Abstract: A thermostatically controlled system is disclosed in which the thermostat is exposed to a zone of high moisture content. In order to prevent the accumulation of moisture within the thermostat which could cause tracking failures or over pressure failures in the device a vent opening is provided which vents the interior of the thermostat to a zone of low moisture content. The vent prevents the occurrence of vacuum within the thermostat and thereby eliminates the pumping of moisture into the thermostat from the zone of high moisture content. Also the vent allows for the escape of any moisture which might penetrate into the thermostat interior.