Looking at the photo, the furnace discharge vent and the the combined vent are the same size (5"?). At the minimum, a combined vents needs to be one size larger than the larger discharge vent. This isn't the likely the issue if the furnace btu's require a size smaller vent, say 4". This disign is an approved layout if all other criteria are met...Heart and Cooley has charts and details on their sight. You're better to look 'em up than me try to explain the many details. http://www.hartandcooley.com/tools/down ... 9094a11d63 PAGE 15...and ...Fig 4-D (pg-9) If the link is inop. Go the H&C/ Download literature/ Vent & Chimney/ H&C Literature/ Sizing Guide...
