Aged rooftop heating unit damaged by small ignition explosion

Incident Investigation

Aged rooftop heating unit damaged by small ignition explosion

February 1, 2018


Reference Number:


Incident overview

Small explosion on roof on building from rooftop unit.

Regulated industry sector



Nanaimo rooftop unit

Investigation conclusions

Site, system and components

Roof top unit supplying heat to building. Thermostat in lobby calls for heat, rooftop unit starts the heating cycle, pilot lights, if flame present pilot flame sense sends electrical pulse back to control module which then opens main gas valve allowing gas to flow to burners, pilot flame ignites main burner. When thermostat reaches the required temperature setting the call for heat signal is turned off shutting down main burner and pilot extinguishing all flame.

Failure scenario(s)

Call for heat from thermostat called for pilot to light, main gas valve opened, main burners did not light immediately allowing a small gas build up, when burners finally ignited a small explosion occurred.

Facts and evidence

The burner management system was working correctly, the combustion chamber had developed numerous rust holes due to deterioration where gas and or products of combustion could escape during light off or operation led to a buildup of explosive gas
outside of the combustion chamber.

Causes and contributing factors

Improper maintenance and age of equipment


  • Injury
    • Qty injuries: 0
    • Injury description: 
    • Injury rating: Minor
  • Damagew
    • Damage description: Small delayed ignition explosion, minor damage to unit
    • Damage rating: Minor

Incident rating


297.35 KB