Arkansas HVACR NewsMagazine November 2021

HVACR NewsMagazine November 2021

Tech News

grills are necessary. The curved blades take advantage of the Coanda effect that keeps the air discharged from the register bumping along the ceiling. Just as a fluid follows the curved surface like a spoon, discharged air stays in contact with the ceiling. We should never deliver air toward the interior of the home.

see perimeter duct systems in older homes, we see the same in new homes.

Today with force air delivery, we should never direct air flow speeds above 50 feet per minute against walls or windows. The delta between delivered air to the indoor side of the window compared to ambient

temperatures outside can easily be more than 130 degrees even in the south. Durability of any window is questionable when this extreme stress is placed on the module and cycled over and over during daily operation. Trim and painted surfaces will suffer with extreme heat or cold cycled over their surface’s day after day. previously mentioned material is of little use if we leave the All the

Speeds associated with terminal device delivery are generally between 600 to 800 feet per minute. Above 800 feet per minute and we cause noisy operation. Below 600 feet per minute and we may fail to reach the targets for delivery. Where we design air flows to be terminated at a wall or window, air speed rates should not exceed 50 feet per minute. It should be remembered that low entry, perimeter duct systems were introduced when fan forced heating was nonexistent. Gravity furnaces in the basement, directed low amounts of air against outside wall beneath windows. Due to early construction practices the relocation of duct systems with forced air furnaces were expensive if not impossible without extensive remodeling, so we still see perimeter duct systems. Because we

duct system in poor conditions. Just because the existing duct system is sheet metal, doesn’t mean the design is correct. The existing system may have been sized for heat only and had AC added. Where flexible duct is used, look for compressed duct, damaged vapor barriers, excessive lengths, the use of duct board wyes, and poorly sized duct. While we have been discussing supply side air issues we have to keep in mind, little air in, less air out! Media filters are a must, and two filters are a must above three-ton nominal sizing. One final note. Retrofit AC and Heating change outs give us a 40% gross margin. Add duct renovation to your repertoire and employ a 70% gross margin tool.