One cannot just take the vent off an 80% efficient furnace and run it out a side wall. It would auxiliary power inducer. The combustion air flow through the furnace must be controlled or its efficiency will decline. The barometric damper will prevent pulling additional excess air through the furnace.

High-efficiency furnaces use sidewall vents for breathing in outside air for fuel combustion, and exhaling cooled exhaust gases nearly depleted of heat.

Follow the step to inspect the flue pipe 1) Remove the flue cap to check for blockages such as debris or birds' nests 2) Replace and secure the cap tightly to the end of the flue pipe 3) Look for signs of blockages by examining the pipe itself water streaks or rust on the flue pipe may indicate a blockage, causing moisture to back up.