Automatic multispeed switching - Prevents false calls for fan operation - Simplified wiring - Contractor friendly PC board layout - Built in time delays - Pump exercise timer - 100% Factory tested - The Hydro Air Fan Control is an interface between the thermostat and air handler. It also has an isolated end switch to start the boiler and/or pump. When the thermostat calls for heat, the Fan Control energizes the end switch relay and allows the fan to operate at low speed when the water is above the optional aquastat setting. When the thermostat calls for cooling, the Fan Control energizes the condenser and operates on high speed. The Fan Control also includes three built in fan time delay options, two selectable pump exercise modes, a secondary aquastat connection for freeze protection and the ability to switch a pump and / or boiler.