All About Ducts

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I thought it was high time to write an article about residential heating and air conditioning ducts, so here goes.

Ducts are simply a vessel to deliver air into your home from the heating and/or air conditioning system. The ducts are a crucial component of all conventional heating and air conditioning systems. If the ducts are of an acceptable material, sized properly, designed and installed per accepted industry standards, they will deliver the correct amount of air quietly into your home. Yet, if the ducts are too small, too large, or designed or installed improperly, the system will not operate as it was designed, may be noisy and will cost more to operate. The grilles also have an effect, but that’s a different article.

Although the duct system is so important, there are currently no regulations that govern the sizing of the ducts. I find this distressing. There are rules pertaining to the amount of insulation required around the ducts. There are rules that regulate what means must be used to connect the ducts. Even though the sizing of the ducting is critical to proper equipment performance, efficiency, and even longevity, there are no regulations that protect a homeowner from a contractor who installs ducts that are smaller than recommended.

The first “central” heating units installed in the Los Angeles area were known as gravity heaters. Some of you may have lived in a home built in the 1920s to 1940s that had one or more gravity heaters located in the basement. These systems typically used round sheet metal ducts to distribute the heated air. These ducts (commonly known as hot air pipes) were sometimes insulated with asbestos. These ducts worked well for this type of heater. They were unusually large, as gravity heaters had no blower motor to push air through the ducts. Although many attempts have been made, these ducts will not work when connected to a modern forced-air system.

In the 1950s, some new homes here were equipped with forced-air furnaces. The ducting was initially tin or galvanized sheet metal that may have been insulated with asbestos. Since tin ducts rust when subjected to cool, moist air, they are not recommended for use with air conditioning systems. These sheet metal ducts were generally small, relative to today’s duct sizes, because heating-only systems did not need to move as much air as an air conditioning system. Since asbestos is a known carcinogen, these ducts cannot be altered in any way and must be removed by a licensed asbestos abatement contractor. In addition, techniques used at the time to connect these ducts allowed for substantial leakage, which is not acceptable with the new state duct sealing regulations.

Fast forward to today. New ducts are flexible and insulated with encapsulated fiberglass. They are sealed using new methods that reduce the leakage rate. The minimum insulation required on ducts today is 2 inches, but 3 inches is a better choice. The outer covering is now plastic, usually gray, or black. Some have a foil-type (Mylar) covering, which is more durable and resists damage from sunlight.

For even more detailed information about ducts, join us at our next Homeowners Workshop. All questions will be addressed, so bring your list. There is no cost, so bring your friends, neighbors, and relatives. If you are interested in attending, call (818) 886-2600 to make a reservation. Join us for a couple of hours and you’ll take home some useful information.

Jim Berry was a heating and air conditioning service technician prior to becoming a Sales Consultant for Kahn Air Conditioning. Kahn hosts free monthly workshops to help homeowners learn more about heating and cooling systems. For questions regarding heating or air conditioning, send an e-mail to Jim at jberry@kahnair.com.