The Aprilaire® Steam Humidifier delivers humidity in the form of steam to the conditioned space via the HVAC system duct. The humidifier generates steam by energizing two electrodes that extend into a disposable canister filled with water. Current flowing between the electrodes causes the water to boil, creating steam. Water is introduced to the humidifier through a fill valve to a fill cup located in the top of the cabinet. The fill cup serves as an overflow reservoir and provides an air gap between the humidifier and water source. The steam canister is filled from the bottom. The canister is seated in a drain cup assembly which includes a drain valve. The drain and fill valves work together to maintain water level in the canister to satisfy the demand of the modulating control based on the electrical conductivity of the water.