Since we brought it up in another thread, I though i'd elaborate on it. I hate auto drains. Despise them. I have repaired many systems for freeze damage that have auto drains. You can not visually see them draining. Blowing out the system ensures there is no water left inside the pipes. Auto drains also waste water. With our water shortage in Colorado this year, everydrop is crucial. Everytime the pipes drain, they need to be filled back up. So as the water rushes to the end of the line and slams into the last fitting, there is stress put on the fittings. I Have seen many systems with drains that have a lot of water hammmer due to this. There is no resistence on the water as it is rushing 200' to the end of the line. I have seen systems that because so much water is rushing through the pipes with no resistence, that the PVB will "chatter" and dump water. That means there is not enough water or pressure to seat the bonnet and poppet together so water dumps out. But then it seats once the lines are filled up. This happens with every zone. It also makes a lot of noise. I was once called to a repair that had a leak in a zone. System was at least 20-30 years old. Dug to find the pipe and found it in a trench with 3 other PVC pipes. Of course it was the bottom pipe that was broken. And it was 1 1/2" PVC. And the PVC shattered 60' in length. If you've never seen a PVC freeze break it would amaze you the first time. PVC will shatter. Normally it will stop at a glue joint. This one didn't and and went throung 3 of them. I had to dig up 60" to replace. The system had auto drains and it didn't open up that year.