Siemens LME11 / LME21 / LME22 fault 3: Air pressure switch faulty
Three red blinks mean the burner control did not get the air pressure signal it expects. The data sheet lists loss of the air pressure signal after the specified time (t10), the air pressure switch welded in its normal (no-load) position, and a fault during valve proving when an LDU11 valve proving unit is used.
Diagnose this with AI
Try Free Boiler Diagnostic Upload a keypad photo, the fault history or a parameter export from your Siemens LME11 / LME21 / LME22. InduQT compares the evidence and suggests the next useful checks.
When it happens
- After the specified time for the air pressure signal (t10), once the fan has started
- During prepurge or burner operation, if the air pressure signal is lost
- During valve proving (only with LDU11)
Where it sits in the burner
Supervised functions: Combustion air proving, Purge, Valve proving and proof of closure.
| Component | Signals to look at | How it fails |
|---|---|---|
| Combustion air proving switch | Combustion air proving switch, Combustion air pressure | Pressure or position switch out of adjustment; Welded, jumpered or miswired contact; Restricted combustion air path |
| Combustion air fan (FD / ID) | Combustion fan running, Combustion fan speed, Combustion air pressure, Fan motor current, Fan vibration | Restricted combustion air path; Fan motor or drive fault; Bearing wear or misalignment |
| Main safety shutoff valves | Main fuel valve output, Valve proof of closure | Valve seat leakage or slow closing; Welded, jumpered or miswired contact; Fuel valve not opening fully |
| Burner management controller | Burner sequence state, Controller supply voltage | Controller internal fault; Supply voltage, frequency or grounding problem; Configuration or commissioning data changed; Electrical noise on signal wiring |
Likely causes and checks
1Fan not building air pressure
- Points to it
- Fan does not start, runs slowly or is noisy; motor or capacitor recently replaced.
- Check
- Watch the fan at start; check the fan motor, its capacitor and the motor output of the burner control.
2Air path or sensing line blocked
- Points to it
- Fan runs but the switch does not make; dirty fan wheel, blocked air inlet or flue, kinked or wet sensing tube.
- Check
- Measure the air pressure at the switch sensing point with the fan running and compare with the switch setting; clean the fan, inlet and sensing tube.
3Air pressure switch set too high or failing
- Points to it
- Intermittent 3 blinks, worse at some air settings or on windy days.
- Check
- Compare the switch setpoint with the burner maker’s setting and the measured pressure; replace a switch that does not switch cleanly.
4Air pressure switch welded in normal (no-load) position
- Points to it
- The switch never changes over, even with good air pressure.
- Check
- With power isolated, check the switch changeover contacts for continuity in both states.
5Fault during valve proving (LDU11 only)
- Points to it
- An LDU11 valve proving unit is fitted and the lockout comes before the fan sequence completes.
- Check
- Check the LDU11 indication and have the gas valves tightness-tested according to the LDU11 documentation.
Telling it apart
- 3 blinks is air pressure not proven (or lost) when it is needed. 5 blinks is the opposite: the switch already in its working position when it should be at rest.
- Per the data sheet, a simulated air pressure failure during operation gives an immediate non-alterable lockout, while a switch welded in its no-load position locks out about 180 seconds after t10 has elapsed. The time to lockout helps tell a failing switch from a stuck one.
Evidence to collect
- Blink count after pressing reset for more than 3 seconds
- LME type (t10 is 5 s on LME21… and 3 s on LME22… types)
- Air pressure at the switch with the fan running, and the switch setting
- Whether the fan runs and the time from start to lockout
- Whether an LDU11 is fitted
Work on fuel trains, burners, pressure vessels and electrical equipment must be done by qualified personnel following the manufacturer’s documentation and local codes. Never bypass a safety device or interlock.