Double L Inlets help to keep your air clean, lower heating costs, and improve pig performance. Attic Inlets are structured to take warm air from the attic to heat the living area of the hog barn. The air that you and your pigs breathe cannot be seen and is easily taken for granted. Clean air is one of the most important parts of your livestock operation. The idea of using warm air from the attic to heat the living area was proven by university testing to save money on propane expenses and improve environment conditions.