Abstract: An air filter monitoring device for determining the condition of the filter in a forced air system having a two speed blower, wherein the device comprises a pair of air pressure sensing probes, one probe being downstream of the blower and filter, and the other probe positioned to detect ambient air pressure, an air pressure differential sensor which includes a first air pressure differential sensor member adapted to be actuated by a first differential pressure and a second air pressure differential sensor member adapted to be actuated by a second differential pressure, a control circuit selectively interconnected between the blower and the first or the second air pressure differential sensor members for interconnecting the first air pressure differential sensor member to the blower when it is operating at a first speed and for connecting the second air pressure differential sensor member to the blower when it is operating at a second speed, and a pair of clogged air filter indicating members which are respons