This investigation report concerns a 1960 Weil McClain hot water heating boiler that had a BTU input of 625,000. The explosion outlined below took place in February 2013.
The subject of the accident, the 1960 Weil McClain boiler, was originally connected with another boiler of the same size. In 2000, the second boiler was removed. In 2001, the facility created two separate heating systems for the buildings. This allowed them to install two additional, smaller boilers, on a separate system, to heat a portion of the building while the 1960 Weil McClain boiler alone heated the rest of the building. After the change, because the separated boiler systems were less than 750,000 BTUs, this location was no longer required under Minnesota law to have a licensed engineer operate the boiler, and not required to have an inspection by a commissioned boiler inspector.
According to the investigation, the heating system for the subject boiler had a leak somewhere, and employees were constantly adding water to the system. One of the maintenance personnel believed he had fixed the leak so he isolated the make-up water to the boiler. In doing so it is also believed that the low-water cutout was isolated from the boiler. This allowed the low-water cut-off to remain full of water. The boiler system still had a leak and the boiler continued to slowly drain.
Even after the boiler was empty of water, the boiler continued to fire. It is undetermined if a zone valve opened, but water made it back to the now red-hot boiler and caused the pressure boundary to fail. This resulted in a catastrophic explosion causing extensive damage to the building.
While there were no injuries because of the explosion, the 10 children that were at the daycare facility left the building only 40 minutes prior to the blast.