whats the code recommendation for discharge pipes when installing a water powered back up sump pump with a primary sump pump. Separate discharge or can you use a Y and connect the backup to the primary discharge. Whats best?
The water powered sump pump, for all intents and purposes, can be considered a second sump pump in parallel with the first, and can be plumbed that way.
They can share a discharge pipe as long as the pipe is rated to carry the load of both pumps running simultaneously. Even if they should never, in practice, run at the same time, it would be inappropriate to attach both to a discharge pipe that could only handle one at a time. Most pumps I've run across seem to be installed with larger piping than they absolutely need, though, so you're probably ok combining them. Be sure to use two check valves, one for each pump, before they combine into the one discharge pipe.
Varies by town. In our town, we were required to have a separate discharge line for the water-powered back up sump pump. Reason was, if main line clogs, the backup has its own line that would be clear.
Also, we were required to put a backflow preventer on the water supply line, and have it tested, certified. Required a permit and final plumbing inspection by our township.
Check with your town's code to be safe.