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HEPA Air Purifiers Vs Upgrading Your HVAC System

How to Choose the Right Ventilation System to Reopen Safely

COVID has brought increased attention to indoor air quality concerns however more needs to be done to raise awareness for better ventilation in buildings

In this article we will highlight the pros and cons of a HEPA air filtration system as compared to upgrades to your HVAC ventilation system. The decision for which is best depends upon your situation and what you are looking to achieve. In some cases, a HEPA air purifier will be best and in others the HVAC.

teacher and students staying safe from covid in classroom

How to Achieve Good Indoor Air Quality

The best guidance we have seen on improving indoor air quality comes from the EPA. 

There are three strategies: source control, ventilation and air cleaning.

Source control means removing the source of the pollutant. For example, smoke outside instead of inside. What we see often is that source control is not possible.

The next step is to improve ventilation.  This means bringing in fresh air from the outside.

For a residence this means opening windows since most home air conditioning systems do not bring in air from the outside but rather recirculate the air within the home. This can work well provided you do not bring in higher/lower temperatures, humidity, pollen, air pollution from traffic, etc.

For an office or classroom, you can improve ventilation through a properly sized HVAC system. Guidance from experts at the Harvard University School of Public Health recommend 5 air changes per hour.

A limitation to a commercial HVAC system is that they generally use lower efficiency air filters that do not filter out fine particles such as viruses.  Keep reading below for a further explanation.

The use of an air purifier is third strategy and most often utilized when source control and proper ventilation cannot be achieved.  This could be due to cost, an older building where ventilation improvements are not reasonable, higher levels of air filtration are required (ie COVID, allergies, smoke), or areas where proper ventilation cannot be achieved.

 

Clean the air to reopen schools, classrooms, buildings

What it Takes for Proper Indoor Air Cleaning

Air cleaning comes down to two principles: the efficiency of the air filter and moving enough air for the room size.

The efficiency refers to the percentage of particles of a given size that are removed by the filter with each air pass. If you want to clean the air of tiny particles like viruses or fine dust, a high efficiency filter is needed otherwise the particles will pass through the filter and be recirculated in the room.

 

How Ventilation Systems are Designed for Air Filtration

The air filter in an HVAC system is designed to keep dust from accumulating in this system and not for air cleaning the air in a space for the benefit of people. These air filters often have a lower efficiency to collect large dust particles while minimizing the air flow resistance.

An HVAC is designed to cool or heat a certain sized area.  The problem with later introducing a higher quality air filter is that the air flow will decrease to where you do not get proper air flow for heating, cooling or ventilation. It also can result in the AC or heat running more often to maintain the temperature which has higher energy costs and creates more stress on the system.

 

When to Choose to Upgrade the HVAC

You are better off upgrading your HVAC system when you have very large spaces to clean.  While an air purifier can get you the higher levels of air filtration, they often do not do well in larger spaces (larger than 1,500 sq ft). A way to get around this with portable HEPA air purifiers is to use multiple units.

Upgrading the HVAC is also a good idea if you mainly need to increase ventilation and not as concerned with removing fine particles.

 

When to Choose a HEPA Air Purifier

Since an air purifier has a fan it is able to use higher efficiency filters to collect the finer airborne particulates.  The smallest particles are the most dangerous since they stay suspended in the air the longest and most likely to be breathed into your lungs.  A ventilation system filter will not capture these and a HEPA air cleaner is your best bet.

In commercial buildings we most often see air purifiers used in addition to the outdoor air ventilation system.  As mentioned above, the guidance for proper classroom or office air quality is 5 air changes per hour.  You can get there by supplementing your HVAC system with an air purifier. So, if the ventilation has 3 air changes per hour (ACH) you only need 2 ACH from the air purifier.

 

What to Look for in an Air Purifier

 The guidance from Harvard University states:

  • Look for a portable air purifier with a HEPA filter
  • Look for a high Clean Air Delivery Rate (CADR)
  • Avoid add-ons (ionizers, UV lights, etc)

The reason for choosing a HEPA air purifier with a high air flow for COVID is that the virus is passed through the air.  You need to clean the air quickly to remove the virus particles.  Only a HEPA filter can capture a virus and do so with enough air flow.

Other technologies like UV require contact time of hours and minutes and that doesn't get the job done when you need to remove airborne particles. Air cleaners that focus on electronic filtering are not suggested and in some cases schools are removing systems that were purchased in the fall.

The CADR that they reference represents the volume of clean air that comes out of the air purifier.  There are three different CADR’s and look for the smoke CADR when sizing a room.

Learn more about selecting the right air purifier for your room size

 

Reaching Ventilation Needs By Opening Windows and Air Cleaning

As outlined in this New York Times article another option is to open window and use air purifiers to achieve proper ventilation levels.  Opening windows is a great way to bring in fresh air to a school. Be sure to check out this article. The 3D graphics paint a very clear picture for how a virus spreads in the air in a classroom.

Considerations for opening windows include the weather (hot, cold, humid, raining), outdoor air quality (air pollution from a highway or urban center), and whether a particular room has windows to open.

Recent guidance from the CDC also recommends opening windows where possible and using portable HEPA air cleaners.

 

Resources

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