Effects of Return Air Inlet Locations on Cooling Season Thermal Comfort in an Energy-Efficient Home

The objective of this field investigation was to compare the impacts on thermal comfort due to vertical location of return air inlets in a residential forced-air system operating in the cooling mode. Design guidelines for forced-air cooling systems recommend placing return air inlet locations above
the occupied zone in order to improve circulation of stagnant air and reduce thermal stratification. A companion study of the heating mode is under way. Results from the research will be used to recommend optimal return air inlet locations for yearround performance.
An energy-efficient three-story house in Pittsburgh, Pennsylvania, was used as the test site. A single HVAC system, zoned by floor, was operated at a constant setpoint temperature of 72F (22.2C) throughout the investigation. The vertical locations of return air inlets were modified using temporary cardboard ductwork to create two distinct cases for the investigation : one with high returns and one with low returns.
Two central data acquisition systems were used to measure system parameters and multiple space temperatures throughout the house on a minute-by-minute basis. The room air temperatures were compared to the requirements of ASHRAE Standard 55 (ASHRAE 2004b) for thermal comfort and recommendations of ACCA Manual D (1995) for uniform temperatures between rooms. Room air temperature stratification for all rooms and all hours was found to be well within ASHRAE
guidelines. Although pressure imbalances between spaces were not the focus of the investigation, they appear to have been the driving force of air leakage at zone temperature sensor locations, contributing to room-to-room temperature differences that exceeded ACCA guidelines. Evaluation of the data leads to the conclusion that high return air inlets help reduce stratification in spaces with floor diffusers. However, vertical location of return air inlets appeared to have no impact on cooling performance in spaces with ceiling supply air outlets.