Abstract: A fire extinguishing system, particularly, but not exclusively, for use in extinguishing chimney fires. The system comprises a container having a pressurized fire extinguishing product therein. A release nozzle is installed in the chimney flue and has a conduit connecting it to a pressure release valve associated with the container. The valve is operated by a piston member which is connected by a wire cable to a fusible element which melts when the flue attains a predetermined temperature. A tension adjusting device is associated with the wire cable to adjust the tension in the cable and the position of a piston head relative to a trigger element associated with the pressure release valve.