- Indoor air quality is the result of three levers: filtration, ventilation with outdoor air, and source and moisture control. HVAC influences all three, but not all equally.
- A higher MERV filter is only useful if the fan can still move the required air, the filter rack seals, and the filter is changed before it loads.
- Wildfire smoke needs a written plan: when to switch filters, when to reduce outdoor air, and how to inspect equipment afterward.
- Humidity is a quiet driver of both comfort complaints and mold risk, and it is controlled by system sizing, coil operation, and envelope tightness.
- Records (filter changes, readings, complaints, corrective actions) turn air quality from opinion into something you can manage and show.
The three levers of indoor air quality
Complaints about stuffy air, odors, dust, or allergy-like symptoms get filed under indoor air quality, but the causes vary. In practice, HVAC and building operations have three ways to influence what people breathe.
- Filtration removes particles from air passing through the system.
- Ventilation dilutes pollutants generated inside by bringing in and exhausting air.
- Source and moisture control prevents contaminants and dampness from existing in the first place.
Filtration gets the most attention because it is easy to buy. Ventilation and moisture often matter more, and they are where mechanical problems hide. We cover the first lever in detail below, then the others.
Understanding MERV and choosing a filter
MERV (Minimum Efficiency Reporting Value) is a rating of how well a filter captures particles of different sizes, on a scale commonly running from 1 to 16 for general ventilation filters. Higher numbers generally capture smaller particles, and they typically come with more resistance to airflow unless the filter is designed with more media area.
| Range | Typical captures | Typical use | Consideration |
|---|---|---|---|
| Low (about 1-4) | Larger particles such as lint and coarse dust | Basic protection of equipment | Does little for occupant air quality |
| Moderate (about 5-8) | Mold spores, dust, and some pollen | Common in standard commercial HVAC | Often an acceptable baseline |
| Higher (about 9-13) | Finer dust, many allergens, much of visible smoke particles | Offices, schools, clinics, retail seeking better air | Check fan and rack capacity; use deeper media if possible |
| High (about 14-16) | Very fine particles | Healthcare and sensitive spaces | Usually requires specific air handler design |
Before upgrading the rating
- Check the fan. Rooftop units are designed with a certain external static pressure. A dense filter on an undersized fan reduces airflow, which can lead to frozen coils or tripped limits.
- Check the rack. Gaps around filters let air bypass the media, which defeats the rating entirely.
- Use deeper filters where possible. Pleated filters in deeper frames offer more surface area and lower resistance.
- Change them on condition, not just by calendar. Differential pressure gauges across the filter bank take the guesswork out.
Ventilation rates and outdoor air
Buildings need outdoor air to dilute carbon dioxide, odors, and other contaminants generated by people, furnishings, and processes. The amount required depends on occupancy and space type and is set out in codes and standards that are periodically updated. We do not quote numbers here, because they depend on the code edition and the use; the design engineer or the local code defines them for your space.
What we do see in the field is the gap between design and reality. Economizer dampers stuck closed, outdoor-air dampers throttled down to cut load, minimum-position settings that were never commissioned, and spaces whose occupancy changed after the system was designed. Each of these leaves the building under-ventilated while the equipment appears to run fine. An outdoor-air check is a standard part of a thorough rooftop service visit.
Carbon dioxide sensors can indicate whether ventilation is keeping pace with occupancy. Elevated readings in a conference room, classroom, or waiting area suggest ventilation, not necessarily poor filtration, deserves attention. Sensors need calibration, so treat readings as a trend rather than a verdict.
Humidity and moisture
Sustained high humidity encourages mold and dust mites, while very low humidity irritates occupants and increases static. For most commercial occupancies, holding relative humidity in a moderate range is the goal, although specific spaces such as labs, museums, or healthcare areas have their own requirements.
- Oversized cooling equipment short-cycles and removes less moisture, leaving spaces cool but clammy.
