Improving Indoor Air Quality: Heating, ventilation, air conditioning (HVAC)

What is HVAC?

Ventilation is the process of bringing fresh air into a building and exhausting stale indoor air out of the building. HVAC systems are the tools used in most schools to control fresh air intake, exhaust old, stale air, and control indoor temperature and humidity. In the process of bringing fresh air into a school and expelling stale indoor air, a properly functioning HVAC system can control odors and reduce pollutants that cause most IAQ problems inside buildings.

Why is HVAC an IAQ issue?

In the past, efforts to reduce energy usage led some school planners to seal buildings tightly. Some buildings that are tightly sealed do not provide adequate ventilation for building occupants. Common IAQ problems in schools are often the result of inadequate ventilation in buildings that are too tightly sealed.Without adequate ventilation, pollutants can build up inside the building and result in IAQ-related complaints.

Common HVAC system problems and related IAQ issues include:

  • Closed outdoor vents or dampers that do not allow for adequate fresh air intake (sometimes vents are closed to save energy) – Without adequate fresh air, a building may develop a variety of IAQ problems.
  • Obstructed outdoor screens on the fresh air intakes – When screens are blocked, obstructions can prevent adequate fresh air, or debris in the screen can bring outdoor pollutants into the building.
  • Filters that have not been changed on a regular basis – Filters only work to prevent potential contaminant sources from entering the building if they are changed regularly.
  • Moisture in drip pans and ductwork – Moisture can lead to mold growth. If mold grows in one part of the HVAC system, it can be distributed throughout the school.
  • Poor outside air quality – If the air outside the school is contaminated with chemicals, smog, mold, pollen, and/or vehicle exhaust, the outdoor air intakes can bring contaminant sources into the school.

Getting Started

HVAC system

School HVAC systems should be designed and operated to provide a minimum outdoor air ventilation rates consistent with the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) 62.1 Standard which for classrooms is about 15 cubic feet per minute (cfm) of outdoor air per person.

Natural ventilation

Designers should consider the use of natural ventilation and operable windows to supplement mechanical ventilation.Consider outdoor sources of pollutants (including building exhausts and vehicle traffic) and noise when determining if and where to provide operable windows.

If operable windows will be used to supplement the HVAC system, ensure that:

  • openings for outdoor air are located between 3-6 feet from the floor (head height);
  • the windows are adjustable and can close tightly and securely;
  • the windows are placed to take maximum advantage of wind direction, with openings on opposite sides of the building to maximize cross-ventilation.

To find more information on improving indoor air visit:

Tools for Schools the IAQ Tools for Schools Program is a comprehensive resource to help schools maintain a healthy environment in school buildings by identifying, correcting, and preventing IAQ problems. This program offers no-cost, low-cost solutions to reducing indoor environmental pollutants.

Reference: EPA Indoor Air Quality Design,