very small current passed through the flame. The flame acts like a highly resistive rectifier

No, if it's not rectified, the FSC doesn't recognise it as flame on.

The system's universally used on domestic gas boilers (except those ones which don't use a thermocouple, that is)

That's what I'm on about

No, that's right, but it's very small, and nowhere large enough to generate interference

The flame sensing is still there so that it can turn off the gas valve if there's e.g. a blow out.