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While I was working on your mystery wiring you posted your update, well done Sir! The correct wiring only has one, but there are more than one mistake varieties on wirings. I also believe the engineer made a wrong re-connection, but haven't worked out which two were swapped. Anyway, you had a...

Is it a decorative cover which is not a cover for air chamber to combustion? Don’t understand what is wrong with some engineers!

The 2 is better than 1, as the pump speed for heating is proportional to the heating output. There is almost no difference on gas consumption.

Solid tap? Unusual! It is a system boiler, and heat only, unless you have e-bus controls.

@Bunnyman why not print letters on A4 papers and put them inside envelopes with windows? By the way, so far the best home office printer is Epson XP-850. I can't refill the ink cartridges myself so the running costs are next to zero.

When sparking is happening, the gas valve is supposed to open so is supposed to get a power from the board. So he should have measured this output from the board. Either gas valve, or PCB board.

link live to the two commons in the heatlink. Use a pair of existing cable between current programmer and thermostat, T1 and T1 to the new Nest stat.

Switched live to RT Pump to switched live

I know there is an anticipator in most of mechanical room stats, but didn't know the theory behind. Thanks!

Get a boiler man to re-charge your expansion vessel and check PRV for not leaking.

Vaillant ecoTEC 6 series don't need external auto by pass. If there are not too many radiators, the internal expansion vessel can cope with.

Old boards will be scrapped. Apart from basic checking on the resistances of pump, fan and gas valve, and visual inspection on diverter valves etc, at boiler cold standby status, we normally don't have ability to go further on board diagnosis, ie, element level repairs. So it is better to have...

Once approaching the setting temperature, the stat switches on off boiler more to get a slow, non-over shoot, approrach.

Honeywell CM907 then.

Honeywell 6360B1036.

TPi over on off switch, you will have an extra 1% saving on gas bill, so why bothered?

OP, you have been lucky so far, as many experienced boiler men on here tried to help you, particularly by @Steelmasons, @The831Bunny, and @picasso. It might be a simple fix, but it is always better by our own diagnosis on site. So it might be the time to call Vaillant their own engineer in.

@bernardgreen is the right person to ask for. lol Unless it is an instant blow off, we don't investigate further apart from a visual check, just replace with another board.

Don't go for Salus, crap on quality. If you don't like TPi by Honeywell, go for a weather compensation control. But it comes a point, what is your boiler make and model, is it a condensing boiler?