An operating, gas-fired water tube boiler (57 tonnes/hr, 24 bar) located in an urban heating plant stopped subsequent to an electrical power outage. The other boiler, which was also in service at the time, did not shut down. Following malfunction of the pilot valve on the spring-activated expansion device for limiting ignition circuit pressure, the technician tried 3 times to restore service before pressurisation by adjusting a tap opening and obtaining permission for activation from the control console. While shifting from start-up to normal mode, an explosion occurred shortly after opening the main feed valve. The combustion chamber was totally destroyed, and the roof along with one building wall were damaged, yet no victims were reported.