Holmes HAWF2041 9" Twin Window Fan with manual thermostat creates relief from the hot weather by drawing in cool air from the outdoors or exhausting stale indoor air. Comfort control thermostat helps to maintain your personal comfort level by cycling on and off to the set level. Dual blade operation; Designed to fit into most double hung and slider windows; Slim design allows for use with window screen in place; Independent electronically reversible motors allows for air intake, exhaust, or air exchange; Three speed settings; Adjustable extender screen and bonus extender panel help to secure fan into larger windows.