Cardboard supply ducts in slab

[Richard Moore]

Ok...this is new for me. 1951 rambler, on a slab (which is rare around here). The furnace heat supply ducts appear to be no more than cardboard form tubes embedded in the slab and cut out at each register opening. No signs of any coating (asbestos, etc), just a lot of dust and dirt. Normal return ductwork, through the attic. Anyone seen this before?

Very good air flow at all registers, but it just don't seem right. Comments?

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PS...the Pergo installed over the carpet was a nice touch (and a separate issue).

Finally, the Coleman furnace appeared to be brand, spanking new but the serial numbers don't seem to conform to anything I've seen on the various forums. Model FC85080A12DH11B, Serial # W0B7424908. Anyone able to shed some light there?

[Bill Kibbel]

Very common here for mid 1940s through 1960 slab homes. If'n it aint asbestos it's paper ducts. Almost always eaten by termites and collapsing. When you turn on the blower, the house smells like a dirt crawlspace.

[Richard Moore]

Hi Bill,

I'm recommending duct cleaning, not only because they were filthy, but to ensure no blockages. It's doubtful our NW types of termites (subs & dampwood) would find the conditions particularly appealing. My thinking is that as long as there's good airflow and it's been this way for 50+ years, it's really not a big deal. Even if the cardboard has deteriorated, there's still a "tunnel" in the slab. What do you do when you find these?

[Bill Kibbel]

The slabs aren't that thick here. Most of the tube is in contact with damp soil. The soil is kept warm all winter so the termites are happy to extend their season.

I remember one that had decent air flow, but the air lost all its heat before arriving at the vents.

[Richard Moore]

Here's a photo of another area of the house.

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The bottoms of the tubes looked to be in good condition with only the cut-out areas kind of ragged. I don't actually know if the slab extends down around them though.

[Jim Morrison]

Sounds like an allergen farm to me. My experience with these things (they are rare in MA) is nearly identical to Bill's. I can't remember one with decent air flow. I wonder if the duct material would survive a good brushing and vacuuming?