DHW mode fan start up error
The S11 error code on Vaillant boilers indicates dhw mode fan start up error.
Vaillant S11 means: dhw mode fan start up error. The usual causes are fan motor failure (seized bearings or burned windings), fan speed sensor fault or wiring fault between fan and pcb. You can safely reset the boiler once — if S11 returns, do not keep resetting. A qualified heating engineer should diagnose the root cause.
What Does S11 Mean
The S11 code is shown by the boiler’s control board when it detects dhw mode fan start up error. The boiler then stops instead of continuing to run in that condition: the code is the appliance reporting a problem, not proof that the boiler is broken.
| What you see on the boiler |
|---|
| The display shows S11 and the boiler stops until the fault is cleared |
Common Causes
Causes documented in 1 technical sources.
| Cause |
|---|
| Fan motor failure (seized bearings or burned windings) |
| fan speed sensor fault |
| wiring fault between fan and PCB |
| air pressure switch fault (stuck open or closed) |
| blocked flue or air intake restricting airflow |
| condensate blocking the siphon or trap |
Safety Warning
S11 involves electrical or pressure-related components. Do not attempt repairs without proper qualifications. Ensure the boiler is electrically isolated before any inspection. If you notice water leaks, unusual sounds, or pressure anomalies, contact a qualified heating engineer immediately.
Parts & Prices
Prices shown are indicative. Actual prices and availability vary by region.
Step-by-Step Diagnosis
1. Listen for the fan — it should start before ignition. If silent, check power supply to the fan motor. 2. Spin the fan manually (with power off) — if it feels stiff or gritty, the bearings are worn. 3. Check the wiring connections between the fan and PCB. 4. Test the air pressure switch — with the fan running, the switch should close. Disconnect and test with a multimeter for continuity. 5. Check the flue for blockages — inspect the full length including the terminal. 6. Verify the condensate trap is clear. 7. Measure the fan speed signal at the PCB (Hall sensor output)
How to Fix It
Replace the fan motor assembly if bearings are worn or motor is not running
replace the air pressure switch if it is stuck or reading incorrectly
clear any flue or condensate blockage
repair or replace damaged wiring
if PCB is not providing the signal, replace the PCB
Refer to the Vaillant service documentation for the detailed procedure. Only a qualified engineer should carry out these repairs.
Diagnostic Tools
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Affected Models
Vaillant models that can display fault code S11:
| Model Family | Output Range | Frequency |
|---|---|---|
| All Vaillant models | — | Possible |
Frequently Asked Questions
Replace the fan motor assembly if bearings are worn or motor is not running. According to the technical documentation, this is the first check on the list for S11.
S11 is not classified as a high-severity fault, so the boiler can be reset once. If S11 comes back, stop resetting: the underlying cause is still there and needs a qualified heating engineer.