I just changed out my old Armstrong (missing shroud and not working) for a Carrier AirV. It bolted right in the space. I had to make a spacer for it to go between the aluminum inside and outside. The standard opening size is: 14 inches by 14 inches. dale '76 25ft. Caravanner; '69 31 ft. Sovreign and '77 31 ft. Sovreign