- Boilers
- Gas boiler
345
Viessmann 345 on a Vitodens boiler: temperature limiter tripped
Risk level
Medium
Proceed carefully and stop if the check is unclear.
Urgency
Medium
Quick answer
Vitodens 100-W code 345 means the temperature limiter has responded: the burner locks out and restarts on its own once the boiler cools. Check the pressure on the display and call a technician if the message keeps coming back.
Safety first
Stop and check this first
- If you smell gas while code 345 is showing, do not touch switches or plugs, avoid flames and sparks, close the gas valve, open windows, leave the home and call the gas supplier and service company from outside.
- Even though the Vitodens restarts by itself after 345, the limiter tripped because of excess temperature: if it repeats, stop forcing operation and arrange an inspection.
Safe checks you can perform
- Record when 345 appears
- Read the system pressure on the display
- Wait for the automatic restart
- Professional hydraulic inspection
Leave these tasks to a qualified professional
- Do not attempt this professional task: Check fill level and expansion vessel
- Do not attempt this professional task: Inspect pump, diverter valve and heat exchanger
What a technician must be able to justify
Technician visit
These are the steps they must carry out and show you. Tick them as they work: if steps are skipped, do not accept a part swap.
These are not instructions for opening or handling the equipment yourself. They are the checks that should support a technical conclusion.
Check fill level and expansion vessel
What they doA qualified technician checks the fill level and sets the expansion vessel pre-charge to the pressure the system requires.
They must show youthe measured system pressure and vessel pre-charge.
In writingthe values found, adjustments made and how they relate to 345.
Inspect pump, diverter valve and heat exchanger
What they doThe technician checks pump flow, 3-way diverter valve operation and scale in the cylinder or plate heat exchanger.
They must show youthe flow achieved and the diverter valve movement.
In writingwhich component limited heat removal and the follow-up test.
Before accepting a part or repair
Nothing has been shown yet. Do not accept a part swap based on the code alone.
Follow these steps
Note: Follow the documented order. An error code identifies the affected system, but it does not prove by itself which part has failed.
Record when 345 appears
- Who:
- you, watching the display.
- How:
- note 345 and whether it appeared while heating radiators or while you were using hot water; that difference guides the technician.
- What matters: telling whether 345 is linked to central heating or hot water on the Vitodens.
- If the code persists:
- if 345 keeps appearing in the same mode, tell the technician exactly that.
- If it has cleared:
- if 345 has not returned, keep the note with the date and what the boiler was doing.
- Evidence to keep:
- note of 345 with the time and active mode.
Evidence: Viessmann Vitodens 100-W/111-W, instrucciones de servicio para el usuario de la instalación (6135869 ES 03/2026) · Viessmann Vitodens 100-W B1HF, B1KF, installation and service instructions for contractors (6167586 GB 06/2026)
Read the system pressure on the display
- Who:
- you, using the button that shows the pressure gauge symbol.
- How:
- the Vitodens user manual lets you view the pressure on screen; below 1.0 bar it says to add water or inform your installer, who should tell you the required pressure.
- What matters: knowing whether low pressure may be behind 345 on your Vitodens.
- If the code persists:
- if the pressure is correct and 345 returns, do not hunt for the cause yourself: call the technician.
- If it has cleared:
- if the pressure was low and 345 stays away after correction, note the reading and how long it holds.
- Evidence to keep:
- photo of the displayed pressure next to code 345.
Evidence: Viessmann Vitodens 100-W/111-W, instrucciones de servicio para el usuario de la instalación (6135869 ES 03/2026) · Viessmann Vitodens 100-W B1HF, B1KF, installation and service instructions for contractors (6167586 GB 06/2026)
Wait for the automatic restart
- Who:
- you, without repeated reset attempts.
- How:
- Viessmann states that with 345 the burner is enabled again once the boiler has cooled down; wait for that restart rather than forcing the appliance.
- What matters: checking whether the Vitodens resumes service by itself after 345.
- If the code persists:
- if 345 comes back again and again, switch off only as the manual allows and call the technician.
- If it has cleared:
- if the boiler restarts and 345 does not repeat, mention the episode at the next service.
