Air quality complaints: a field diagnostic approach

An air quality complaint is a symptom, not a diagnosis

In a commercial or institutional building in Greater Montreal, “the air feels heavy on the third floor” is one of the most common requests — and one of the most poorly handled. It’s tempting to open the dampers, push the airflow up or change a setpoint just to make the complaint go away. But an air quality complaint is a perceived symptom, sometimes subjective, and the technician’s job is to turn it into objective measurements before touching a single setting.

The method below is a diagnostic tree: rule out the real hazard first, measure second, and only trace the cause once the numbers are in hand. That discipline is what separates a lasting fix from a patch that simply moves the problem from one floor to another.

⚠ Safety — carbon monoxide comes before everything else

Carbon monoxide (CO) is odourless and colourless: it isn’t part of the “perceived” symptoms of a comfort complaint, but it can be present whenever a combustion appliance is involved. Before any other measurement, check for CO. A stable, repeated reading means stopping the comfort diagnosis, evacuating if necessary, and searching for the source — chimney backdraft, a mistuned appliance, an air intake contaminated by exhaust. Anything involving a combustion appliance ties directly into gas heating and combustion expertise, which is inseparable from air quality.

Tools required

• A reliable, calibrated CO detector (safety first);
• A multi-parameter IAQ instrument measuring CO2, temperature, relative humidity and air velocity — for example a commissioning-grade tool such as the testo 400, TSI or Fluke 975;
• A way to measure or estimate fresh-air rate (velocity at the damper, damper position, economizer sequence readout);
• Access to the control sequences and the maintenance log for the system.

Step 1 — Document the complaint before measuring

Before reaching for the instrument, frame the problem. Where exactly? What time? Which symptoms: stuffy air and drowsiness, odours, dryness and static, or headaches? Each points in a different direction. Drowsiness and “heavy” air late in the afternoon usually point to CO2 rising with occupancy; winter dryness points to humidity; a localized odour points to a source or an air short-circuit. Always cross-reference the complaint with the occupancy schedule and the system run schedule: a zone that’s only uncomfortable when it’s full is almost always a fresh-air supply problem.

Step 2 — Measure CO2, temperature and humidity

CO2 is the best indirect indicator of ventilation per occupant. Outdoor air now sits around 420 ppm. ASHRAE sets no regulatory limit on indoor CO2, but its Standard 62.1 recommends the indoor concentration stay no more than about 700 ppm above outdoor air at steady state — often a practical target around 1,000 to 1,100 ppm in an office. Beyond that, it isn’t a toxicity question (the RSST workplace limit is 5,000 ppm over 8 hours, far from the comfort threshold), but a clear signal: fresh-air supply isn’t keeping up with occupancy.

Measure temperature and relative humidity too. The comfort range generally falls between 30% and 60% relative humidity; keep it below 60% to limit mould risk, and above 30% to avoid dryness and winter static. Always take a reading in a complaint-free reference zone: a clear gap between the affected zone and the rest of the building localizes the problem, while uniform values point to a global setting.

Step 3 — Verify fresh-air supply

If CO2 climbs with occupancy, trace it back to the source. Is the outdoor-air damper in the right position? Is the economizer sequence working, or stuck at minimum? Is the damper seized? On systems with a heat recovery ventilator (HRV), a fouled or mis-sequenced recovery unit cuts fresh air as much as energy efficiency. The goal isn’t to push airflow to the maximum — over-ventilating in winter dries the air and drives up heating costs — but to match supply to actual occupancy.

Step 4 — Inspect filtration and distribution

Enough fresh air, poorly distributed, still produces complaints. Check filter loading: a clogged filter cuts airflow and unbalances the network. Verify supply/return balance and look for dead zones (air that doesn’t circulate) and short-circuits (supply air going straight back to the return without sweeping the occupied space). These distribution faults often explain why a single zone complains while the system, overall, delivers enough air.

Step 5 — Correct, then re-validate

The diagnosis doesn’t end at the fix. After reopening a damper, correcting a sequence, replacing a filter or rebalancing a network, re-measure in both the affected zone and the reference zone to confirm a return to target ranges. Without that re-validation, you don’t know whether the complaint is solved or simply relocated. Most of these causes — filters, dampers, recovery units, sequences — fall under rigorous preventive maintenance: a serviced system generates far fewer complaints than one that’s only touched in reaction.

Normal values at a glance

• CO: no meaningful indoor reading expected; any stable value means an immediate search for the source (RSST limits 35 ppm / 175 ppm short-term are ceilings, not targets).
• CO2: aim below roughly 700 ppm above outdoor air at steady state (often ~1,000–1,100 ppm in an office); beyond that means insufficient fresh air.
• Relative humidity: 30% to 60%, staying below 60% to limit mould.

This is the level of rigour the Montréal Combustion team brings to every intervention.