I recently purchased a 2" BACK FLOW VALVE For a Pentair 120 heat pump. The rest of the piping for the pool equipment is 1 1/2". If I use the 2"flow valve and reduce it on the outlet side of the valve will there be any problems with the valve working properly.
No, there should not be an issue. The flow through 1.5" pvc is capable of opening up the valve.