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Can a home’s HVAC hurt indoor air quality?

Mold on Duct

In a prior blog post, I discussed how a home’s heating, ventilating, and air conditioning (HVAC) system can aid in controlling a host of indoor air quality issues.  Now let’s take a look at how the HVAC can hurt indoor air quality (and hopefully see how to avoid them)!

Lack of proper ventilation in HVAC can lead to poor indoor air quality

Ventilation is a key component in keeping our homes healthy.  One of the best ways of ventilating a home is simply opening up a window to allow good air in and bad air out.  Unfortunately, in many parts of the country, there are extremes in temperature, not to mention rain and snow. It is not always desirable to have the doors and windows open.  

Typical HVAC systems in residential homes use air to heat and cool the home, but airflow is not the same as ventilation.  The air ducts are typically a closed loop that merely recirculates air inside the house. Problems with this system occur when the indoor air becomes stale and contaminants are trapped inside the home.  

What is the solution?  Bring in outdoor air! Believe it or not, most homes do not have a system that brings in outdoor air.  This would require a mechanical ventilation system which helps improve indoor air by bringing outdoor air into the home.  Depending on the system, it may provide a constant supply of fresh air while sending stale air outdoors. Other systems have mechanical ventilation that only runs when the thermostat is calling for heating or cooling.

HVAC systems that are too large provide poor humidity control

When you have an oversized cooling system in a residential property, you run the risk of having elevated humidity indoors during warm months.  That’s a bit counter-intuitive. Why is this the case? Oversized systems can make the temperature in the home drop more quickly when the AC is turned on.  But the faster the temperature drops, the less time the system is running, and the less moisture is removed from the air. What results is that although the home may be cool, the occupants still feel uncomfortable and clammy from high humidity.

A potentially greater danger can occur when warm, humid indoor air can hit a cold surface and create condensation.  This, in turn, can create a ripe environment for mold growth. This is a problem both in the winter and the summer.  The picture above is an example of what happens when warm humid air lands on a cold surface, in this case, a drywall soffit with cold air conditioning ducts running through.

Basements often times need additional dehumidification because the HVAC system has a harder time handling the humidity.   Having a dehumidifier in the basement can help reduce the relative humidity and the possibility of mold growth.

Lack of proper filtration of small particles

Poor filtration can lead to elevated amounts of particulate matter (PM).  What is particulate matter? Particulates are very small particles that can include dirt dust, soot, and smoke.  Because of their small size, the body’s natural filtering system (the nose) cannot block these very small particles from entering into our bodies and can create a number of health issues. HVAC’s filters help with this problem, but if you do not change the filters on a regular basis, the indoor air can get pretty bad in short order.  It is important to regularly change the filters and verify that they have the appropriate MERV (Minimum Efficiency Reporting Value) rating.

Dirty systems in HVAC can hurt indoor air quality

Dust can accumulate on cooling coils in an HVAC system and lead to clogged and/or moldy parts.  Bacteria can also form on these coils, which can cause a dirty sock smell in the system. Proper maintenance and cleaning is a good idea in preventing these items from being blown throughout the house.

When dust and mold accumulate in the ductwork, you run the risk of these unhealthy items blown throughout the home.  Although the EPA does not specifically endorse duct cleaning, they do recognize its importance if the ductwork has blockages or when there is the existence of mold growth and bacteria. 1

HVAC and comfort

Last (and certainly not least)  we address the area of comfort. Comfort is usually measured by the temperature and humidity in a home.  Humidity issues were addressed a few paragraphs above, but what about temperature? Many HVAC systems result in one room being colder or warmer than others.  Compound this problem with the fact that people have their own sliding scale of what is considered comfortable.  In most residential homes that have a single heating and cooling zone, this can be just a fact of life. However, some homes can provide a more even temperature with multiple heating and cooling zones.  This is a more expensive option, but heating/cooling costs can be reduced by only maximizing the HVAC in the rooms currently being occupied. Larger homes will typically have more than one zone (or HVAC system) which can help with comfort and costs.


Health and comfort are important issues to address with any home.  Trained professionals can help diagnose if you have an HVAC issue and can provide the best solutions in allowing you to have a safe home environment.

  1. https://www.epa.gov/indoor-air-quality-iaq/should-you-have-air-ducts-your-home-cleaned#Summary

Ian Cull

Ian Cull is a nationally recognized expert in the field of indoor air quality. He is the Chief Science Officer of Indoor Science, a company he started in 2004. He speaks around the world on air quality topics and is a training provider of the Indoor Air Quality Association. Mr. Cull is a Licensed Professional Engineer (PE) and Certified Industrial Hygienist (CIH). His degree is in Environmental Engineering from the University of Illinois - Urbana Champaign. Mr. Cull has developed 50 air quality related courses for the IAQA University and is the author of the book, “Fundamentals of Mold Remediation”. In his words… “Besides being passionate about indoor air quality, I enjoy cycling, music, the Chicago Bulls, and having fun with my three kids.”

2 thoughts on “Can a home’s HVAC hurt indoor air quality?

    Thank you for writing this article. It sheds light on indoor air quality and written so the lay person understands. I look forward toothed next one. Keep up the good work.