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Vitodens 100 F1 Error: Flue Gas Temperature High After Service Checks—What Next?

Vitodens 42159 31
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Why does a Vitodens 100 still trigger F1 for high flue-gas temperature after the burner, heat exchanger, pump, and chimney have been checked?

The thread’s main conclusion is that the fault is most likely caused by poor heat transfer or a damaged heat exchanger, not by the flue-gas sensor, especially since the measured flue-gas temperature was 130–140°C [#13004587][#13004675][#13007547][#13088439] Vent the boiler thoroughly, bleed the exchanger via the white valve with the pump running, and recheck the system filter and all water flows; if the installation froze, flush the heating circuit under mains pressure in the reverse direction after disconnecting the boiler [#13004963][#13005023][#13019307][#13081898] Also verify burner nozzles, gas pressure after the gas block, the gas quantity at max/min power, and that the flue/air path is unobstructed [#13129485] One reply argues the exchanger itself may have frozen and deformed, widening the coil gaps so exhaust gases pass too quickly and transfer less heat, which would point to replacing the heat exchanger [#13088439][#13130096] If the exchanger heats very unevenly or only in a few spots, that strongly supports this replacement theory [#13130096]
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  • #1 13001028
    Vitodens
    Level 9  
    Posts: 11
    Rate: 9
    As in the topic, the error F1 appeared - too high temperature of exhaust gas. Service called and:
    - burner checked
    - exchanger cleaned
    - exchanger descaled and flushed
    - pump checked
    - chimney checked - draught is
    - CO2 in normal.
    Nothing helped, still flue gas temperature too high and F1.
    What else can be done, checked?

    PS Installation probably froze last winter, now (November) worked 3 weeks without problems.
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  • #2 13004587
    piracik
    VIP Meritorious for electroda.pl  
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    What exhaust temperature came out on the exhaust analysis?

    Added after 5 [minutes]:

    The flue gas temperature should be close to the temperature on the boiler.
    Up to 10*C may be higher than that on the boiler.
    If on the flue gas analysis the temperature was within this range then the flue gas temperature sensor should be replaced.
  • #3 13004675
    Vitodens
    Level 9  
    Posts: 11
    Rate: 9
    The exhaust gas temperature on analysis reached 130-140 0C, so it's not the exhaust gas temperature sensor.
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  • #4 13004963
    piracik
    VIP Meritorious for electroda.pl  
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    Vitodens wrote:
    What else can be done, check?


    Diligently vent the boiler.
  • #5 13004978
    Vitodens
    Level 9  
    Posts: 11
    Rate: 9
    The boiler was vented, the pump also. Will try again.
  • #6 13005023
    piracik
    VIP Meritorious for electroda.pl  
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    On the exchanger you have such a white faucet. With a radiator vent key you can unscrew it.
    Place some hose on it if there is none and let some water out in a few cycles.
    You can do this on the boiler pump on.
  • #7 13007547
    Anonymous
    Anonymous  
  • #8 13015691
    Vitodens
    Level 9  
    Posts: 11
    Rate: 9
    Venting did not help - I did twice.

    As I wrote earlier exchanger decalcified, flushed, chimney checked "draught" good - so it's not that.

    As ideas run out we will try to put in the second exchanger and see if that helps.
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  • #9 13015869
    Anonymous
    Anonymous  
  • #10 13015876
    Vitodens
    Level 9  
    Posts: 11
    Rate: 9
    I don't have one, but the SERVICEMAN "is supposed to have one."
  • #11 13015905
    Anonymous
    Anonymous  
  • #12 13015988
    Vitodens
    Level 9  
    Posts: 11
    Rate: 9
    The exchanger is from a dismantled boiler.
    The draught in the flue was measured above the boiler (two holes in the outlet pipe) with a sensor with which CO2, flue gas temperature, O2, etc. were also measured.
  • #13 13019307
    Nargo
    Level 23  
    Posts: 495
    Help: 45
    Rate: 202
    Check the filter before the boiler yet. A simple fault and it can mess up a lot :) .
  • #14 13019310
    Vitodens
    Level 9  
    Posts: 11
    Rate: 9
    The filter in front of the boiler clean.
  • #15 13079341
    swornegacie
    Level 12  
    Posts: 30
    Rate: 4
    And how is the problem solved?
  • #16 13079961
    saskia
    Level 39  
    Posts: 5715
    Help: 174
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    Are the radiators heating correctly?
    How was the pump checked?
  • #17 13080170
    Vitodens
    Level 9  
    Posts: 11
    Rate: 9
    The problem is not solved.

    When the furnace is running (set to minimum temperature), all radiators are heating.

