Indoor Environmental Engineering
High air speeds in the occupied space but low airflow in the ceiling plenum return for the ventilation system. Why is this occurring?
The Building: The space investigated was on the north half of the 8th floor of a office building which was built around the year 2000. The occupied space is configured with private offices along the perimeter and administrative areas in the core of the floor. The supply air system is a variable volume under-floor system. Air is delivered to the core space from the under-floor plenum through circular diffusers in the floor and to the perimeter zones through small under-floor fan systems that draw the supply air from under the floor plenum and discharge out supply diffusers by the windows. The main return air intakes are located on either side of the core mechanical shaft, one pair in the ceiling plenum above the north core offices and one pair at in the central hallway between the north and south sides of the 8th floor. The suspended ceiling above the north side of the 8th floor is equipped with return air grills but the central hallway has a high ceiling so does not utilize a ceiling plenum return air strategy but instead drawls air directly from the hallway.
The Condition: This investigation was implemented in response to discomforts experienced by employees regarding air drafts. The occupants who experience discomforts sit in cubicles in the corner of the building along the hallway between the perimeter private offices and core administration areas.
Air speed measurements showed that the air speed was up to 4 times greater than the American Society of Heating Refrigeration and Air Conditioning Engineers (ASHRAE) recommendation, with the doors to the perimeter offices open in the area of the complaints, and dropped to below the ASHRAE recommendation with the doors closed to the perimeter offices in the area of the complaint.
Space pressure measurements and air current detector smoke showed that, when the perimeter office doors were open, supply air to the perimeter offices flowed through the office door and down the hallway but, when the office doors were closed, flowed up through the perimeter office return grills into the ceiling return plenum and back down through the suspended ceiling into the occupied space in the adjacent core area.