- Evidence to keep:
- time to restart and number of times 345 appeared.
Evidence: Viessmann Vitodens 100-W/111-W, instrucciones de servicio para el usuario de la instalación (6135869 ES 03/2026) · Viessmann Vitodens 100-W B1HF, B1KF, installation and service instructions for contractors (6167586 GB 06/2026)
Professional hydraulic inspection
- Who:
- qualified heating and gas technician.
- How:
- checks fill level, expansion vessel pre-charge, pump, 3-way diverter valve and, where relevant, the plate heat exchanger, following the table for 345.
- What matters: the technician finding what prevents heat removal on your Vitodens.
- If the code persists:
- the technician should correct the cause and run a full cycle without 345 returning.
- If it has cleared:
- ask for the test to be recorded even if 345 does not appear during the visit.
- Evidence to keep:
- job sheet with vessel pressure, flow and the valve checked for 345.
Adjusting the expansion vessel, venting the pump or removing the Vitodens diverter valve is technician work.
Evidence: Viessmann Vitodens 100-W B1HF, B1KF, installation and service instructions for contractors (6167586 GB 06/2026)
Still not solved
If the error persists, stop here. See what the technician must do and show you.
Where this code applies
A code is meaningful only inside the right product and version context.
Primary scope
- System
- Boilers
- Brand
- Viessmann
- Product type
- Gas boiler
Known code variants
- Viessmann 345
- Viessmann error 345
- Vitodens 100-W 345
- Vitodens 345
- Vitodens fallo 345
What it means
In the Viessmann service table, 345 appears when the temperature limiter responds. The burner is locked out and enabled again automatically after the appliance cools, with no reset needed. The manufacturer links the message to inadequate heat transfer and calls for checks of system fill level, expansion vessel pre-charge pressure, circulation pump flow and the 3-way diverter valve. If it happens during hot water heating, the cylinder or plate heat exchanger should be checked for scale and contamination.
Warnings and stop conditions
- Criticalsource verified
If you smell gas while code 345 is showing, do not touch switches or plugs, avoid flames and sparks, close the gas valve, open windows, leave the home and call the gas supplier and service company from outside.
Evidence: Viessmann Vitodens 100-W/111-W, instrucciones de servicio para el usuario de la instalación (6135869 ES 03/2026)
- Highsource verified
Even though the Vitodens restarts by itself after 345, the limiter tripped because of excess temperature: if it repeats, stop forcing operation and arrange an inspection.
Evidence: Viessmann Vitodens 100-W/111-W, instrucciones de servicio para el usuario de la instalación (6135869 ES 03/2026) · Viessmann Vitodens 100-W B1HF, B1KF, installation and service instructions for contractors (6167586 GB 06/2026)
Symptoms and causes
Probable causes · in order
- Low system water level or pressurePossible
Viessmann includes the heating system fill level and the diaphragm expansion vessel pre-charge pressure among the checks for 345.
Evidence: Viessmann Vitodens 100-W B1HF, B1KF, installation and service instructions for contractors (6167586 GB 06/2026)
- Insufficient water flowPossible
The table asks whether the circulation pump delivers enough flow and calls for venting the system, because heat cannot be carried away without circulation.
Evidence: Viessmann Vitodens 100-W B1HF, B1KF, installation and service instructions for contractors (6167586 GB 06/2026)
- Faulty 3-way diverter valvePossible
Operation of the 3-way diverter valve is one of the measures for 345, since it routes heat between central heating and hot water.
Evidence: Viessmann Vitodens 100-W B1HF, B1KF, installation and service instructions for contractors (6167586 GB 06/2026)
- Scale or dirt in the hot water circuitPossible
If 345 occurs during hot water heating, Viessmann directs a check of the cylinder or plate heat exchanger for scale and contamination.
Evidence: Viessmann Vitodens 100-W B1HF, B1KF, installation and service instructions for contractors (6167586 GB 06/2026)
Virtual technician
Does 345 still appear after these checks?
Answer a few questions to narrow down the likely cause and the safest next action. Guidance only - it never replaces a professional.
Sources and technical references
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