We have recently purchased a Laing Autocirc Instant Hot Water Pump. We are confident that it is correctly installed including the power supply.However, when we turn on the pump after the initial hook-up the pump runs for only a few seconds and stops and does not start up again regardless that the temperature in the hot line remains below 60 degrees.Is it possible that there would be a back-flow check valve between the pump and the hot water heater? If so, where would we find it? or what else could it be? Help!