outside intake air vs inside - InspectionNews

Ross Neag · #1 (**permalink**) 05-16-2008, 08:01 PM

Does anyone have a chart or reference as to the increased efficiency of using outside combustion air on Cat IV furnaces versus inside air (non hooked up combusition air kits)? This would help me settle an issue with a builder who claims its only 1% and helps being fresh air in...yada 3x.

Bob Harper · #2 (**permalink**) 05-17-2008, 06:18 AM

As indoor air is consumed by a furnace, replacement air must come from somewhere. You can allow the furnace to inhale indoor air instead of piping it in and the lower part of the house will depressurize enough to draw in makeup air from the paths of least resistance. This will cause cold air infiltration in places they probably do not want it. As cold air enters the living space, it will cause a call for heat and run the furnace anyway. Piping in the combustion air reduces cold air infiltration while reducing the chance of backdrafting any remaining atmospherically vented appliances.
Tell him to pipe in the air then spend his money on sealing the ducts and weatherizing the home. These unpiped Cat.IVs can be dangerous.
Bob

Jim Luttrall · #3 (**permalink**) 05-17-2008, 10:37 AM

Quote:

Originally Posted by Ross Neag

Does anyone have a chart or reference as to the increased efficiency of using outside combustion air on Cat IV furnaces versus inside air (non hooked up combusition air kits)? This would help me settle an issue with a builder who claims its only 1% and helps being fresh air in...yada 3x.

Can't help you Ross. Have you tried the manufacturer?

Markus Keller · #4 (**permalink**) 05-17-2008, 06:21 PM

I don't have a link for fact info. I have looked into it and read various things. The efficiency difference from what I have read is minimal, a few % 1-3,4. I've never seen anything that states some high rate.
I don't believe that efficiency output is the real issue or concern, either in the field or by the manufacturer's from what I have read. I have to agree with Bob that the real issues area safety and proper operation.
There are usually two points I discuss with the client:
1- starving the unit for air and poor operation; You know when they build the furnace into those nice tight little closets with solid doors.
2- creating negative pressure in the house as Bob discussed