On Thu, 18 Feb 2010 09:48:43 -0800 (PST), "hr(bob) "
wrote:

I don't understand cleaning ductwork. If the ducts start out clean and
then gradually accumulate dirt, that dirt either stays put, or blows
out into the room. If they start out clean, and you have a filter, how
does the ductwork get dirty? And, if it is dirty, once the loose
dirt blows out, how does more loose dirt occur to get blown out. That
new loose dirt will get blown out whether the duct is clean or has a
build-up of dirt that isn't loose, so cleaning the ductwork is only
needed if it gets so bad that it impedes air-flow. What am I missing
besides enriching all the ductwork cleaning firms?

Nothing. Once I had ducts cleaned as my bride was sneezing often,
thinking allergies we had the ducts cleaned.

The crew re-installed grills crooked, misaligned screws, scuffed wall
paint where the shop vac bumped corners. Never again.

Hell, I can do that much damage.