The Field Controls CAS-34UP Fan-in-a-Can is a combustion air system designed for 24V gas boilers with automatic vent dampers. It mechanically draws fresh outdoor air into the boiler room and disperses it near the burner to support proper combustion. When the thermostat calls for heat, the fan operates and establishes airflow; its proving switch then permits the boiler burner to fire. https://www.partshnc.com/cas-34up-field-controls-fan-in-a-can-for-24v-gas-boilers-with-an-automatic-vent-damper-603400701-replace