SPRINKLER TALK FORUM - You Got Questions, We've Got Answers

Depending on your location, you may not want to add a thing. The original standard configuration would have a drain hose bib on the low point of the plumbing, just after it exits the house, and upstream of the PVB. Nowadays, this faucet could be frowned upon, as a possible cross-connection, where the codes are making the assumption that some idiot will hook up a hose and connect it to something nasty. One way around this is to use a simple PVC plug in place of a boiler drain faucet. Remove plug, connect air, winterize, replace plug.

It is more than likely that local service companies already have any adapters they need to connect their air to a PVB testcock, which means the indoor drain doesn't have to be utilized for winterizing.

install the drain after the backflow. they can also use the test ports on the side of the backflow, but a threaded connection after the backflow is the preferred method.

Downstream installation has the advantage of not presenting the possible cross-connection that upstream installation does, but we always appreciated the convenience of having a low-point drain valve that was open all winter, so water leaking past the sprinkler system shut-off valve had a means to safely drip away, and prevent freeze damage.

He already has a low point drain valve in the basement. We see it that way here in Colorado more often than not.

The outdoor low-point drain would have the advantage of not dripping water into a basement, when left open all winter. Modern installs don't really have a good way to deal with a leaking system-shutoff valve, if it's passing any quantity of water.