Viessmann Furnaces Error Codes

F1

Burner lockout, furnace will not start, red fault LED on control, no flame established

Ignition failure / no flame signal

F10

Low system pressure warning, boiler may not fire or may shut down frequently

Low water pressure / insufficient system fill

F11

High system pressure fault, pressure gauge near or above 3 bar, safety valve may discharge water

Overpressure / expansion vessel or fill fault

F12

Fan speed error, boiler does not ignite, fan either not running or running at incorrect speed

Combustion fan / speed feedback fault

F13

Condensate fault, boiler shuts down with drain or condensate error, possible gurgling noises

Condensate drainage blockage

F14

Gas valve error, boiler does not fire, control indicates gas valve or modulation fault

Gas valve control / modulation fault

F15

Communication error between main control and user interface or external Vitotronic module

Control bus communication fault

F16

Domestic hot water (DHW) temperature sensor error on combi or indirect systems

DHW sensor fault

F17

Flow switch or flow sensor error on combi boiler, no hot water despite demand

DHW flow detection fault

F18

Power supply or internal voltage error, boiler may reboot or lock out intermittently

Internal power supply / low voltage fault

F19

Sensor plausibility error, temperature readings inconsistent or out of range

Sensor plausibility / multiple sensor disagreement

F2

Furnace attempts to start but shuts down quickly, control shows flame error, intermittent heating

Flame detection fault / unstable flame signal

F20

Safety chain open, boiler will not start, safety loop error

Safety interlock / limit circuit open

F22

Dry fire / low water content detected, boiler shuts down quickly after start

Low water content / dry firing protection

F23

Flow/return temperature difference too high, boiler shuts down to protect heat exchanger

Insufficient circulation / high delta-T fault

F24

Rapid temperature rise detected, boiler shuts down shortly after burner starts

Overtemperature / poor circulation fault

F25

Flue gas temperature too high, boiler shuts down with flue overheat fault

Flue gas overtemperature / heat exchanger fouling

F26

Gas valve modulation error, boiler may run only at low power or not at all

Gas valve modulation / actuator fault

F27

Flame detected when there should be none, boiler locks out for safety

False flame detection / ionization fault

F28

Ignition attempt fails, no flame established, multiple retries then lockout

No ignition / gas supply fault

F29

Flame lost during operation, boiler shuts down with flame failure code

Flame dropout / unstable combustion

F3

Furnace locks out with overheat indication, supply temperature extremely high, burner stops

High-limit temperature trip / safety temperature limiter (STB) activated

F30

Room thermostat or external demand signal error, boiler does not respond to heat calls

External control / thermostat fault

F4

No ignition, control shows ignition fault, burner does not light at all

Ignition transformer or ignition circuit failure

F5

Furnace shuts down with flue gas sensor error, may run briefly then lock out

Flue gas temperature sensor fault

F6

Furnace displays supply temperature sensor error, unit may not fire or may run erratically

Flow temperature sensor fault

F7

Return temperature sensor error, boiler may short-cycle or shut down

Return temperature sensor fault

F8

Outdoor temperature sensor error on systems with Viessmann weather-compensated control, heating curve not working

Outdoor sensor circuit fault

F9

Furnace shows flue gas recirculation or combustion air fault, may shut down with safety lockout

Combustion air / flue gas recirculation fault

Symptom

Boiler display is blank or intermittently turns off, but mains power is present

Control power / display fault

Symptom

Boiler locks out frequently during windy conditions, especially with sidewall flue

Wind-induced flue disturbance

Symptom

Condensate pipe outside frequently freezes in cold weather, causing boiler shutdowns

Frozen condensate discharge

Symptom

Domestic hot water temperature fluctuates or is too hot/cold on a Viessmann combi or indirect system

DHW temperature control issue

Symptom

Furnace emits unusual smells (burning dust, metallic, or flue gas odor) during operation

Odor / combustion or contamination issue

Symptom

Furnace is running but some radiators remain cold or only partially warm

Hydronic balancing / circulation issue

Symptom

Furnace makes loud banging or knocking noises when starting or stopping (kettling)

Boiler kettling / localized boiling in heat exchanger

Symptom

Furnace runs but house does not reach set temperature, long run times with poor heat output

Underfiring / incorrect boiler output

Symptom

Furnace short cycles (frequent on/off) without maintaining stable temperature

Short cycling / control or sizing issue

Symptom

Furnace vibrates excessively or makes humming/rumbling noises during operation

Mechanical vibration / mounting issue

Symptom

Radiators get hot even when there is no call for heat from the thermostat

Unwanted heating / valve or control fault

Symptom

System pressure drops to zero over days or weeks, requiring frequent topping up

Chronic pressure loss / leak or expansion fault

Symptom

Water leaking from the bottom of the Viessmann furnace or from attached pipework

System water leak / condensate or heating circuit leak