What would cause both the pumps in my duplex booster system to come on at the same time?
It depends on the type of sequencing the system has. With current sensing relays, if the threshold setting on the first current sensing relay was set too low, this would happen. With a pressure sequencing system, if the pressure switch to sequence P2 on was set too high, this would happen. Both pumps will also start if the plumbing system is being filled (at startup). One further cause of both pumps starting would be if you have a drawdown tank and the booster pumps are relatively small. In this case, occasionally the tank will fill fast enough that the second pump is called on by the demand.
