Clogged flue chamber causes explosion in hot water boiler combustion chamber

A clogged flue chamber created a buildup of unburnt (raw) gas, when ignited caused an internal explosion in the combustion chamber. This caused excessive internal heat and damaged the boiler external control wiring.

A few days prior to the incident the combustion burner door was lifted off to access the motor fan blades for inspection as the air flow combustion efficiency had been noticeably reduced. At this time the ignition electrode probe was removed to be cleaned but may have been incorrectly installed by the operator. It was also noted by the operator that there was a build-up of soot on the fire-tubes openings, but no maintenance was completed to clean the tubes. The combustion burner door was then closed up and the boiler was returned to service. A few days later the combustion incident occurred. The way the ignitor probe was reinstalled could have allowed large amounts of unburnt gas into the combustion chamber before ignition could occur. Once ignited, the pressure created within the chamber had nowhere to go because of the clogged fire tubes. This created the blow back of the firing gases causing the damage to the internal canister and external control wiring.

Upon investigation the area within the internal combustion chamber where the fire tubes are located were found heavily clogged with soot.
These areas of high soot potentially reduced the of ignited gas flow into the fire tubes.

Also the ignitor probe was noted to be out of position where it created time for gas to flow without being ignited. When the gases were ignited it likely created an internal explosion in the combustion chamber.