I have a BAXI Solo 2 50RS gas boiler. Working fine except that it seems to cause a lot of interference in the house - very loud buzzing on any radio that's on.
What's up? Should I get the thermostat replaced or maybe a suppressor has failed?
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.)
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.
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
formatting link
need manufactures part no 232156 which you can search on.
or you search through appliance type, baxi, thermostat and then listed as
41-077-78 solo Ii 50 rs
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.
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:
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