How Residential HVAC Impacts Air Quality

Feb 26, 2019

When I do an indoor air quality assessment in residential properties, one of the first things that I look at is the heating, ventilating and air conditioning system (HVAC).  Mechanical systems can greatly aid in improving indoor air quality. I’ll discuss how in this blog post.


Ventilation is important in maintaining air quality in that it helps remove or dilute airborne pollutants coming from indoor sources (such as paint, cooking, stains, sanding, etc.) .  It also helps in making the air less stale and can reduce various odors.

One of the best (and cheapest) ways to provide ventilation into your home is simply to open all your windows and let the breeze move air throughout.  Unfortunately, during seasons of extreme heat and cold, this is not always the best (or most comfortable!) way to ventilate. This can also be a problem if the outdoor air quality is poor. There are a few different methods of mechanical ventilation that can overcome these challenges.  

HRVs and ERVs

Heat recovery ventilation (HRV) and energy recovery ventilation (ERV) both provide the home with a steady flow of fresh outdoor air.  During the winter, an HRV takes stale, warm air that is sent out of the building and the heat recovery core warms the incoming fresh, colder air before it is distributed to the home.  During the summer, the heat exchange is reversed and the incoming outdoor air is pre-cooled when the exhaust air picks up heat on its way out of the building.

Energy Recovery Ventilation (ERV) is similar, with the notable exception that it also keeps the humidity level on the inside of the home stable. In the winter, it transfers indoor humidity from the exhaust air into the outdoor fresh air; during the summer the humidity transfer is reversed.   

What is the primary difference between an HRV and an ERV?   Both are similar devices in that they supply outdoor air to the building and exhaust stale indoor air while recovering energy from the exhaust air. The difference is that an HRV system transfers heat while an ERV transfers both heat and moisture.

Outdoor air intake

A less sophisticated method of mechanically bringing in outdoor air is to have a duct that connects an outdoor air intake to the return duct.  This method typically only brings in outdoor air when the central air is running. However, there are smart controls that can force the system on to provide sufficient ventilation.


Air Filters and MERV

Air filters on an HVAC system are an important way to remove dirt, dust and other particle-based contaminants from the indoor air, but not all filters are created equally.  The Minimum Efficiency Rating Value (MERV) is an industry standard rating system for media air filter efficiency and allows the consumer to have a better understanding as to the quality of various filters.  The rating has a numerical value ranging from 1 (lowest efficiency) to 16 (highest efficiency). But be careful, higher efficiency filters can increase the pressure drop and actually damage the air-handling unit.

MERV values 1 through 4 are only able to remove larger particles and would be a poor choice in filtering indoor air.  MERV 8 through 13 air filters are often a good balance between efficiency and pressure drop. When selecting a filter for your air-handling unit, it is important to consider both filter efficiency and pressure drop.  It’s important to note that having too high of a MERV rating can place more stress on the HVAC system and cause particles to bypass around the filter. Generally, the wider the filter, the lower the pressure drop. However, most homes only have a 1-inch slot for a filter.  The safest bet is to discuss air filtration with a licensed mechanical contractor.

Humidity Control

Typically when people think of humidity, they think of the summer and how much harder their air conditioner needs to run in order to have a more comfortable home indoors.  But low humidity can also affect your home during the winter by making the air too dry. To receive the appropriate indoor comfort, it is important that humidity is not too high or too low.  But that only gives one side of the story.

Humidity can have a major effect on indoor mold growth and on personal health.  When warm, humid air reaches a cold surface, condensation is likely. Condensation greatly increases the chances of mold growth occurring when coupled with a food source (paper, wood, dust, etc.).  We recommend having relative humidity not exceed 60%. However, when outdoor temperatures are below freezing, we recommend that the humidity level should not exceed 35% to further prevent possible condensation.   

Being able to control humidity in your home can best be achieved by having a portable dehumidifier to use during hot and humid months and an HVAC system that includes a humidifier to use during cold and dry months.  


HVAC systems can greatly aid in making indoor air quality healthier and more comfortable.  But a poorly designed system can take a potentially bad situation and make it much worse. In my next blog, we shall investigate how HVAC systems can make the indoor air quality significantly worse.