Weather compensation sensor problem

cully · 11 Mar 2026

Please advise, I have a Vitodens 100-W boiler and the weather compensation sensor is giving incorrect reading
The boiler isn't firing but everythig else is working so I presume the sensor is the issue.
Will the sensor disconnected from the boiler enable the boiler to fire up or do I have to change something on the menu first?
Thanks

vulcancontinental · 13 Mar 2026

Not positive about Viessmann but with most external sensor software, if there's a problem the heating works without restriction. Just disconnect one wire and if the heating works you'll know it's a boiler not sensor issue.

cully · 13 Mar 2026

Thanks,
Did that on Wed afternoon and the result was for the boiler, when heat was called, went to a constant 70* c
Normally I would see the temp around 55*
An interesting observation
Last time the boiler was serviced the heating engineer said that with this boiler I didn't need a mixing valve as the boiler, with the weather compensation did away with this requirement.
Now the weather compensator is out until I replace it, the mixing valve does the temp control!
Interesting.

vulcancontinental · 13 Mar 2026

cully said:
Thanks,
Did that on Wed afternoon and the result was for the boiler, when heat was called, went to a constant 70* c
Normally I would see the temp around 55*
An interesting observation
Last time the boiler was serviced the heating engineer said that with this boiler I didn't need a mixing valve as the boiler, with the weather compensation did away with this requirement.
Now the weather compensator is out until I replace it, the mixing valve does the temp control!
Interesting.

And I learned the Viessmanns do the same as other boilers when WC is disconnected.

Anyway, you're not cold.

Agile · 13 Mar 2026

You could measure the resistance of the weather sensor.

For Viessmann about 10k typically if I remember correctly.

More likely to be a connection somewhere rather than a faulty sensor.

cully · 15 Mar 2026

Thanks

desktop987456 · 15 Mar 2026

cully said:
The boiler isn't firing but everythig else is working so I presume the sensor is the issue

I have the same issue, mine is an old boiler (dial type) see my post on the issue which I posted in Nov25. Haven't been able to sort it out, the sensor is brand new and the resistance values are within range of a working sensor as per MI. Mine just stays that way as per photo, no flame symbol, i have waiting for quite sometime in hope that it will fire but it doesn't. Contacted viessmann tech support but they couldn't help either.

Agile · 16 Mar 2026

I am wondering if you guys are properly setting up the boiler to use weather compensation.

This video seems to cover the procedure.

cully · 17 Mar 2026

Just checked the resistance, getting around 13 OHMs replaced wiring, and back to same problem, temp at boiler fluctuates up up to 30+Deg
Removed weather sensor wire and boiler working normal at 70 deg
So, although the sensor is giving the above resistance I will change the sensor as the connections at both ends looks good, presumed that with the sensor giving a reading the wiring would be ok.
Question, do I need to use a Veissman sensor as the one that was in use appears to be the origional installed with the old boiler 24 years ago

Agile · 17 Mar 2026

Most boiler weather sensors are about 10k ohms. Thats 10,000 ohms.\

Are you sure you are quoting us the right meter reading?

vulcancontinental · 17 Mar 2026

All sensors I've sent are 12kohms at reference 20⁰C

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Weather compensation sensor problem

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