Mixed-Mode Retrofit on a Busy Street
A 1985 three-story office retrofit houses 120 staff. Two facades have operable windows; the west facade faces a six-lane arterial where continuous PM2.5 monitoring reads 32–38 µg/m³ during the 7–9 a.m. and 4–6 p.m. peaks, falling to 8–12 µg/m³ overnight. Mechanical ventilation is provided by three AHUs with MERV 8 filters; the engineer reports two AHUs cannot accept MERV 13 without fan and motor upgrades. Conference rooms reach 1,150 ppm CO2 by mid-afternoon; open-office CO2 averages 750 ppm. Staff report afternoon headaches and stuffiness, and facilities has responded by taping the west windows shut. The owner wants a phased plan that improves perceived air quality within one budget cycle without promising a full HVAC replacement.
1. Rank the four interventions - source control, outdoor-air ventilation, filtration, and the window policy - and justify the order using the measurements.
Read the worked answer
Order: source control → ventilation → filtration → window policy.
- Source control first. The conference rooms at 1,150 ppm are the only readings clearly outside a reasonable target, and they are occupancy-driven. Reducing design occupancy density or adding demand-controlled outdoor air to those rooms addresses the cause; filtration and taped windows do not.
- Ventilation second. At 1,150 ppm with roughly 500 ppm outdoor air, those rooms are short on a per-person outdoor-air basis. The open office at 750 ppm is acceptable, so do not resize the whole system - target the conference rooms.
- Filtration third. MERV 13 on the two upgradeable AHUs reduces indoor PM2.5 from recirculation, but only in zones those AHUs serve, and it does nothing for the CO2 problem.
- Window policy last and conditionally. Taping the west windows removes dilution and occupant control. Replace the tape with a written rule: west windows closed when the street monitor exceeds about 25 µg/m³; east windows open when outdoor PM is low. This keeps dilution without importing peak traffic PM.
2. The owner can fund either MERV 13 upgrades to two AHUs or CO2-controlled ventilation for the six conference rooms. Which do you recommend, and what evidence would change your recommendation?
Read the worked answer
Recommend CO2-controlled ventilation first, for three reasons:
- The complaint pattern is stuffiness and headaches in occupied rooms, and the conference rooms are the only spaces with a measured ventilation failure (1,150 ppm vs 750 ppm elsewhere).
- A MERV 13 retrofit on two of three AHUs leaves one-third of the floorplate unimproved and does not touch the ventilation deficit.
- CO2 control is cheaper to verify: you can watch the same sensors that produced the baseline.
What would change the answer: If zone-to-AHU mapping showed the conference rooms are served by the two AHUs that cannot be upgraded, filtration becomes a prerequisite for any recirculation-based fix. If outdoor PM2.5 at the air intakes were consistently above about 25 µg/m³, filtration would move ahead of ventilation because added outdoor air imports pollution. If the conference rooms were already near 800 ppm at design occupancy, the CO2 signal would be weak and the case for filtration would strengthen.
3. Where would you place monitors and what would you compare to prove the fix worked?
Read the worked answer
Monitor placement
- CO2 at breathing height (1.1–1.2 m) in each conference room, away from supply diffusers and doors, plus one open-office reference.
- PM2.5 in at least one west-facing and one east-facing occupied zone, plus one outdoor reference at the west facade.
- Log at 5-minute intervals for at least two weeks before and two weeks after each phase.
What to compare
- Conference-room peak and 95th-percentile CO2 during occupied hours, normalized to the same meeting occupancy - not raw averages, because occupancy drives the number.
- PM2.5 indoor/outdoor ratio during the 7–9 a.m. and 4–6 p.m. peaks, to test whether window rules and filtration actually reduced infiltration.
- The complaint log, using the same survey wording before and after.
Do not claim success from a single afternoon of low readings; the baseline problem appeared mid-afternoon.