FITS 3 in - 6 in CHIMNEY UP TO 100,000 btu/hr, MADE IN THE U.S.A. TJERNLUND AUTO-DRAFT® Inducers assure positive draft when restricted boilers and furnaces, poor chimneys or slight negative pressures in buildings prevent proper exhaust of combustion gas. The venturi action of Tjernlund Auto-Draft Inducers starts air moving smoothly. These units are quick and easy to install and completely automatic in operation. Tjernlund's unique design and durable construction makes them trouble-free and reduces maintenance to a minimum. The Vari-Draft Control permits adjustments to the individual job requirement.