At 03:25 Tower 15 was dispatched mutual aid along with Company 2 to Kelly Township at River Woods for an automatic fire alarm. The alarm was coming from a HVAC unit on the roof. Truck 2 and Tower 15 put their aerials in service and checked all units on the roof with nothing found while Rescue 2’s crew cleared the interior. Units returned approximately 1 hour later. Units responding and on scene: Chief 1501(Stump), Tower 15, Truck 2, Rescue 2, and MICU 2.