Bad Air

I read with interest the article titled "Insulation in the Heating System--A Source of Black Stains," (Nov/Dec '98, p. 17). It has always been my understanding and experience that staining is largely due to particulate matter in the room air and pushing limits of diffuser exit velocity and the temperature differential between fin and discharge and room temperature in the case of baseboard heaters. In many cases, the diffuser selection might be satisfactory but the duct approach not ideal, so that the diffuser does not deliver symmetrically on all sides. In this case, the higher velocity delivers more air and accompanying particulate into the discharge stream, to be deposited at the corners of the diffuser, where the discharge streams diverge and a "backwater" is created.

With electric baseboard heaters, it's possible to achieve greater particulate output per unit length--than with hydronic or steam baseboards--by increasing the operating temperature. The baseboard generally has better access to particulate matter at floor level and sucks harder than the lower-temperature hydronic ones.

Jim Were
Building Services Engineer
Life Member, ASHRAE
Canberra, Australia

Other Things that Go Bump in the Night

I appreciated your article "Black Stains in Houses: Soot, Dust, or Ghosts?" (Jan/Feb '98, p. 15), but I would like to add that there might be yet another source for the black stains such as molds, particularly the Aspergillus niger mold. The Aspergillus niger mold leaves an excrement of black material, which is often described as soot. To properly identify this mold, nearly pristine samples need to be grown on a Rose agar, which then needs to be examined under a microscope.

Keith H. Rhea
Certificate Member of Refrigeration Service Engineers Society

Sweaty Ducts

I read with interest the article titled "Drowned Duct Rescue," (Jan/Feb '00, p.7). I am the mechanical supervisor for the city of Jacksonville, and I have experienced similar flex duct sweating in a well-ventilated attic in the middle of summer. The duct sweated where two flex lines were touching and where it was supported by a nonventilated strap. The only way to fix this problem was to ensure that no two ducts were touching and to have a ventilated strap. We screwed the metal strap to the top of the metal duct and then insulated over it. It worked for us.

Eric Pardee
Mechanical Supervisor
Jacksonville, Florida

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