For two boilers, a bad wired connection
The boiler (in the photos below) is one of two boilers in the boiler room. The temperature controls on the No. 2 boiler failed because it was wired in series with boiler No. 1. When the No. 1 boiler was electrically disconnected it also disconnected the temperature controls for boiler No. 2. This caused boiler No. 2 to run continuously.
Boiler No. 2 continued to run until it reached a temperature that melted the solder in the joints of the piping above the boiler. This caused the piping to fail. The licensed engineer heard the failure of the piping and immediately secured boiler No. 2.