I’ve always had concern that my boiler would overheat at night and I wouldn’t know about it. I have a power fail alarm but that doesn’t cover some problem with the boiler system. Some years ago, I had the spring coupling break and jam/stall the circulator. Fortunately, it was during the day and I was there when it happened. For over temp, I have a Honeywell L400B aquastat that provides a closed contact (I set the temp at 220 degrees). My boiler is in the basement and I wanted an alarm upstairs when I was sleeping. My simple solution, that someone has probably all ready done, was to use a doorbell alarm. The one I purchased was a Fimei wireless (to 1000 Ft.) doorbell kit. I modified the pushbutton by removing the circuit board and soldering two wires to the button switch contacts. The two wires connect to the Honeywell L400B over temp aquastat. I also clipped out the LED, that illuminated the pushbutton, since I didn’t need it and I didn’t want any unnecessary current draw from the 12V battery in the pushbutton circuit. The wireless transmission works great from the basement up to my 2nd floor bedroom. The alarm has 4 volume settings and is plenty loud when on maximum. John M.