- Condensate drains and pans that hold standing water can become a microbial source and an odor source.
- Dirty evaporator coils harbor growth and reduce dehumidification.
- Envelope leaks and open doors bring humid air in; the HVAC system then fights a losing battle.
- Coastal humidity adds load to cooling coils even on mild days.
Where humidity problems persist, a root-cause diagnostic usually uncovers the combination of sizing, controls, and envelope factors behind it.
Coil and filter loading: the slow degradation
Dust builds on filters and coils gradually. As the filter loads, airflow falls, the fan works harder, and the space may lose capacity before anyone notices. Dirty evaporator coils reduce heat transfer and can shed particles into the supply air. Condenser coils clog and raise head pressure. A planned inspection cycle, with a record of what was found, keeps this from becoming a failure. See the preventive maintenance guide.
A wildfire smoke response plan
Southern California residents know the pattern: fire in the region, smoke and ash for days, then a clearing. For buildings, a plan prepared in advance beats improvisation. Elements vary by building type, but a reasonable outline looks like this.
- Decide your triggers. Use local air-quality information to decide when the plan begins, and designate who makes the call.
- Upgrade filtration where the system allows. Install the highest-rated filter the fan and racks can handle without losing airflow.
- Adjust outdoor air. Reduce outdoor-air intake to a minimum acceptable amount during heavy smoke, recognizing the ventilation trade-off, and restore it afterward. Where codes or health-care uses require continued ventilation, follow your engineer's guidance.
- Check seals and doors. Leaky doors and damaged gaskets defeat the efforts at the air handler.
- Monitor filters closely. Loading will be fast; plan extra changes.
- Inspect after the event. Clean coils, check drains, replace the smoke-loaded filters, and note readings.
- Consider a clean-air room for occupants with respiratory vulnerability, such as a space with dedicated filtration.
Schools and clinics have particular responsibilities here. Facilities in the schools category often write the plan into their emergency procedures.
Documentation: making air quality manageable
An air-quality program that lives in one person's head disappears when that person leaves. A modest record set is enough.
| Record | Why it matters |
|---|---|
| Filter type, size, rating, and change dates per unit | Shows consistent maintenance and helps predict loading |
| Differential pressure readings | Detects loading and bypass |
| Outdoor-air damper position and test results | Shows ventilation is working as designed |
| Humidity and CO2 trends where measured | Documents conditions over time |
| Complaint log and corrective actions | Shows responsiveness, and highlights recurring zones |
| Smoke-event actions and post-event inspection | Demonstrates preparedness and supports insurance or audit requests |
Each unit's record in the DaVinci Portal can hold exactly this kind of history, accessible by QR code on the unit. For building-wide scheduling, see building controls and automation.
Frequently asked questions
Should I just install the highest MERV filter available?
Not without checking. A very dense filter on a fan not designed for it can reduce airflow and damage equipment. Match the filter to the unit's capacity, and use deeper media where possible.
How often should commercial filters be changed?
It depends on the environment and runtime. Many sites change quarterly, but dusty, coastal, kitchen-adjacent, or smoke-affected sites need shorter intervals. Pressure gauges help decide.
Does closing outdoor air during wildfire smoke hurt air quality?
It reduces smoke entering but also reduces dilution of indoor contaminants. Use it for limited periods and follow engineer or code guidance for your building type.
Can HVAC fix a musty smell?
Sometimes. Drain pans, wet coils, and humid conditions are common causes, and cleaning or fixing drainage can resolve it. If moisture comes from the building envelope, HVAC adjustments alone will not.
What documentation should I keep for an audit or insurer?
Maintenance logs, filter change records, ventilation checks, and corrective actions for complaints. Consistent, dated records are more persuasive than a one-time inspection.
Have a site this applies to?
Davinci Mechanical is the commercial and union division of Scottish Tom's Heating & Air. Send us the equipment list or the problem and we'll tell you what we'd check first.