Abstract: A deluge fire sprinkler system for suppressing fires occurring in an air environment, the air having an atmospheric pressure, the deluge fire sprinkler system including a network of water pipes having an interior volume, a water input port opening the interior space, and a plurality of water outlet ports further opening the interior space, the network of water pipes having an internal fluid at a pressure reduced below that of the outside atmosphere; a plurality of blow off caps for resisting aspiration of air into the water outlet ports and for permitting emission of water flowing under pressure from the water outlet ports, the blow off caps covering the water outlet ports, the blow off caps being adapted to permit air to aspirate into the interior space of the water pipe network upon exposure of such network to heat; a valve actuatable by a pressure rise, such valve operatively connected to the water input port; a pump for lowering the internal fluid pressure of the network of water pipes below that of the a