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Combustion air / condensing furnace


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High efficiency condensing furnace installed in interior closet. The combustion air is drawn from the closet which only has an upper combustion air opening to the ventilated attic space. Should this furnace be treated as a typical forced air unit with upper and lower combustion air openings if not drawn directly from the exterior? Or, is the installation PMI?

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An attic is considered an outside source per F2407.6.1(1 & 2). A single opening from the closet into the attic space is OK if it's in the upper 12-inches of the space and is 1 square inch per 3,000 Btu and is greater in area than the sum of the area of the vent connectors F2407.6.2). It can be to an attic (F2407.6.2). Obviously, the opening has to end above the insulation - six inches minimum, I think - and they aren't allowed to install any screen over it in the attic (F2407.1.1).

So, you've got to measure it to ensure it meets the size requirement and you have to ensure it ends above the insulation and isn't screened. If it meets those requirements there is no need for a second opening and you're going to end up with egg on your face.



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I don't think I have ever seen a Cat II furnace.

PMI ?? per manufacturer instruction?

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Mike O is right, however, if you want the AMI/PMI you would have to look at the manufacture's instructions. The diagram I provided is from Trane and it calls for two openings to the attic or outdoors.

Download Attachment: icon_adobe.gif Cat IV Install Instructions.pdf

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