Baxi 105e flame modulation

17 Jan 2021
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United Kingdom
Hi all

I have a Baxi 105e that came with the house we moved into a few years ago. First winter it was quickly apparent there were a number of faults. All I have been able to address with the exception of one.

With both DHW and CH demand, the boiler is cycling. DHW is far quicker and cyclic if a tap is fully open (every 20s or so, whereas CH can be closer to +5mins)
My observation and I believe cause is that the flame is not reducing and therefore it's hitting target temp,likely creeping over and shutting down. As soon as it cools even slightly (as quickly as 10secs) it will then fire back up until it hits temp, so on and so forth.

In order to test this theory, if I create a demand and then pull one of the black wires to the gas valve, the flame drops to a small burn and the boiler does not cycle, temp is maintained. As soon as both black wires are reconnected large flame kicks in, starts cycling.

I've seen people saying that min gas settings on the valve can cause this but I can't see how that's the case here. To my naive mind it's the fact that no reduction in flame is occuring in the first place. The fact it happily runs with just one black wire I assume means it's running at min fine.

During the various other fixes a plethora of parts have been changed so this probably rules a few possibles out immediately.

Parts replaced within last 2yrs:
-PCB (was on original, replaced with latest available version)
-Overheat stat
-DHW/CH thermistors
-Diverter valve and associated gubbins
-Plate heat ex (this being the only part I've replaced as part of this final problem. I'd already removed and cleaned out with nothing coming out of it suggestion it was gunked up months back, but for the cost I just replaced with new earlier this week)

So I guess two questions. Are my findings logical, and more importantly what the hell controls the high/low flame settings (voltage I'm guessing rather than gas flow)?

If it's a gas related fix I won't be going near it, but if I get an engineer out, I want to be fairly confident they aren't going over old ground and costing me a small fortune in wasted parts or time.

Appreciate any pointers or guidance, even if it means telling me I've overlooked something ridiculously basic :)

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That boiler is probabley 20 plus years old. You are throwing good money after bad. Get a new condensing combi.

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