A/C Duct Cleaning

Have a 41 yr old home that in the past summers when it has been very hot during the summer, has developed black stuff on top of air vents and ceilings in several of the rooms.
Has anyone used a reputable air duct cleaning company to clean out the vents from an old house like this?
I have used diluted bleach water and scrubbed the area but it comes back again and also a solution of water and borax, but the ducts need cleaning where we can't get to.
I have asthma and use hepa filters and a good a/c filter regularly and change it when I am supposed to. We have had an extremely dry spring and summer and now the rains are starting with the fronts and it has been in the triple digits in temps until last of Aug here in SCentral Texas and now it has been very humid and dry all this week and it seems the a/c has some condensation.
Please, if anyone has used the duct cleaning, respond.
Thank you, Michelle
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Michelle Moreland Orlando wrote:

In Florida they must be licensed. Talk to past customers.
If an A/C stays on a lot, water can condense in ducts. That causes dust to cake and provides an environment for mold and bacteria. It's especially bad if the lining is fiberglass. It seems the only way to clean it is to open ducts to use a brush. Even then, mold may return.
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On Sat, 23 Sep 2006 15:24:55 GMT, Michelle Moreland Orlando

This is more of a question than a helpful response, but I was wondering how long you have used the Hepa filters in that home. I would thank that the long term use of the Hepa filter would have to significantly reduce the amount of muck in the vents. But, I guess, even if it only lets through 0.09% of muck, that would still build up over time. FWIW, I had my vents cleaned by a "professional" service. The house was "used" and we had just moved in. It seemed to help decrease the bad odor of the previous owner. In addition to vacuuming my ducts, they also "disinfected" them. I am sure they use that term very loosely. Anyway, if you get it done, I'd recommend a basic job. Mine was pretty cheap - maybe $100 IIRC. However, they try to pitch all these expensive add-ons that I didn't go for. Not very helpful I'll admit, but I try. Also, I've seen a lot of people get razzed in the Home Improvement Newsgroup for having their vents blow/vacuumed.
eVe GiiDiiOn
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We live in Houston so Ac stays on almost year round. We had ours cleaned a few years ago. My husband was having allergy problems. The truck was cute beyond belief with these little vacuum stacks on top. It took almost all day (our house is not large, about 1900sq. feet) They seemed very thorough, it cost about $5oo. To be totally honest I could not tell any difference. It turned out my husband had polyps anyway and had to have surgery. The people who did the duct cleaning said it would never have to be done again, which made no sense. But we were trying to do everything we could to help him. I think the most important thing is changing the filters often. Very often.
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