ENGINEER’S TOOLBOX
BASICS OF RESIDENTIAL DESIGN TAHIR RIAZ, PEG LLC
T
he Air Conditioning Contractors of America (ACCA), has for years published protocols and procedures that have been the standard best practice method for calculating residential heat loss, gain, duct sizing, equipment and register selection for buildings. These protocols are required by building codes. A well designed mechanical system can be energy efficient and save the consumer money, in the long run. After the load calculations for a lot are complete and the air flow for each room has been calculated, the next step is to choose the right mechanical system. Air conditioner, heat pump, furnace, or boiler; all these selections are made after this point. ACCA has a protocol built around this process, known as Manual S. This protocol describes various parameters for numerous mechanical systems. These parameters need to be utilized in order to select the most appropriate system for the house. Selections are based on the heating and cooling requirements of the house. After the system has been selected the next step is, duct design. You can have the most accurately done load calculation in the world but if it the air
is not distributed properly in the building, the performance of the system goes out the window. Manual T and D protocols cover duct design in great detail. Manual T goes over HVAC accessories and their proper placement. Good accessory selection can help alleviate comfort issues. Air stratification, whistling registers, and rattling dampers are common examples of issues that can be prevented by following this protocol. Supply registers have a greater role in comfort than return grilles, due to the Coanda effect. Coanda effect is described as a low pressure area between the air and the ceiling at or near a supply outlet. The low pressure causes the air flow to cling to the ceiling surface, which increases the supply throw. On a ceiling, any obstruction will interfere with the airflow being maintained. Changes in the direction of airflow can also have an impact on the Coanda effect. Air always takes the path of least resistance. For return grilles, one should be placed on every floor of the conditioned area. For heating only purposes, a lower level return is ideal. This is valid regardless of the supply registers location. Hot air rises, but in reality, during the heating season, cold air remains stagnant and always closer to floor than to the
ceiling. In cooling only seasons the air is stagnant near the ceiling, so it’s ideal to put a high side return. For both heating and cooling seasons, the most efficient location needs to be selected. A single return grille can have some performance issues – unless it’s a high-low return grille combination. Usually the return grille location is opposite to the supply registers. Adding a return to every room keeps the delta between conditioned space low, which is ideal. Poor air circulation can lead to performance and distribution issues in a house, the return grilles should be placed away from the supply registers to prevent short circuit of air flow. Moving on, to Manual D, this is the protocol that guides you with engineering standards to produce a proper duct design. This protocol can help balance the air delivery for the correct amount of air against the friction rate of the ducts and the static pressure required by the system fan. It also goes over different fittings and their impact on the duct system. It’s a very useful tool to calculate the friction rate and equivalent length of the duct system, two factors that can have a serious impact on the effectiveness of your duct design. The quality of ductwork has a significant effect on the performance
“You can have the most accurately done load calculation in the world but if it the air is not distributed properly in the building, the performance of the system goes out the window.”
38
BUILD MARYLAND
July/August 16