Pump discharge and city water both supply the same cold water main plumbing line which then feeds water heater, faucets, shower, toilet etc.
Turning pump on while connected to city water will help maintain steady constant flow. If city water pressure is lower than pump discharge pressure, check valve in city water connection will close until pump discharge pressure decreases enough to allow city water check valve to open.
If city water pressure is higher than pump discharge pump will not run.
Pump only takes suction from fresh water tank.
