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

Given where we live, I have to "winterize" ours ever year. The first thing I do is turn the water off to the system. Second I remove the back-flow preventer and place it in the garage or basement. There are some sensitive gaskets in there that can deform and cause it to leak when you turn the water off. Lastly, you want to make sure that all the water is out of the system. We have a fair amount of elevation in ours, so all I have to do there is cycle the valves a few times and the entire system drains. I simply turn on the timer to open the valves for a few days after I've turned the water off.

If you don't have a good low point in the system, you can take an air compressor, open the sprinkler valves and blow air through the system until all or most of the water is out of the pipes. Make sure all the valves are open as you can cause problems if you put too much pressure on the system.

That's about it. Even with all of that, I still have to replace a few heads and sometimes repair lengths of pipe almost every year.