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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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  • #31 13130096
    Vitodens
    Level 9  
    Posts: 11
    Rate: 9
    _uki_ wrote:
    at worst, the motherboard



    It could always be worse - to replace the heat exchanger.

    After firing the furnace, the exchanger heats up very quickly and strongly in two places. These are places where it was suspected that the gaps between the coils are too large. The rest of the exchanger barely warm. It appears that the coil may be deforming, and the cause was probably the freezing of the system.

    Theoretically the problem solved.

    PS
    I think I will add a little and buy a new furnace, at least I will have a warranty and almost a set of spare parts.
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  • #32 21325771
    benrahtjohn
    Level 1  
    Posts: 1
    Hi all, today I beat the F1 on the Viotodens 200 WB2B. I did the exchanger service including removal, pump service, 3-way valve, all hoses, vessel etc. 5 hours, vented through a faulty vent!!!! Such a trivial thing to do and it gave a hard time. Good luck, hope it's useful.
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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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