re: How to protect an underground sprinkler system from freeze?

You don't have to do all of that. Just shut the valves off on both side of the backflow preventer and drain the water out of it with the little release valves on the side. There are usually 2 small valves on the side of the preventer. That is the only thing that will freeze. If you want to take it a step further, just make sure you have the exposed lines in the preventer loop insulated.