Combustible Air Duct Size

Mike Finley

I need to redo the existing combustible air duct returns to a furnace in one of my rental props.

2 ducts, one high and one low, right?

6 inch okay for both?

MechanicalDVR

In saying one high one low you mean inlets I hope. What you need is one square inch of fresh air for every 1000 btu's of burner. As in 200 total square inches (100 low and 100 high) if you had a 100,000 btu burner. And don't just figure the face of a grill, you need to know free area of the grill, to calculate square inches.

Mike Finley

No wonder after 30 minutes of reading the 2003 IRC I still couldn't find a simple answer!

mdshunk

By the way, if your intention is to provide the unit with combustible air, you'd better evacuate the building. If you want to provide it with only combustion air, you're good.

Mike Finley

Thank MD, no wonder it was getting expensive. I can return those tanks of hydrogen now.

coolmen

A standard stamp faced grill will provide 75% free area . so a 14x14 grill = 196 sq ,75 % = 147 sq free area

coolmen

Home depo in the book and mag area should have a code check hvac field guide manuel. very easy illustrations and examples. this is a must have for the hvac guy with info on cumbustion,ducting,venting,chimneys appliances ect,,
This comes in handy when the inspector starts questioning my work and the codes.