We have a relatively new furnace (but old pipes and radiators). It is maintained by:
Alex Magee & Sons Ltd
Occasionally the alarm will go off and it will shut down. Pressing the “Alarm Silence and Manual Reset” button on the control panel (see below; click on photo to enlarge) will reset the furnace and it should start working again.
According to Karen Ingham: “the reset button should be fine for now. If it keeps alarming, check the water level (it will be higher than the indicator, Alex Magee raised the level because the low water alarm kept going off). Sometimes it just does that. If it is doing it constantly, you will have to call them but I suspect as it cycles on and off, there may be just a blip in the power which is all it needs to give that message.”