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Discuss Viessmann 100 WB1B-26 F4 fault on warm days in the Central Heating Forum area at

I have a problem with my Viessmann Vitodens 100 WB1B-26 boiler. It has had this problem since it was installed (Dec 2012), but is worse this year due to the very warm winter in my area (Philadelphia, PA USA). The problem goes like this. From midnight to 10 AM, the boiler works fine. But when the outside temperature gets up to 45-50 deg F, heat is not needed since the house is warm so the boiler stays off and cools down. Then about 7 PM after the sun goes down, the outside temp is colder, and the boiler should turn on, but the F4 fault occurs instead. Once a day in the evening, I reset the boiler (power off then on), the boiler whines for a few seconds then stops, there is a thump, and the boiler turns on. Everything is fine for the rest of the evening and morning of the next day. Then at 7 PM, it goes thru the cycle again.

During the winter months when the boiler never gets cold, everything works great.

The installation was the first and only one by the HVAC technician, so I suspect it problem is due to a bad installation. He will never work on a Viessmann again. I have already fixed one problem having to due with drain hose being full of water. The nearest Viessmann service technician is 95 miles away, so I am on my own.

I know that F4 is lack of flame detection. But since everything works fine in the winter, I'm assuming there is not a bad part (ignition lead, spark generator, flame sensor, etc.).

Is there any way moisture could be causing the problem?
I can send pictures or a video if helpful.

Any help would be appreciated. Ron
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No, it has never been serviced. The closest technician is 90 miles away, and he was reluctant to make the trip. Also, the boiler has been acting this way since installation (although not as bad) Thanks for the quick reply.

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