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

Follow mfr instructions for winterizing and this sort of thing does not happen.

Replace the ball valves.

Might also be time to replace the shut off valve in the basement as well. Especially since this happened a few years in a row.

Even with a replacement shutoff valve installed in the basement, it is always best to work from a standpoint of not trusting the shutoff valve to be 100 percent effective, and to have an open drain in a low point to allow water to drip out before it gets into the expensive parts of the outdoor plumbing.

Does the pipe exiting the house "elbow" up into the backflow, or does it have a "T" that needs to be plugged? Regardless, both styles of plumbing, like wetboots stated, should have a drain in the basement. The elbow style always has a drain, the T style rarely does. In any event, the technician either forgot to drain back into the basement, or the shut off valve is allowing water to weep past it. Or, he drained it, closed the drain, and the weeping shut off valve refilled the line.

I always wanted the drain to be outdoors, with a supply pipe exiting the basement above grade. That way, a drain could safely drip all winter, if need be.

The problem we find is that even with a 3/4" drain outside, it can freeze closed and still cause damage. I see it more often when the home owner installs a boiler drain in place of the plug.
We install our backflows with a T below the backflow with a 3/4" blow out fitting. Remove the plug and drain everything outside. We do blow through the backflow. Now, before anyone jumps on me for it...99% of our backflow and plumbing repairs are due to hard water build up, leaky shut off valves, or blow outs scheduled to late. I just repaired a Febco 765 that had 15 years of hard water build up on it, that only needed a cleaning and an O-ring. It's been blown through 15 times.
I've been toying with the idea of adding a basement drain to the installs, but the 1% of the 1% that need one, doesn't justify the time. A good service tech. can tell when a shut off needs replacing. And a homeowner should realize that icicles hanging from the backflow is not a good thing.

Of course, I should mention that a boiler drain at an outdoor low point needs to be an "old-school" model with a large opening, which is not a thing you can get unless you specifically look for it, and even that won't help if there is any plastic plumbing between it and the basement. Heat conduction is the thing. Houses radiate heat, and basement warmth works its way into outdoor copper plumbing.