Crack in bottom part of backflow preventer can you just replace this part or do you have to buy entire unit??
Thanks,
Eileen 
Crack in bottom part of backflow preventer can you just replace this part or do you have to buy entire unit??
Thanks,
Eileen 
The cracked part isn't part of the backflow preventer proper; it's simply a quarter-turn ball valve that's been put there to allow the preventer to be shut off as part of winterization or testing procedures. Simply replace it, preferably with a version that has a drain cap on the side for winterization purposes, and you'll be good for a good, long time afterwards. Note that you'll need to undo the union below the valve in the process of replacing it in order to be able to unscrew it.