Fire sprinkler test water is water used to periodically test a building's automatic fire sprinkler system. Instead of pumping the water used to periodically test a building's fire sprinkler system into the sewerage system, a closed system is a system where this water can be stored in a storage tank (or similar) to reuse in the next periodic fire sprinkler test.

If the water is partially or completely diverted to a tank for purposes other than fire sprinkler tests (such as garden watering), the system cannot be considered as a closed system.

Fire spinkler. Source: Department of Planning & Environment