Aprilaire 4332 Damper Assembly Aprilaire #4332 damper assembly - includes damper blade, spring and knob. fits models 600, 500,400 The Aprilaire 4332 humidifier damper assembly is a damper assembly kit that includes a damper blade, spring and knob The Aprilaire 4332 damper assembly is designed for Aprilaire models 400, 500, 550, 550a, 558 and 600 humidifiers. The Aprilaire 4332 humidifier damper assembly is a damper assembly kit that includes a damper blade, spring and knob