I am like most contractors and techs. I've heard about HEPA for a long time, but I never looked into how it could be integrated into an A/C IAQ strategy until now.

First, here's what HEPA is:

HEPA (High-Efficiency Particle Arrestance) is an ASME, U.S. DOE standard that specifies a capture rate of 99.97% of particles sized at 0.3 microns.

Because the HEPA standard isn't something is regulated or policed, there are many filters and products that use the term in one way or another. Even the term “True HEPA” has no scientific meaning and is often used to market products that don't meet the DOE standard.

For the sake of simplicity, let's just stick with the DOE standard and call anything that has been tested to capture 99.7% of 0.3-micron particles “HEPA.”

Let's start by answering the question on everyone's mind at the moment:

HEPA has been effectively demonstrated to capture particles the size of coronavirus.

So much so that in the airborne infection isolation rooms, the CDC allows air to be circulated out of the rooms when it has been filtered through a HEPA filter.

HEPA filtration is effective on many different particle sizes and has been shown to be even MORE effective when particles get smaller than 0.3 microns due to some weird behaviors of tiny “nano” particles called Brownian Motion.

Don't believe me? This is a great article on the topic by our friends over at Smart Air Filters.

HEPA filters are rated at 0.3 microns because particles of that size are dangerous to human health and are in the size range that has been demonstrated to be the hardest to remove.

HEPA Challenges

The issue with HEPA is that it works best with lower air velocity, AND they are SUPER restrictive. Running all of the air for your A/C system through HEPA is going to be tough, if not impossible. That is why manufacturers have often turned to other technologies like UV, electrostatic, cold plasma, and oxidizing ions to “assist” less restrictive filters.

Some of these products may hit the 99.7% number, but they may not do as well at filtering the smaller than 0.3-micron particles. They may also generate other byproducts like ozone (O₃) or formaldehyde or have costly maintenance. That isn't to say these other products are always a bad idea, but many aren't as tried and tested as HEPA.

Bypass HEPA

In the photo above, we are installing a bypass HEPA filter on the return of a system that serves a dentist's office. In this case, the blower runs continuously, so we decided to use a HEPA filter to filter 300 CFM of the 1700 CFM system airflow constantly. This HEPA unit comes with a carbon prefilter to help with odors and VOCs. The location has multiple filter back ceiling returns throughout the structure with pretty low face velocity due to the number of them.

We added 20 x 20 x 1 MERV 8 carbon filters to all of the filter back returns and the bypass HEPA system. We added MERV 11 filters in the existing 4″ media filter racks.

This did result in a static pressure that was slightly higher than we would like in a perfect world (0.7″wc), but we performed a full MeasureQuick system test and found that it was still performing well.

All of this was done at a price point similar to many of the high-cost UV/Ionization/PCO-type solutions but without some of the question marks those can bring, especially in a medical environment where the risks of chemical interactions and viral spread are higher.

Alternative Methods of Install 

In an ideal circumstance, the air from the bypass HEPA filter would be pulled from a central location inside the space and then discharged into the return AFTER the equipment filter to effectively reduce system static pressure and prevent the unneeded step of filtering air through the system filter after it has been HEPA filtered.

Like most things, the existing design will often dictate what is practical rather than ideal, and one of the beauties of HEPA is how many different ways you can install it.

Also, running the blower continuously at a low speed will help with filtration but may or may not be a good idea, depending on moisture load and equipment dehumidification capacity.

In Summary 

We are now offering HEPA solutions because it has been demonstrated to be effective above the other options, is cost-effective, and can be applied in a variety of applications without adding system static when installed in a bypass fashion.

Sure, it doesn't replace the existing system filter, but neither do many other options being sold today.

—Bryan

P.S. – As a final word on the topic, I know I will get messages about hundreds of other air-purifying technologies out there. Before asking more questions, please have a look at these resources.

Oxidizing Air Purification 

Why HEPA Works

EPA Guide on Air Purifiers

Berkley Air Purifier Study

 The HomeChem Study YouTube Playlist