Fault code F5 on a Viessmann Vitodens 100

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.

  1. Duncan Miller said:

    Can’t believe it, just came home from a 5 week trip. Viesmann boiler was giving a f5 fault. November in Scotland, and a Sunday. Tried the hair drier trick, with in 3 min by boiler fired in to life. You gust got to love the Internet and the people that put helpful post on it like this. Can’t thank you guy’s enough.

  2. Mohammad Khalaf said:

    thanks a lot, it works 🙂

  3. maciej said:

    it helped me too! thanks a lot!

  4. Rhys said:

    Thank you so much. This worked for me as well, I have been struggling with the F5 fault for some months now and I have historically managed to get around this by opening the water top up valve. This usually does the trick although you get a lot of alarming gurgling noises before the boiler cuts in but this time around it simply did not work. But cometh the hour cometh the HAIRDRYER.

    Lewis my man, thank you.

  5. Kate said:

    Thank you! My hairdryer fixed my F5 too, in less than 5 minutes. What’s so cool about a Viesmann? I HATE the one that was installed in the condo we bought 7 yrs ago for the summer months. There hasn’t been a winter that it hasn’t shut itself off for some reason.

  6. Adam said:

    You sir are a god send. Came home from a weekend away, freezing, knew i’d be in work for the week and unable to be there for a repair man. saved me a lot of money on out of hours call out fees! Nice.

  7. Mick said:

    I had an F5 this morning I pushed the reset button and got a rubber mallet and tapped the inside of the boiler,, she fired up and still going

  8. Moid101 said:

    I got F5 this morning (freezing frosty morning outside), i have boiler break down cover but they could not get out until the next day, then i read this and thought i would give it a try and hey presto if fired up. Still going to have them come out and replace the part with the new version that prevents it from repeating the problem…

  9. Tony said:

    Cheers man!

  10. Bradley said:

    Who would have thought that a 30 year old hairdryer would be the answer!!! Thank you so much. Got back from a two week camping holiday, boiler not working, shower needed, no hot water!! Hubby has just spent two hours taking the boiler apart. Then he found this!! Worked a treat. Only needed a couple of minutes too!! Thank you for your amazing post!!

  11. Susan said:

    I had the F5 error too but pressed the reset button and it worked again for awhile. Can anyone tell me where to buy this air pressure switch and put a photo on where it’s located. My unit is the 100w combo for natural gas

    • Susan said:

      I talked to a Viessmann rep and he said that when the heater is serviced the technician should be replacing the flapper which is a $28 part.

      • Claes said:

        Hi Susan. Good to hear that your boiler is working again! Did the Viessmann rep tell you what the part number was?

  12. michel giguere said:

    ca marche parfaitement Merci!

  13. Paul said:

    Woohoo – Lewis is my hero! I’d like to offer him my unmarried daughter as a gift.
    And thank you to Claes for this blog to publicise the info.

    Viessmann’s website offers the part:

    Flue damper Vitodens 7823838, gemischklappe, flapper Summary
    Viessmann Flue damper 7823838. This is the new none sticking type. Gemishklappe. (flap) flue flapper, Viessmann vitodens 100 wb1a mixture fuel damper. Cures repeat F5 Faults

    The things they didn’t tell you when you were young…….

    Part is £16.20 plus postage to UK, also on eBay.

    Just off to find my wife so she can show her gratitude in no uncertain terms!

  14. Sarah said:

    Thank you OP – worked a treat. Easy to do and only took a few minutes. (For those worried about getting the front of the boiler off… no screws etc., just two clips underneath.)

    Why does the heating always break down on long weekends, when it’s freezing cold and you can’t get it repaired for days? This happened yesterday, Saturday afternoon of new years weekend, and we’d be extremely lucky to get it repaired by Tuesday but more than likely it would be Weds/Thurs. If it’s still working on Tuesday we’ll gamble and leave replacing the flap til the next service.

    This quick fix has saved us days of misery with our one electric heater and has also given me the added bonus of bragging rights 😉

    Thanks again x

  15. Brad said:

    Lewis… You’re a godsend and an absolute beauty of a human being!

  16. Jeremy said:

    Just got back from a 2 week holiday to find the boiler not working and on an F5 fault. I had a quick look on google and found this. At first I thought it wasn’t going to work but tried once with the hair dryer on the area you specified and in less than 10 mins the boiler was up and running.

    Thanks for the post.


  17. Tony said:

    Worked for me after being away a week

  18. Debs said:

    We’ve had the same problem a number of times..when coming back after holidays etc..but it happened a couple of weeks a few times..reset button flashes..we have always turned the hot tap on and after a few try’s it works..last week woke no heating..couldn’t get it to work even using hairdryer.. so got engineer out..the fuel damper flap wasn’t opening at all.. he took it out cleaned it,now working fine..said he will order us a new one..I wonder if we could fit it ourselves??? Only £20 plus vat..

  19. Sam said:

    MAYBE IM SPEAKING TO SOON. But this guy is a genius!!! Ive had no hot water for 3days ive spoken to countless ‘professionals’ saying we need a new boiler, its n air-lock, a new Fan said viesmann. We’ve had people saybits the diverter valve. It’s the gas pressure its water pressure. Its ridiculous!! I almost threw 5K down the drain trying to fix this n all i needed was a £15 travel hairdryer bought 8yrs ago to fix this problem.. viessmann actually hung up on me because i was out of the 5yr warranty and that noone could see me for 2 WEEKS if im not gonna book n engineer and no longer in warranty we don’t want to help.. thanks alot to this guy n his trick! Absolute LEGEND. I owe you a beer!!

  20. Dave said:

    Over the last five years I have been conned out of hundreds of pounds by engineers trying to fix this problem and they have replaced just about every part that can be replaced its got to be a great money maker for anyone in the boiler/gas safe trade.
    I done the hair drier thing and it worked a treat hats off to the gentleman who posted the work round I bet the gas engineer hates you. Strange that now most people are aware this fault and how to fix that Viessmann have come out with a mod if it was a car it would have been recalled and modified free of charge

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