Compact 18-24 alarm A03: boiler or pellet hopper overheating
This article elaborates on the information given in the article 'Standard Alarm Codes and Fault Finding on MCZ RED Boilers', and should also be read in conjunction with the installation and user guide for the product.
There are two principle reasons for alarm A03 bring raised: Either the boiler water temperature has reached 95 degrees, or the pellet hopper temperature has reached 110 degrees. In either incidence, the effect will be that the boiler will be immediately shutdown to prevent either the water reaching boiling point, or the pellets in the hopper from igniting.
The alarm is triggered by one of two thermostats: One located on the top right front corner of the boiler (95 degrees) and one attached to the base of the pellet hopper, close to the rear of the brazier (110 degrees). The thermostats are contact switches and are normally closed. If they detect a temperature that exceeds their set point, the switch opens and it is the breaking of this circuit that triggers the alarm.
The thermostats are of the 'Klixon' type or similar and are described in MCZ literature as 'Clickson'. Appearance is as follows, or similar:
Provided that the underlying cause of the boiler / hopper reaching these overheat temperatures has been identified and resolved, and if the alarm keeps tripping when there is evidently no excessive temperatures, the following checks can be done on the components themselves.
Firstly, on the current 2012 version Compacts, the thermostats will have a non-self resetting cut out button - see photos above (also applicable on some 2011 models). If resetting this has no effect, or was not out, then for each thermostat in turn, remove the wires from the two terminals and bridge across the wires. If the stove is now restarted and the alarm is not raised again, the relevent component is at fault and will need replacing. Similarly, with the wires disconnected from each sensor, perform a continuity test across the sensor: If the reading shows open circuit, the sensor is faulty and need replacing.