Mystery duct

**Wayne Edwards** · 02-14-2014, 06:07 AM

Long time reader, first time poster. I came upon a standard propane mid efficiency furnace yesterday that had a large duct coming from the supply duct looping back into the return air plenum below the furnace. This duct was about 3 feet long and was probably a 10 inch vent. It had a manual damper which dropped open when the system fired up. I asked my (more experienced) boss if he had seen anything like this before. I have done over 1000 inspections and he has done 1000's more than me. Neither of us could figure out its purpose????
And, of course, my image is unable to be resized to post the picture. Any suggestions in the mean time while I figure out how to post the picture. Thanks for any insight.....