Performance Analysis of a Modular Small-Diameter Air Distribution System

TO5 2.2.3: Modular Small-Diam. Air Distrib. System

Previous work by the U.S. Department of Energy's Building America research team IBACOS and other Building America teams has shown that for all but the most simple house geometries conditioned air must be supplied to each thermal zone. Standard methods that use traditional duct geometries have several drawbacks, which include difficulty in bringing ductwork into conditioned space because of the necessary bulkheads, long duct runs, large duct sizes, and a larger surface area to seal. The homebuilding industry is interested in alternative space conditioning systems that can address these concerns and provide thermal comfort and energy efficiency.
This report investigates the feasibility of using a home-run manifold small-diameter duct system to provide space-conditioning air to individual thermal zones in a low-load home. This compact layout allows duct systems to easily be brought within conditioned space via interior partition walls. Centrally locating the air handling unit in the house significantly reduces duct lengths. The plenum box is designed so that each connected duct receives a similar amount of airflow regardless of its position on the box. Furthermore, within a reasonable set of length restrictions each duct continues to receive similar airflow. For the plenum box investigated in this report, duct runs longer than 10 ft and shorter than 25 ft had airflows of 20-16 cubic feet per minute (CFM).
The design method uses an additive approach to reach the total required zonal airflow. Once the airflow rate needed to satisfy the thermal load of a zone has been determined, the total number of duct runs to a zone can be calculated by dividing the required airflow by the standard airflow from each duct. For example, if a zone requires 60 CFM of conditioned air to meet the load and the base airflow from each duct is 20 CFM, three ducts would be specified for that zone. This method differs from that of traditional duct system design, which relies on varying the diameter of the trunk and branch ductwork to manage static pressure (SP) and to supply the desired airflow to each thermal zone. The additive approach greatly simplifies the design effort and reduces the potential for duct design mistakes.
By setting up equivalent branch runouts and measuring the air velocity in a representative sample of ducts, the research team found two plenum box designs that maintained satisfactory uniform airflow from each branch duct (+/-5%).
After choosing the ideal box geometry, a full mockup duct design was then laid out for laboratory testing of airflow, SP, and power consumption of a ducted mini-split heat pump (MSHP) blower. The duct system was designed such that it could be installed in-and meet the airflow requirements of-the second floor of an example 3,600-ft2 production home.
Measured results indicate that this plenum design can satisfy the heating load (8,280 Btu/h). However, the total airflow falls short of satisfying the cooling load. The SP inside the plenum box, which is 51.5 Pascals (0.21 in. water column), limited the total airflow of the MSHP blower, which limits the total thermal capacity. A slightly oversized MSHP ducted unit could overcome the elevated SP. Fan energy consumption is kept to 0.22 watts per cubic feet per minute or lower by using short duct runs and smooth duct material.