Flo-Control Valves 1. To prevent gravity circulation and reverse flow in the system, install Flo-Control valves on the supply side relatively close to the boiler. 2. You can get gravity circulation in a single pipe! You may need a second Flo-Control valve on the return to stop this. 3. Flush the system before starting it up to keep installation debris from lodging on the valve's seat. Electric Zone Valves 1. Slow-closing zone valves are less likely to create water hammer noise in a closed hydroponic system.