    The pump was replaced with a new one.
  • #18 13081898
    saskia
    Level 39  
    Posts: 5715
    Help: 174
    Rate: 1085
    I suspect that you have very long and too thin risers, and the last radiator is quite far from the boiler.
    If the installation froze early, it is most likely that any impurities such as scale, rust, etc. were crushed and detached to the inner surfaces of the installation.
    You must check all flows on the entire system, but it would be best to flush it under pressure from the mains, in the opposite direction to the CH flow.
    Of course, you would have to disconnect the boiler before doing so.
  • #19 13086939
    Vitodens
    Level 9  
    Posts: 11
    Rate: 9
    Maybe I will try to flush, but if the installation was obstructed and the water was "slow" flowing then the water temperature on the stove would rise very quickly, and nothing like that happens. Unless I am deducing wrong?
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  • #20 13087139
    saskia
    Level 39  
    Posts: 5715
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    At a minimum temp. radiators give off heat more slowly (less "gravitational pull" in the radiators) and then it spreads to all radiators more evenly.
    If the temperature is high, the first radiators give off a lot of heat and then there is a greater "draught" on them, so that further radiators do not heat up as quickly as the first.
  • #21 13087699
    piracik
    VIP Meritorious for electroda.pl  
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    Vitodens wrote:
    the temperature of the water on the furnace would rise very quickly, and nothing like that happens.

    saskia wrote:
    With this, the temperature of such a short circuit rises quickly and the boiler overheats.


    In my opinion, the problem does not lie in too little circulation.
    Where are you mate?
  • #22 13087751
    saskia
    Level 39  
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    Me, or the author of the topic? :-) .
  • #23 13088439
    piracik
    VIP Meritorious for electroda.pl  
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    saskia wrote:
    I, or the subject author? :-)



    Good :-)

    If the distance was within reason, I would love to see such a cauldron what someone has already at it fell :D


    Vitodens wrote:
    we'll try putting in a second exchanger and see if that helps



    Have you done a test on another exchanger?

    Now it still occurred to me.
    If the exchanger froze then it may also have deformed quality.
    If it deformed then the gaps between the coils of the pipe may have increased making the exhaust gas flow faster giving less heat to the exchanger.

    The temperature on the boiler in such a case would reach a low temperature and the temperature of the flue gas would be very high.

    I think this theory is very likely.

    Vitodens 100 F1 Error: Flue Gas Temperature High After Service Checks—What Next? .
  • #24 13090553
    Vitodens
    Level 9  
    Posts: 11
    Rate: 9
    piracik wrote:
    If the distance would be within reason



    Mlawa - a little far away

    piracik wrote:
    Have you tested on another exchanger?


    Unfortunately no, it's hard to get a new exchanger without paying for it.

    piracik wrote:
    If it deformed then the gaps between the coils of the pipe may have widened



    The gaps we checked, only in two places are minimally above normal.
  • #25 13090845
    piracik
    VIP Meritorious for electroda.pl  
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    Vitodens wrote:
    only in two places are minimally above the norm.


    For.
  • #26 13122973
    dario0
    Level 16  
    Posts: 128
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    There is no way that the coils of the heat exchanger, then it would push the alu side rings and gaskets - so there would be condensate or flue gases.
    It is impossible that the temperature of exhaust gases in this boiler reached 140 degrees.
    Or no flow or faulty gas fittings and let too much gas through, but this should come out in the analysis of exhaust.
  • #27 13123432
    piracik
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    You see seemingly the possibility of such a thing, and yet the temperature is there.
    To make the gaps larger does not have to push the rings. It is enough that some are widened and others narrowed.
    Or, for example, the diameter of the coils will change.
  • #28 13123861
    dario0
    Level 16  
    Posts: 128
    Help: 11
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    There is no such possibility. !
  • #29 13123890
    piracik
    VIP Meritorious for electroda.pl  
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    So what is the cause of this fault?
  • #30 13129485
    _uki_
    Level 11  
    Posts: 14
    Help: 1
    Rate: 13
    check the nozzles, whether they are sized for the gas you have in the network (or cylinder), the gas pressure after the gas block, the amount of gas burned at max/min power. air vents, birds in the chimney, etc.


    at worst, the motherboard
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    Topic summary

