Heating, Cooling and HVAC

The selection and design of air-conditioning ducts plays an important role in providing good indoor air quality. For years, conscientious HVAC contractors have been insulating the insides of galvanized ducts with rough fiberglass insulation. They had good reasons, too: The rough surfaces reduce air noise, and the insulation isn’t as easily damaged as external insulation wraps. However, these same rough internal surfaces can trap moisture and dirt and create indoor-air-quality problems.

Leave well enough alone?
There are many companies in the business of cleaning HVAC ducts, a service that makes some sense to homeowners with allergies or concerns about indoor air quality. However, duct cleaning may do little good when it comes to improving indoor air quality–and it may actually do some harm.

Do ducts ever need to be cleaned? That depends on their design, type, and age:

* Ducts that are smooth (uninsulated) on the inside seldom need cleaning, because they provide little medium for the growth of mold or bacteria. In fact, galvanized surfaces actually kill microbes, because the zinc surface is toxic to them.
* Air-conditioning ducts don’t see much accumulation of debris, either; it is all caught in the air filter and the cooling coil before it can reach the discharge ductwork. The exceptions would be unfiltered return-air ducts or old ductwork that has previously been attached to just a heater or furnace.

But how can duct cleaning create indoor-air-quality problems? It has more to do with the fiberglass insulation inside the ducts than with the ducts themselves, and it is more common in older homes or where union sheetmetal companies installed the ducts. When the vacuum and brushes are applied to a fiberglass-insulation surface, the integrity of the insulation surface is weakened. Small particles of itchy fiberglass may be blown into the home for many years after the cleaning.

So what’s the best ductwork to use? Here are my personal favorites:

* Externally insulated galvanized ducts are the best, due to their anti-microbial properties. Any ducts that have internal smooth-surface insulation, such as a foil covering, provide little surface for growth of microbes.
* Flex duct, since its internal surfaces are plastic, may also be used in short, straight runs. But if the flex duct has pockets where moisture and dirt can accumulate, it’s a growing ground for mold and bacteria. And it’s very difficult to clean contaminated flex ducts satisfactorily once they are contaminated, so proper filtration is essential.
* Coated duct board, while not as robust as metal ducts, can be a good idea, as long as both the inside and outside are foil-covered.

So while HVAC contractors should insulate the insides of galvanized ducts, they should use foil-lined insulation. Rough-surface antimicrobial coatings may help improve the house’s IAQ, but they don’t last forever, and they still provide pockets for dirt and moisture to accumulate.

Total systems using flex-duct, which was originally designed for use as diffuser runs from a central rigid duct system, are common in many tract houses today. While it is possible to design good total flex-duct systems, they demand careful installation. Otherwise, the dozens of low spots in the sags are potential microbial breeding grounds.

Source:hgtvpro.com