Siemens LME21 blinks 5 times and locks out after a minute, what to check?
My Siemens LME21 gas burner control blinks 5 times and locks out after about a minute every time I try to start the burner. The fan is off, but the air pressure switch seems to be made. What should I check first?
A Siemens LME21 gas burner control blinking 5 times indicates an 'Air pressure switch time supervision' fault. This means the air pressure switch was already in its working position when the burner control expected it to be at rest, preventing startup. A non-alterable lockout occurs after 65 seconds if this precondition is not met. This fault is distinct from 3 blinks, which signifies the switch not making when air is needed.
Likely causes and checks
1Air pressure switch contacts welded in working position
- Points to it
- The air pressure switch contacts are stuck in the closed (made) position, even when there is no air pressure from the fan. The controller expects the switch to be open before the fan starts.
- Check
- With power isolated and the fan stopped, check the switch contacts for continuity. If continuity is present, the switch is welded and needs replacement.
2Short circuit on the air pressure switch wiring
- Points to it
- A short circuit in the wiring can cause the controller to 'see' the air pressure switch as made, even if the switch itself is open. This can be due to damaged cables or recent work on the switch or its plug-in base.
- Check
- A qualified electrician should check the air pressure switch connecting lines (terminals 3, 6, and 11) for short circuits with power isolated.
3Air still moving with the fan off
- Points to it
- External factors like a strong chimney draught, wind, or another fan sharing the flue can create enough air movement to activate the air pressure switch even when the burner's fan is off. This can be an intermittent issue.
- Check
- Measure the pressure at the switch sensing point with the fan off and compare it to the switch's set point.
4Fan running when it should be off
- Points to it
- The fan is running during the waiting time (tw) when it should be off, causing the air pressure switch to be made prematurely. This could be due to a fault in the fan motor supply or an external contactor.
- Check
- Check the fan motor supply and any external contactor that might be keeping the fan running when it should be off.
Next check: First, isolate power to the burner and check the air pressure switch contacts for continuity. If they show continuity with the fan off, the switch is welded and needs to be replaced.
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