Reznor’s X duct furnaces are designed to provide 80% thermal efficiency for indoor application with gravity venting. They are certified for use with natural or propane gas, as specified, in sizes from 75,000 through 400,000 BTUH input. These models are used as heating components in heating, heating/cooling or makeup air systems and require a separate blower system for air delivery. The furnace has a Thermocore® aluminized steel heat exchanger with venturi-design tubes. The die-formed burners are of aluminized steel and include flared ports with a stainless steel insert. The model X furnace is approved for a temperature rise range of 50°F to 90°F and includes “finger-baffles” for proper air distribution at these lower air volumes.