When we were on a 12-day vacation in November we had programmed our thermostat (this was before the tado°) to not heat up our apartment until the day before we were returning. When we got home it was 13.5℃ in our apartment.

As far as the thermostat was concerned, it had told the boiler to turn on the heat. However, the Viessmann Vitodens 100-W WB1A boiler had malfunctioned and was showing a F5 fault code. According to the manual (page 78) this means that the air pressure switch is faulty. Apart from being a very anti-climactic ending to our sunny vacation, it looked like this was going to be expensive.


Vitodens 100-W WB1A displaying fault code F5

After calling our plumber and getting voice mail I did a Google search to see if there might be anything I could do about the problem myself. I eventually found this forum post from 2010 on a hideous DiY site that looked like it might work:

dont know whether this helps anyone but i had this problem friday just gone 10-12-2010 f5 flashing red light i took the front off the boiler and to the right in the top half of the unit is the fan it sits right at the edge and works its way back to the middle of the unit this is where the flap is sticking, i got the misses hairdryer put it on the hottest setting and ran it over the fan area and all the metal bits that are attached to the fan this allows the sticky substance that keeps the flap stuck open to release itself i left the hairdryer on for about 5 mins and turned the water on halfway through to help it to start up if it doesnt start up use the rubber end of a screwdriver and gently tap the fan and one of the other units this should sort it hope this helps msg me if your sucsessful (dont touch any wires as they will shock you 😉

December 2010

I gave the hairdryer advice a try and amazingly it worked. Thank you Lewis.

Below is a photo with a green highlight of the area that I heated with the hairdryer. After applying the heat for five minutes, I reset the boiler, it made a gurgling sound, and then it started working again. More than a month has passed now and there have not been any more problems.


Heat the area highlight in green

It should be noted that this will probably come back and bite us if we go on vacation again when it is cold and turn down the heating. The next time the boiler has its scheduled maintenance we will have the malfunctioning part replaced.

Update: It’s been a year now and the boiler is still working without any problems.

Update 2014-12-30: After coming home from a seven-day trip with the heating turned down, the boiler was showing the F5 fault code again. After five minutes of heating with the hair dryer I tried a reset, but it started to show the F5 fault code again after a few minutes. I applied the hair dryer for three more minutes and then tried another reset. The heating flame icon was shown on the boiler display as it should and the boiler has been working flawlessly for three days now.
While we were away the temperatures had been around zero for a several days. We were lucky. Although the thermostat was set to turn on the heating if indoor temperatures reached 5℃ it would not have worked because of the malfunctioning boiler. The pipes could have frozen. We are going to have the malfunctioning part replaced.

Update 2019-10-05: In 2015 we replaced had our plumber replace the malfunctioning part and after that we never any problems with the boiler (we moved out in 2018). I don’t have any documentation about the part number, but maybe this technical manual will help someone: Vitodens 100-W Installation and service instructions. It looks like it could be part 058 that was the problem, but I’m not sure.