A firefighting medium is allowed to escape from a nozzle through apertures, the nozzle being provided with an internally arranged valve in sealing contact with a valve seat until the valve is released from its sealing position. The release is caused by a heat sensible release mechanism acting on the valve, whereby the sealing effect of the valve is relieved. A locking force required for maintaining the sealing effect by a removable cap or cover is provided, attached to the nozzle front, the locking force being established between at least two interacting surfaces. By releasing the sealing effect of the valve by removing the locking effect between the two interacting surfaces, the cap or cover is allowed to drop off its seat on the nozzle, relieving the press causing the sealing effect on the valve of the nozzle, the removal of the cap or cover caused by releasing mechanism.