Arkansas HVACR NewsMagazine November 2019

S tate, National, Chapter News Tech News

material used in the construction is obviously a major factor in providing comfort. The levels of insulation, for example, will slow heat loss or gain. This slowing will increase system time off and reduce airflow requirements. This is the reason an assessment is necessary with any system alteration or where problem diagnosis is required. Differing application calls for alternative duct solutions. Do we cool or heat more over the course of the year? If in colder climates, perimeter duct systems may be necessary. Is a basement available? Can I maximize air flow in a particular home with high sidewall grilles throughout the home? Making the decision on what type duct configuration has major impacts on comfort and economy. There is no single type of duct system that outshines another. The important point we need to emphasize here is without proper terminal device placement, the best combination of equipment and duct materials will fail to deliver comfort or economy. During cooling, a properly placed device will deliver low energy air to stay in contact with energy laden surfaces like an insulated ceiling, exterior walls or windows. During heating, high energy air is directed to those same surfaces that now maintain low levels of energy. The terminal device when properly

selected will also mix entrained air within the space. Mixing the air is the hard job. Placing the air is easy with proper selection. Often a perimeter duct system is installed, confusing distribution with basement heating systems while distributing air back into the interior of the home. In some instances you will find perimeter duct systems in an attic with terminal devices pushing air to the exterior of the home. This practice puts 600 fpm air flow directly against a perpendicular surface to kill performance on all levels and adds resistance to air flow with excessive length. We never want to see velocities greater that 50 fpm impacting a perpendicular surface. This practice can degrade construction materials and can place stress on window components. As an example during winter conditions, imagine directing a 130 degree stream of high velocity air on a window where outside conditions are 20 degrees. We repeat this cycle several times an hour if wind loads are high or insulation levels are low in the structure. Can anyone say premature window failure? The conditions we note are not extreme. You see these conditions across the entire south. What happens when we drop those ambient conditions to single digits temperatures? In the summer,