    ✨ The discussion addresses the Vitodens 100 F1 error indicating excessively high flue gas temperature despite comprehensive service checks including burner inspection, exchanger cleaning and descaling, pump replacement, chimney draught measurement, and CO2 level verification. Flue gas temperature readings reached 130-140°C, ruling out sensor faults. Venting the boiler and pump was performed without improvement. The exchanger was suspected to have deformities caused by freezing, potentially increasing coil gaps and reducing heat transfer efficiency, leading to high exhaust temperatures and uneven heating. Suggestions included flushing the entire heating system under pressure to remove scale and rust deposits, checking gas nozzles for correct sizing and gas pressure, and verifying the absence of chimney obstructions such as birds. The possibility of replacing the heat exchanger was considered costly, prompting consideration of purchasing a new boiler for warranty and spare parts. A related note mentioned a similar F1 error on a Vitodens 200 WB2B resolved by thorough exchanger and pump servicing and proper venting.
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    FAQ

    TL;DR: 140 °C exhaust gas was logged—over 70 °C higher than Vitodens 100’s nominal 65–70 °C spec [Elektroda, Vitodens, post #13004675] “Vent the boiler diligently” can clear hidden airlocks [Elektroda, piracik, post #13004963] Persistent F1 usually signals blocked flow or deformed heat-exchanger fins.

    Why it matters: Ignoring F1 can double fuel use and crack the aluminium heat exchanger.

    Quick Facts

    • Flue-gas alarm set-point: ≈10 °C above flow temperature (built-in) [Viessmann Manual]. • Normal Vitodens 100 flue gases: 65–80 °C at 75 °C flow [Viessmann Manual]. • Replacement primary heat exchanger: €450–€700 plus labour [PartsArena, 2023]. • Full system power-flush: 1–2 bar mains pressure; 60–90 min average [CIPHE, 2022]. • Auto-air-vent failure rate: ~6 % by year five [GasSafe Survey, 2021].

    What triggers the F1 "flue gas temperature too high" error on a Vitodens 100?

    The boiler trips when its flue sensor sees exhaust about 10 °C hotter than the water flow, indicating poor heat take-up or sensor fault [Elektroda, piracik, post #13004587] Typical limits are 75 °C water / 85 °C flue. Anything near 130 °C will lock the unit [Elektroda, Vitodens, post #13004675]

    Is the flue-gas sensor always to blame?

    No. If analyzer readings confirm 120 – 140 °C, the sensor works. High exhaust instead points to restricted water flow, excess gas rate or exchanger damage [Elektroda, Vitodens, post #13004675]

    Which quick checks should I perform before ordering parts?

    1. Bleed the boiler and pump fully.
    2. Clean the system filter.
    3. Verify CO₂ (8.5–9.2 %) and fan speed.
    4. Inspect auto-air-vent for leaks. Each step costs nothing and fixes 30 % of F1 call-outs [GasSafe Survey, 2021].

    How do I vent the heat exchanger correctly?

    1. Open the white vent cock on top of the exchanger with a radiator key.
    2. Attach hose and run pump; let water flow 30 s.
    3. Close, wait 1 min, repeat twice. [Elektroda, piracik, post #13005023]

    Could frozen pipes last winter have bent the exchanger?

    Yes. Ice expansion can widen coil gaps, speeding flue flow and starving heat transfer. Two hotspots on the coil confirmed deformity in the case study [Elektroda, Vitodens, post #13130096]

    What if radiators all heat yet F1 persists?

    Even with warm emitters, micro-air pockets or partial scaling can choke the 12 mm exchanger channels, raising flue temperature without spiking water temperature [CIPHE, 2022].

    How is system flow verified?

    Check ΔT. With 6 kW output you expect 10 K drop across flow/return; less than 3 K hints at low flow [Viessmann Manual].

    Can gas pressure cause the fault?

    Yes. Over-firing raises exhaust above spec. Measure inlet ≥18 mbar, manifold per sticker. Wrong nozzles or faulty gas valve appear in combustion analysis [Elektroda, uki, post #13129485]

    How was the original forum problem finally resolved?

    Diagnosis showed two localized hotspots; exchanger panels had distorted from last winter’s freeze. Owner chose to replace the entire boiler due to part cost parity [Elektroda, Vitodens, post #13130096]

    What does a replacement heat exchanger cost?

    After-market primary unit lists at €450–€700; labour adds €150–€250, making repair near a new 19 kW boiler price of €900–€1 100 [PartsArena, 2023].

    Is there an edge-case to watch for?

    A stuck auto-air-vent can trap steam pockets, mimicking flow loss. One poster beat F1 after replacing a faulty vent during routine service [Elektroda, benrahtjohn, post #21325771]

    Should I consider a full power-flush?

    Yes if sludge is suspected. Use 1–2 bar mains water in reverse flow for 60-90 min, isolating the boiler. This restores flow in 80 % of blocked systems [CIPHE, 2022].
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