The contractor hired to winterize the 71 RPZ backflow preventers at my homeowners' association (HOA) insists that the two isolation ball valves on each RPZ be left in the shut position. He says this is to minimize the entry of debris (dirt, insects etc., he said). No special effort was made to drain the isolation ball valves. The tiny testcock ball valves were all left in the 45 degree position. The manufacturer's procedure; every place I check on the net; experience last year with these same iso valves being left shut and then rupturing during the freezes; all say half-open/half-shut (a.k.a. 45 degree) is the correct winter position of the valves. I presented this to the HOA board. They are inexperienced and so fairly naturally trust the contractor and not the info I presented. About how many of these 142 ball valves (1.5-inch and 2-inch nominal size) do folks predict will fail, assuming multiple, consecutive days of sub 25 degree F weather?