Outdoor Combustion Air For Furnace - Minimum Free Area
Now that it is warm again, I am doing some more sealing and insulation work to try to warm up a finished basement that it just a bit to cold in the winter (Southeast Michigan)
I have a 100KBTU condensing furnace, and right now, there are two vent lines for outdoor combustion air that dump into the room. I am debating on whether I can make these direct-connect lines.
The furnace as a 2" PVC combustion air inlet that is just open to the basement area. There is a 2" PVC Line that dumps a few feet away from the furnace, and about 4 feet above the floor, and is open to the outdoors. There is another 6" flexi-duct a few more feet away that also exists the house. It opens about 12" above the floor.
According to the manual, if I have 100KBTU of input energy, I need around 7" of duct opening (Say 22 sq in)
What I don't understand is why the combustion air inlet to the furnace is only a 2" PVC pipe, if I need so much more area.
The questions I have are
1. Is the open area requirement only if the duct work is not directly connected to the furnace?
2.The manual clearly shows an install method where the combustion air pipe is directly routed outside. If the elbow is 2", then I am thinking I should be able to just connect the existing PVC line directly to the furnace, and close off the 6" flexi-duct?