Viessmann F5 error (flow sensor open circuit) — complete guide

The error code F5 (with variants F5 0, F5 1, F5 8, F5 9) indicates an open circuit on the flow temperature sensor (heating flow or DHW flow, depending on sub-code). The boiler stops receiving a valid signal and goes into safety lockout.

Symptoms

• Boiler locked out, F5 shown on the Vitotronic
• No heating or no hot water, depending on which sensor is affected
• Sometimes appears after a recent install or service intervention
• Can return after temporary reset — signals wiring oxidation

F5 variants

F5 0 — Short-circuit of the sensor (logically the opposite of open-circuit, but same underlying fault).
F5 1 — Short-circuit of the output sensor (DHW-specific on certain models).
F5 8 — DHW sensor fault — defective NTC.
F5 9 — DHW output sensor fault.

Affected Viessmann models

• Vitodens 100, 100-W, 111-W
• Vitodens 200-W, 222-W
• Vitodens 300, 300-W
• Vitopend 100

Common causes

• Broken or oxidised sensor wiring — loose connector or moisture (40%)
• Failed NTC sensor — ageing, typically after 8–10 years
• Disconnected plug — after vibration or an intervention
• Heavy limescale around the sensor — distorts reading and can break signal
• Control board fault on the sensor read side (rarer)

Initial diagnosis

1. Identify whether the fault is on heating flow or DHW (F5 alone = heating flow)
2. Was the fault sudden or gradual?
3. Any recent intervention (service, descaling)?
4. For boilers 10+ years old: sensor replacement is the most likely cause

Intervention costs

• Diagnostic + sensor test: on request
• NTC sensor replacement: on request
• Wiring / connector repair: on request
• Descaling + sensor replacement: on request

Frequently asked questions

Can I keep using the boiler with F5?

No — the flow sensor is safety-critical. The boiler stays in lockout until the fault is resolved. No bypass is possible.

Can F5 hide a water leak?

Rarely — a leak from a nearby radiator could moisten the sensor wiring and trigger F5. A professional check covers this.

How long does a Viessmann NTC sensor last?

8–15 years typically. Post-2018 models use more robust sensors — longer expected life.

Need an English-speaking Viessmann specialist in Luxembourg?

Weber & Fils is certified on all Viessmann boilers and heat pumps installed in Luxembourg (Vitodens 100–300, Vitopend, Vitocal, Vitola). Their technicians speak English fluently and stock the most common parts.

For quotes and emergency call-outs: weberetfils.lu/en or call +352 20 60 22 22 — 24/7.

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