Your customers are in their homes a lot these days. Because of this, they are more aware of their HVAC systems and the air it circulates. This awareness leads them to doing many internet searches for the best indoor air quality (IAQ) solutions. The odds are high they find a lot of silver bullets.

In stories, a silver bullet is the projectile of choice to kill scary things that we don’t understand. The term also describes a simple solution to a complicated problem. Unfortunately, silver bullets in the HVAC industry rarely hit the mark because we take them out of context and ignore foundational principles.

These principles result in successful solutions when you use them together correctly. They often provide a healthy environment and exceptional comfort at the same time. Let’s look at the top five IAQ principles and how you can use them daily to better serve your customers and address issues important to them.


Equipment Cleanliness

For decades, our industry has promoted the benefits of clean equipment through maintenance. To clarify, I’m talking about thorough cleaning to prolong equipment life and keep it operating at peak performance — not a quick wipe down with a service rag. There’s no thrill in cleaning a blower wheel, but it gets results.

Another key area of equipment cleanliness is the indoor coil and drain pan. These are breeding grounds for all kinds of nasty stuff. In this situation you may consider ultraviolet (UV) light applications on coil and pan surfaces. Do your research and choose wisely before you randomly select a product. Some applications could have side effects and cause damage to system components.

As equipment cleanliness degrades, it’s important to ask yourself, “Why does the equipment get in this condition in the first place?” The answers to this question lead us to other principles. Let’s start at the air filter.


Air Filtration

The air filter is likely the first accessory most of your customers think about when they consider IAQ. Unfortunately, it could contribute to many of their IAQ issues. For a filter to work effectively, you must size it for the air volume you expect to move through it.

An undersized or restrictive filter is ineffective — regardless of its minimum efficiency reporting value (MERV) rating. One factor that influences poor filtration is too much filter face velocity. In other words, the airspeed moving through the filter media.

Most filter manufacturers rate their filter media at different face velocities in feet per minute (fpm). A popular value for most filter types is 300 fpm. When you exceed the rated face velocity, it can cause decreased filter performance.

Remember, air takes the path of least resistance. If air can’t go through a filter, it will bypass it. This is one reason high-efficiency filters can decrease IAQ instead of improving it. You can learn more about how to deal with restrictive air filter issues in my past Duct Dynasty article, “Confronting Restrictive Air Filters,” published March 28, 2016.


Building Pressure

What if dirt and dust don’t make it to the air filter? Some time ago, I had insane dust issues in my home. I was a victim of negative building pressure and the consequences that go with it.

Mother Nature loves balance. She likes it when the same amount of air that goes in, comes out. My colleague Paul Wieboldt refers to this as the “gozinta” and “gozouta” effect. To put it another way, if one cubic foot (cfm) of air blows into the living space, the same amount should come out. So, one cfm in equals one cfm out.

Unfortunately, there are system defects that exist such as duct leakage, door closure, and airflow imbalances that cause air to come from places you don’t want. Air loaded with cellulose fibers from my attic flowed into my home from airflow imbalances (basement supply ducts with no return and no air balancing).

Once we identified these issues and corrected them, it changed the air quality of our home. Fixing the air distribution issues and air balancing played a big role in the solutions. The other principle we used to improve our dust issue was to add an outside air duct. That leads me to the next principle, ventilation.



In the late 1970s, there was an energy crisis. As a result, there was a push to tighten up buildings in the name of energy efficiency. This led to some air quality problems the World Health Organization (WHO) labelled as “sick building syndrome.” One of the major culprits in these buildings that led to their “sick” condition was lack of ventilation.

Over the years, we’ve gotten wiser about adding outside air in buildings. We better understand its ventilation benefits and side effects. The HVAC system is often the primary source that brings in outside air. This is typically through a controlled location, such as an economizer, outside air duct, or a dedicated system such as an Energy Recovery Ventilator (ERV) or ventilating dehumidifier.

In addition to ventilation, outside air can also change the way a building leaks air. Building air leakage is often random and uncontrolled. It is referred to as infiltration (air going in) and exfiltration (air going out).

When you use an outside air duct for ventilation, you also change the source(s) of infiltration and control the air. The last thing you want is air for ventilation coming in from a nasty source. Be aware of how much ventilation you provide, it may affect the next principle, indoor air conditions.


Indoor Air Conditions

The last principle we’ll look at are the aspects that determine indoor air conditions. Pay specific attention to dew point, humidity, enthalpy, and dry bulb temperature. These factors heavily influence the indoor air conditions and their relationship to comfort and air quality. In a future article, we’ll examine these relationships in more detail.

Each building has different load requirements. Therefore, it’s important to size your equipment correctly. It’s hard to achieve acceptable indoor air conditions with oversized equipment that short cycles and cannot remove enough moisture.

It’s not enough to assure the correct equipment size. You also must make sure the equipment operates near its rated capacity to condition the living space at design conditions. If you hook the equipment up to a pathetic duct system, your customers won’t have the comfortable indoor conditions they expect.


Next Steps

Now that you have an overview of these five IAQ principles, I have a reminder. There are no silver bullets, only silver shotgun shells. Each principle above is a piece of silver buckshot in a casing that provides a powerful punch when you use them together.

Learn the principles that truly impact IAQ and understand how they work together. While I briefly covered each item, you can find entire books on each subject. I encourage you to seek them out and do some research.

Realize that a single principle might not provide the intended results. It could even create additional issues that you didn’t predict if you misuse them. However, following these few basics goes a long way towards success:

  • Assure you size filters for the right amount of airflow and acceptable pressure drop.
  • Control building pressures with exceptional duct design, installation, and balancing practices.
  • Provide ventilation to assist with needed air changes and prevent the recirculation of stale air.
  • Properly size equipment so it can handle the building load and maintain comfortable conditions.

Above all, don’t prey on people’s fears. Maintain integrity in your daily operations and serve customers like they are family. Those actions remove their fear quicker than anything.