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jimd60 · Jan 17, 2006, 04:42 PM

I have just finished my basement and my furnace is now in a room about 20 x 20. I had installed double doors (solid not vented) to enter the room which is also being used for storage.

My question is do I need to add some kind of makeup air for combustion by installing a grill in the wall? How do I know how large I need? Is there a way to calculate this? The heater is a 125,000 BTU york furnace about 2 years old.

Thanks for any help.

skiberger · Jan 17, 2006, 09:31 PM

Next to the exhaust vent on the furnace, is there another vent connection? If so this is where a fresh-air vent pipe can be installed and piped to the exterior.
Air from with-in a building
"If the volume of space is greater than 50 cubic feet per 1,000 BTU/h....normal infiltration shall be regarded as adequate...
If not, 2 openings in a wall, with one 12" from the top & one 12" from the bottom. Each opening shall have a free area equal to min. 1" square inch per 1,000 BTU/h input rating of the appliance installed but not less than 100 sqr. Inches." (Internation Res. Code, section M1702.1 M1702.2)

labman · Jan 17, 2006, 09:42 PM

If you have an installation manual for your furnace, it should tell. If not, go with the above rules. Those rules assume average construction. I actually tightened my crawl space up so tight, I had to add a vent for the furnace. I used the space between 2 joists as a duct ending it about a foot short of the furnace since the furnace was not set up for a duct.

jimd60 · Jan 18, 2006, 05:35 AM

Great - Thanks for the info, that's what I was looking for!

caibuadday · Jan 18, 2006, 09:26 AM

Originally Posted by jimd60

I have just finished my basement and my furnace is now in a room about 20 x 20. I had installed double doors (solid not vented) to enter the room which is also being used for storage.

My question is do I need to add some kind of makeup air for combustion by installing a grill in the wall? How do I know how large I need? Is there a way to calculate this? The heater is a 125,000 btu york furnace about 2 years old.

Thanks for any help.

125000 btu= 1.25 cubic ft... 1 cubic of natural gas require 10 cubic ft of air for a complete combustion... generally they use 50% excess of air... the min.excess air is 25%

hvac1000 · Jan 18, 2006, 12:46 PM

If you want to make it real simple for yourself just add a vent to the supply and lock it open. The burner will draw combustion air from the register/duct system and when the blower is running there will be more air than needed. Code actually call for combustion air to be from a source that is 50% hi and 50% low is a enclosed room.