Ventilation and indoor air quality

Ventilation controls indoor air quality by bringing in outdoor air to dilute pollutants and exhausting stale air from the building. Most air quality problems come from too little outdoor air reaching the occupied space, a pollution source that is not being controlled, or a humidity problem, and each can be found with a methodical look at the building and its HVAC system.

What makes indoor air poor

Indoor air carries a mix of contaminants from people, materials and activities:

• Carbon dioxide and body odours from occupants, which rise when ventilation is not keeping up with the number of people.
• Volatile organic compounds from paints, adhesives, furniture, cleaning products and printers.
• Fine particles from outdoor air, wildfire smoke, combustion and indoor activities.
• Combustion gases, including carbon monoxide, from faulty fuel-burning equipment or vehicle exhaust drawn in from parking areas or loading docks.
• Moisture and biological growth from leaks, condensation and wet materials. Our guide on moisture, mould and indoor air covers this in detail.
• Radon, a radioactive soil gas that can enter through foundations.

In a typical commercial building, these contaminants are handled by a simple loop. Outdoor air is drawn in at the air handler, filtered and mixed with return air, and supplied to the occupied space. Some of the room air returns to the air handler; the rest leaves through local exhaust in washrooms, kitchens and copy rooms, pulling transfer air with it. Problems arise wherever that loop is broken or short-circuited.

Three ways to control indoor air quality

1. Control the source. Removing or isolating a pollutant is the most effective step. Examples include choosing low-emitting materials, exhausting printer and copy rooms, and fixing leaks quickly.
2. Ventilate. Outdoor air dilutes what cannot be removed at the source. Local exhaust in washrooms, kitchens and labs removes pollutants before they spread.
3. Filter and clean the air. Filters remove particles from recirculated and outdoor air. See HVAC filters and MERV ratings.

How much ventilation is enough

Building codes across Canada set minimum ventilation requirements, and for most commercial buildings they reference ASHRAE 62.1, which calculates outdoor air based on both the number of people and the floor area of each space. Residential buildings follow separate requirements. The design rate is only a starting point: what matters is how much outdoor air the system actually delivers to occupied spaces, which can drift well away from the design over time.

Common ventilation failures

• Outdoor air dampers stuck closed, or set to a minimum position that was never verified.
• Economizer controls disabled or broken.
• Fans scheduled off during occupied hours, or ventilation reduced to save energy without checking the effect.
• Rooms subdivided or used by more people than the design assumed.
• Supply air short-circuiting from diffuser to return without mixing into the occupied zone.
• Outdoor air intakes placed near exhaust outlets, loading docks or garbage areas.
• Exhaust fans that have failed or have blocked grilles.

What to measure

A focused set of measurements tells you a lot:

Measure during normal occupancy, and over time rather than in a single spot reading. Trends across a working day reveal much more than one number.

A practical approach to complaints

1. Record complaints: who, where, when and what symptoms or odours.
2. Walk the affected area and look for obvious sources, water damage, blocked vents and changes in use.
3. Check the HVAC equipment serving that area: outdoor air dampers, filters, schedules, drain pans and exhaust.
4. Take measurements over time during occupied hours.
5. Fix what you find, then follow up with the occupants to confirm the problem is solved.

Many complaints are solved at steps 2 and 3. If they are not, specialist sampling may be needed.

A typical case

A training room in an office building gets complaints of stuffiness and drowsiness every afternoon, but only on days when courses are full. Mornings are fine, and the rest of the floor has no complaints.

Following the steps above, a CO2 logger placed in the room for a week shows levels rising steadily through full-day sessions and staying low on quiet days, which points to ventilation not keeping up with occupancy rather than a pollutant source. At the air handler, the outdoor air damper is found at a minimum position set when the space was an open office with far fewer people per square metre. The room's supply diffusers are delivering roughly what the original design called for, but the design did not anticipate a training room.

The fix in a case like this may be as simple as increasing the minimum outdoor air for that air handler, or adding demand-controlled ventilation so outdoor air rises when CO2 rises. Either change should be checked by measuring outdoor air at the unit afterwards, and the room should be monitored again to confirm the complaints have stopped. If the air handler cannot supply enough outdoor air for the new use, the designer needs to know before the room is booked full time.

Common mistakes

• Treating a single spot CO2 reading as a verdict on the building.
• Adding air purifiers to a room that actually needs more outdoor air.
• Changing the use of a space without checking its ventilation.
• Reducing outdoor air to save energy without measuring what reaches occupants.
• Ignoring the intake: snow, debris, nearby idling vehicles or exhaust outlets can all affect what comes in.

Balancing air quality and energy

Outdoor air has to be heated in winter and cooled in summer, so there is a real energy cost to ventilation. Heat and energy recovery ventilators recover much of it, and demand-controlled ventilation adjusts outdoor air to actual occupancy. Our guide on HRV vs ERV explains the options.

How ehZee can help

Our mechanical system inspections check whether your ventilation equipment is delivering outdoor air as intended, and trace air quality complaints back to the equipment and conditions behind them. You get a written report with findings ranked by priority.

Common questions