Newps wrote:

Outdoor sprinklers are filled with air in the winter.

Not all, by any means. When I worked for Grinnel Fire Protection, we frequently
designed sprinkler systems which used anti-freeze. These were closed-head
systems. The water was shut off at the main valve and the system drained. There
was a filler port at the top of the system, and it was filled with anti-freeze.
Then the water was turned back on.

If a fire occurred, one or more heads would blow, the anti-freeze would drain
out the open heads, and water would follow from the mains. After the fire was
out, the system had to be drained before it froze.

This type of system was very popular as an outside branch of a larger inside
system; for example, a few heads providing protection for an outside truck
loading dock.

George Patterson
Coffee is only a way of stealing time that should by rights belong to
your slightly older self.