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Often in dry pipe setups there is still a stopper in all of the sprinkler heads that has to melt to let the water out. This is common in places like datacenters or other places where accidentally hitting the sprinkler head would cause major damage from the water.

Basically smoke/heat detectors trigger the pipe to fill, then heat from the fire releases sprinklers wherever it is hot enough to melt the stopper.

But I suppose there are cases where the fire might be expected to spread so fast that they don't put the stoppers in and just let all of them go.