Abstract: Automatic valve assembly connected to outlet from clothes dryer to selectively add heat and humidity to building. Bifurcated exhaust duct assembly has manual valve and filter in first duct to exhaust into building, and automatic valve in second duct to exhaust outside building. Automatic valve is retained closed by releasable latch means which releases valve automatically when filter becomes clogged with lint to such an extent as to generate excessive exhaust pressure on upstream side of valve. Valve plate is journalled on off-centered hinge axis so as to swing to open position when exposed to exhaust flow. Pressure sensing means exposed to exhaust pressure cooperates with latch means to release the latch means when exhaust pressure exceeds pre-determined excess. Steadying means cooperates with opened valve plate to maintain plate in open position which produces negligible restriction of flow past automatic valve.