Auditorium: Front-Row Draft and Rear Stagnation in the Same Room
A 400-seat university auditorium, 80 ft × 60 ft with a 14-ft ceiling over the seating, is served by a constant-volume air handler delivering 8,000 cfm at a 20 °F supply-to-room difference, through sixteen 4-way ceiling diffusers at 500 cfm each in four rows of four. Return air leaves through six ceiling grilles located in the first two rows. The diffusers have 12-in. necks and were selected for a 12-ft throw to 50 fpm at 500 cfm. Complaints: the front rows report cold draughts on neck and shoulders, measured at 55 fpm with 66 °F supply; the rear rows report stale, warm air in the afternoon, measured at 78 °F and 1,500 ppm CO2, against 850 ppm in the front row and 420 ppm outdoors. The air handler resets its outdoor-air damper from a CO2 sensor in the return duct, currently reading 950 ppm. Attendance at the complaint time is 260 people.
1. Explain why the front and rear complaints have different causes, and why increasing airflow would be the wrong response.
Read the worked answer
Front rows: 500 cfm through a 12-in. neck 4-way diffuser with a catalogued 12-ft throw to 50 fpm, in a 14-ft ceiling, puts the 50-fpm envelope roughly 8–10 ft above the floor - precisely at the head and neck of a seated occupant. The measured 55 fpm at 66 °F exceeds the ASHRAE 55 winter limit for 1.1 met / 0.5 clo (about 40 fpm at 70 °F), so it is a real draft, not a perception complaint. The diffusers are behaving as selected; the selection itself was wrong for this mounting height.
Rear rows: 1,500 ppm CO2 and 78 °F while the front reads 850 ppm is stagnant, under-mixed air, not a shortage of outdoor air (see the next task). The return grilles in the first two rows short-circuit supply air straight back to the air handler; the rear of the room is swept only by the tails of the jets.
Raising airflow or fan speed fixes neither. It would increase throw and velocity at the front, making the draft worse, and it would not change the return path, so the rear would stay stagnant.
2. Use the CO2 data to determine whether outdoor air is deficient or whether the problem is distribution. State your assumptions.
Read the worked answer
Steady-state mass balance: Q = G × 10⁶/(Cs − Co), with G ≈ 0.0105 cfm of CO2 per person at light activity.
At the return duct: 950 − 420 = 530 ppm, so Q = 0.0105 × 10⁶/530 ≈ 19.8 cfm per person, or about 5,150 cfm for 260 people. That is above the 15 cfm/person design minimum. Even if the DCV damper were sitting at its minimum position (3,900 cfm = 15 cfm/person), the predicted return rise would be 0.0105 × 10⁶/15 = 700 ppm, i.e., about 1,120 ppm - higher than the measured 950 ppm.
So the total outdoor air is adequate; the rear reading of 1,500 ppm is a local ventilation-effectiveness failure. The return grilles are sampling air that has just left the diffusers, which is why the return sensor reads low and why the DCV never calls for more air. The sensor is measuring the short-circuit rather than the occupied zone.
Caveat: the generation rate and the perfect-mixing assumption are approximations; the conclusion is robust because the gap between predicted and measured is in the direction of more outdoor air, not less. Confirm with a zone CO2 reading at breathing height in the rear before changing the air handler.
3. Recommend specific changes and the acceptance criteria you would use to sign the room off.
Read the worked answer
- Move the return to the rear of the room - low or high, or split front and rear - so return air is drawn through the occupied volume rather than across the ceiling next to the supply. This is the single change that addresses the rear stagnation.
- Change the front diffusers to a higher-induction, shorter-throw type, or add outlets and reduce per-outlet flow, so the 50-fpm throw terminates at no more than about 60% of the distance to the occupied zone.
- Verify coverage at the rear: four rows of four diffusers may leave the last rows outside the jet envelope. Add diffusers or use a linear slot with a long attached throw along the rear wall.
- Relocate the CO2 sensor into the occupied zone (or use several and average), so DCV responds to the people rather than to the short-circuit.
Acceptance: ADPI ≥ 80 over the occupied zone per ASHRAE 113; local air speed ≤ 40 fpm at neck height in the heating case; NC ≤ 30 in the seating area; CO2 in the rear no more than 700 ppm above outdoor at design occupancy; and a post-retrofit trend showing the front-to-rear CO2 difference has collapsed. Re-check throw and ADPI at the turndown condition as well, because the room is not always at 8,000 cfm.