HMI Hoyme Manufacturing Inc. Motorized Fresh Air Damper for Combustion - 9" SF2 for Two Heating Appliances Simple to install, attaches to the terminating end of insulated air supply duct. Interlocks with the safety control system of gas, propane and oil-fired appliances. Certified as required by National Gas Codes; fail-safe open, power closed. Approved for use in applications using two separate heating appliances. Standard connection to 24VAC thermo/aqua-stat circuits.