HVAC Design Project

One of the projects for ME 405 (Thermal Systems Design) was to design a central air-conditioning/heating system for a new house in the Seattle Tacoma area. The scope of the design included calculating the heating and cooling loads of the home, sizing an appropriate heat pump and air-handler, and designing a duct system to deliver the required air to rooms in a quiet and even manner. Considering the regional climate and the house structure, the regional load factor (RLF) method, as specified by the 2013 ASHRAE Handbook, was implemented to determine appropriate equipment to maintain the desired temperature year-round. For both heating and cooling, the envelope, infiltration/ventilation, internal gains, and distribution losses were calculated.

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The envelope loads from all components of the house were calculated using the U-factors of the roof, ceiling, exterior walls, windows, and doors. The relevant fenestration, infiltration/ventilation, distribution loss, and internal gain loads were utilized in these calculations. 

With the total heating and cooling envelope loads determined for the house, a heat pump system was sized to provide the proper heating and cooling loads. A split unit, with a separate outdoor and indoor unit, was selected as it allowed for greater efficiency and more freedom in duct layout.

With a heat pump and air handler selected based on the cooling and heating loads, the duct system to deliver the conditioned air to the living spaces of the house was then designed. First, the areas of house that required conditioned air were determined and split into smaller zones to ensure the house would be heated/cooled relatively evenly. The location and number of registers in each zone were then reasonably determined based on room size. Registers were strategically placed near fenestration to prevent drafts due to the cooling air around the windows/doors.

With register locations decided, the ducting was designed. A trunk and branch system was utilized due to the rectangular shape of the house to reduce the required duct material. In order to ensure the duct system supplied the appropriate amount of conditioned air the air changes per hour (ACH) was determined for the house. The duct and registers were then sized to ensure the correct flow rate could be achieved to register without excessive noise due to large airflow velocities. To maintain a relatively constant airflow velocity throughout the duct system the diameters of the ducts were reduced as the air travels further from the blower.

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Lastly, in order to analyze the performance of the duct system, the head loss of each component (ducts and fittings) was determined along with the total pressure head required from the blower unit. This was done using the energy equation applied along streamlines from the blower inlet to register exit. The designed HVAC system composed of a split unit heat pump and trunk and branch duct system was determined offer excellent performance during use in a new home in a Seattle neighborhood.