Arrived ed for no heat in lobby area. Inspected Carrier furnace electrical, gas components, filters and heat exchanger. Furnace was flashing fault code for high limit switch. Carrier furnace labeled #1 had a filter stuck in blower motor. Filter was very dirty and nothing to hold it in place which led to being pulled into blower motor. Also found a failed heat exchanger as seen in images from surrounding insulation burning to a cracked ring. Beginning section of single wall 4" flue pipe is installed upside down leaving air gaps around crimping for co to leak out. All flue vents for either furnace and water heater are single wall pipe and show rust through out flue. CO detector is picking co levels in utility closet possibly from flue condition/installation and failed heat exchanger or both. While in utility closet faint smell of natural gas was noticeable. Found gas line goin into water heater was leaking and recommended immediate repair. Inspected furnace #2 that showed no issues while