Follow these steps when cleaning alarms:
1. Turn off the mains power to the alarm – green light should go out
2. With the thin nozzle attachment, vacuum around the vents
4. Turn the mains power back on – green light should be on
Alternatively, using a hairdryer on its coolest setting, blow out all the grooves and vents on the alarm then repeat the above steps.
found at http://www.aico.co.uk/How-do-I-maintain-alarms.html