For example, this is a 5-year old American Standard Freedom 90 Cat IV condensing furnace, with the normal PVC exhaust vent to the exterior. Normally when I see similar furnaces of this age, they all have a PVC vent pipe that extends to the exterior for combustion supply intake. Does the manufacturer (or some code) require a direct vent connection to the exterior? I wasn't able to locate a manufacturer's installation guide.

In this case, it is installed in a non-vented (not conditioned) crawlspace, but they did provide one 6" vent at the rim joist for combustion air to the crawl. (Don't need comment on the rust in the attached picture; I have already recommended a service tech to repair any condensate leaks. I just needed a picture for illustration of the furnace type.)

As a side note, I do see older Cat IV furnaces (i.e. 1980s) with no visible option for an air intake vent that would extend to the exteror, so I have to assume some older models just used room air for combustion supply.

Attached Images

rust-on-furnace.jpg (54.9 KB, 60 views)

This was discussed recently. Check out Bob Harpers comments.

Combustion Air Source and Cat IV Efficiency

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Dave

Thanks David. I had scanned prior threads, but missed that one.

It makes sense to me to pull combustion air from the exterior. Another reason is relative humidity. My area generally has a low relative humidity, as say compared to the East coast or Southeast part of the country; we average 20-50% RH, sometimes lower in cold winter months. However once you start measuring interior relative humidities, it can jump 20-40% above the outside, especially in some crawlspaces. Even an indoor furnace (say a basement or other interior room) will have at least a slightly higher RH than outside. One of the reasons I was looking closely at this furnace is because it seems to have an unusual amount of staining and corrision from very small moisture (condensate) seepage at various areas in the cabinet of the furnace, as well as a leak where the exhaust vent pipe passed through the cabinet wall. So I guess another arguement for using exterior sources for combustion air may also include levels of humidity. I would assume higher levels of relative humidity may affect the lifespan of the components if not the efficiency level of the furnace.

I would still like to see an Installation Manual from American Standard if someone can find one.

Owner's Manual - American Standard

Owners manual only. But has contact info.

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Dave