inspectorwill Posted February 4, 2013 Report Share Posted February 4, 2013 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? Quote Link to comment Share on other sites More sharing options...
Marc Posted February 4, 2013 Report Share Posted February 4, 2013 Cat II or Cat IV? Spoken another way: Positive vent pressure or non-positive? Marc What's PMI? Quote Link to comment Share on other sites More sharing options...
Mike Lamb Posted February 4, 2013 Report Share Posted February 4, 2013 Yes. One opening near the ceiling and one near the floor. Quote Link to comment Share on other sites More sharing options...
Mike Lamb Posted February 4, 2013 Report Share Posted February 4, 2013 I don't think I have ever seen a Cat II furnace. PMI ?? per manufacturer instruction? Click to Enlarge 28.78?KB Quote Link to comment Share on other sites More sharing options...
inspectorwill Posted February 4, 2013 Author Report Share Posted February 4, 2013 PMI - Per Manufacturer Installation Cat II appliances are rarely used residential installations and I don't think I have ever seen one either. I am going with upper and lower combustion air sources since it draws from the closet interior. Thanks guys. Quote Link to comment Share on other sites More sharing options...
hausdok Posted February 4, 2013 Report Share Posted February 4, 2013 Splitting hairs but I think the term is AMI - in Accordance with Manufacturer's [b/]I[/b]nstructions. When you say PMI to me, I instantly think of a Preventative Maintenance Inspection. ONE TEAM - ONE FIGHT!!! Mike Quote Link to comment Share on other sites More sharing options...
hausdok Posted February 4, 2013 Report Share Posted February 4, 2013 Hi, 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. ONE TEAM - ONE FIGHT!!! Mike Quote Link to comment Share on other sites More sharing options...
Mike Lamb Posted February 4, 2013 Report Share Posted February 4, 2013 I don't think I have ever seen a Cat II furnace. PMI ?? per manufacturer instruction? Click to Enlarge 28.78?KB 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: Cat IV Install Instructions.pdf 1775.8 KB Quote Link to comment Share on other sites More sharing options...
Eric B Posted February 4, 2013 Report Share Posted February 4, 2013 An attic is considered an outside source per F2407.6.1(1 & 2). I think that that may be based upon an attic that is well ventilated to begin with - at least it's viewed that way by our local gas utilities. Quote Link to comment Share on other sites More sharing options...
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