Ventilation Deficit and Filter Upgrade in a 4,000 ft² Office
A 4,000 ft² single-story office building is served by a 7.5-ton constant-volume rooftop unit. Forty people occupy the space. At 3 p.m. on a mild 72°F day, indoor CO2 reads 1,250 ppm at the return grille, occupants report afternoon headaches, and there is a persistent musty odor near the return. The economizer actuator is seized with the outdoor-air damper at its minimum position, and the minimum-position setpoint has never been adjusted since commissioning. Measured outdoor air at the intake is 300 CFM. Maintenance recently replaced the original MERV 8 filters with MERV 13 in the same two-inch filter rack, and total external static pressure is now 1.4 in. w.c. against a 0.9 in. w.c. rating. The evaporator drain pan holds standing water and the coil face has visible biological growth.
1. Calculate the minimum outdoor-air requirement using the breathing-zone rate of 5 cfm per person plus 0.06 cfm per ft², and compare it to what the building is actually getting.
Read the worked answer
People component: 40 occupants × 5 cfm/person = 200 cfm. Area component: 4,000 ft² × 0.06 cfm/ft² = 240 cfm. Combined breathing-zone outdoor air = 200 + 240 = 440 cfm.
Measured intake is 300 cfm, so the building is short by 140 cfm, about 32% below the minimum. Per person, the space is receiving 300/40 = 7.5 cfm, but the area component is being ignored entirely.
Supporting evidence: 1,250 ppm CO2 is well above the roughly 1,000 ppm ceiling that corresponds to adequate per-person ventilation in a typical office. Afternoon headaches that clear when occupants leave are consistent with a ventilation-rate deficit plus a stuck economizer that never brings in free cooling on a 72°F day.
The correction is not simply opening the damper; the minimum position must be set by measured airflow (balometer or duct traverse) to at least 440 cfm, the economizer actuator replaced and functionally tested, and demand-controlled ventilation considered if occupancy varies.
2. Explain how the MERV 13 upgrade interacts with airflow, and what measurements you would take to decide whether to keep it.
Read the worked answer
A two-inch filter rack designed for MERV 8 has a small face area and high face velocity. A MERV 13 in the same slot typically triples the pressure drop, from roughly 0.15–0.25 in. w.c. clean to 0.40–0.55 in. w.c. The unit is already at 1.4 in. w.c. total external static against a 0.9 in. w.c. rating, so the fan is far off its design curve.
Consequences of running off the curve:
- Airflow drops, lowering sensible capacity and raising the supply-to-return split.
- Evaporator temperature falls and the coil can ice, which further cuts airflow and can slug the compressor.
- Higher static loads the blower motor and can trip thermal overload.
Measurements:
- Filter pressure drop with a differential gauge, clean versus loaded.
- Total external static at the air handler, plus component drops across coil and filters.
- Actual airflow from the fan performance table, or a duct traverse if the table is ambiguous.
- Supply-to-return split and coil temperature to check for icing.
If the fan cannot be re-tapped or the blower cannot carry the load, add filter area with a V-bank or a four-inch media cabinet rather than downgrading filtration blindly. Filtration should be matched to a documented airflow, not swapped by part number.
3. The musty odor and standing water: what is the root cause, and what remediation would you specify?
Read the worked answer
Standing water in the pan with visible growth on the coil face means the coil is staying wet and the pan is not draining. Three contributors are visible in the data:
- Drain failure. The pan must slope to the drain, and the drain must have a properly sized P-trap with a depth greater than the fan's negative static at the pan. A missing or shallow trap holds water in the pan.
- Reduced airflow. Low airflow from the filter upgrade drops the coil temperature, lowers the sensible heat ratio, and leaves the coil and drain pan wetter for longer. That is a direct biological-growth driver, not a cosmetic issue.
- Stuck economizer and poor ventilation. Higher CO2 and occupant load raise internal moisture and reduce perceived air quality, compounding the odor complaint.
Remediation: shut down, clean and sanitize the coil and pan, verify pan slope, install or correct the trap, flush the drain line, replace the filter with a configuration that restores airflow, and add UV-C at the coil only as a supplement after the moisture source is fixed. Then verify airflow, split, and space relative humidity. Odor returns if the coil stays wet.