I "guess" these filters should be installed in the (rear) exhaust vent. I have two Electrolux, one from the 70's and the other from the 80's. Both units match your description of valid units. However, the rear ports of these two devices are 2 inches in diameter. The filters are 2 3/16 inch in diameter. They do not fit. Either I'm trying to install them wrong, or your description of these Chinese antiallergic filters is wrong. Suggestion: Include 1. Dimensions and 2. Installation way in your description. Cheap, too hassle to send back. You lied to me once.