Can Your HVAC System Filter Out Coronavirus?

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Using a high-MERV filter doesn't have to result in a large pressure drop

Yesterday I gathered with a few of my friends and colleagues from the building performance community for a virtual happy hour.  During our chat, Steve Byers of Energy Logic posed the question:  Compared to BC (before coronavirus), how will you be different once the stay-at-home orders are lifted, the coronavirus pandemic is behind us, and we go back to whatever our new normal will be?

My first thought was that I'm going to wash my hands a lot more than I did BC.  That may seem like just a superficial change (sorry), but it's connected to a deeper behavioral change.  I'd like to think my handshaking days are behind me, too.  When I went to Orlando earlier this year for the ASHRAE meeting and AHR Expo, I shook a lot of hands...and one of those hands ended up giving me a cold. 

And then there's the air.  I've been doing building science and indoor air quality work for nearly two decades, and now the coronavirus pandemic has me thinking about particles in the air even more than I did before.  I think I'll never think about the word "droplets" the same way again, especially after seeing so many visualizations of them recently.

But everyone seems to be thinking more about what's in the air around them.  This is a wonderful thing, but it's also likely that some people will overreact, falling for ineffective or even harmful methods to improve their indoor air quality.  So let's take a quick look at filtering the air inside your home.

Spending time at home

With most of us staying at home these days, we're breathing the air in our homes more than usual.  The average American spends about 90% of their time in buildings in normal times, but that times is split between home, work, gyms, bars, and other buildings.  Home was always the highest at about 69%; now it's mostly in the home.

So if you want to filter the air in your home to improve the indoor air quality and maybe remove any coronavirus floating around, what do you need to know?  Well, you need to know what size particles to filter out and what kind of filters will do that.  And you need to know how long the particles carrying the coronavirus will hang out in the air.

A lot of the droplets expelled when a person sneezes, coughs, or breathes are large, from 10 µm up to 100 µm and a few even larger.  (1 µm is one micrometer, usually called a micron, and 1,000 microns equal 1 millimeter.)  They fall out of the air quickly.  Because they don't spend much time in the air, your HVAC system will almost never get a chance to catch them unless someone happens to be standing near a return vent.

The smaller particles, 10 µm or less, are the ones that will float around in the air for hours.  One early study on SARS-CoV-2 has found that the virus on those smaller particles can remain intact and infectious in the air for up to three hours.  That's the stuff you can have an effect on with good filtration.

High-efficiency filtration

The standard one-inch deep fiberglass filter is meant to stop only the larger stuff, like the dust you can see floating around in the air.  The rating that describes filtration effectiveness is Minimum Efficiency Reporting Value, or MERV.  That one-inch filter is about MERV-1.  (Unfortunately, not all filters come with a MERV rating.  Some filter sellers have decided to come up with their own rating systems.  I don't buy those filters.)

The chart below (credit:  John Semmelhack of Think Little) summarizes the percentage of particles trapped in a filter by the size of the particles.  As you can see, MERV-1 does very little. 

Filter effectiveness by MERV rating and particle size  [Chart assembled by John Semmelhack, Think Little]

MERV-13 is the minimum filter rating that we recommend in our HVAC design work.  To get that rating, it needs to remove at least 90% of particles in 3-10 µm range, 85% of particles in the 1-3 µm range, and 50% of particles in the 0.3-1 µm.

MERV-16 filters out  more than 95% of the particles in all three of those ranges.  In comparison, an N95 mask goes a step further and must filter out at least 95% of particles smaller than 0.3 µm.

Caution:  Your HVAC system may not be able to handle a MERV-13 filter

Yes, to get good filtration, you need to be thinking about MERV-13 as a minimum.  I've heard that ASHRAE is planning to recommend a minimum of MERV-14 for better filtration of coronavirus.  But if you have a one-inch filter slot next to the air handler, you may end up causing problems — perhaps expensive problems — if you replace the standard MERV-1 filter with a MERV-13 filter or better.

The problem is resistance to air flow.  That MERV-13 filter will add a lot of resistance to the air flow in your system, which reduces the total amount of air the system can move.  The result may just be lower air flow and everything still works sort of OK.  But it could also reduce the air flow enough to cause problems with heating and cooling.  It could crack the heat exchanger in a furnace or damage the compressor in an air conditioner or heat pump.  See my article on the unintended consequences of high-MERV filters.

Now, it can be done properly so you get good filtration without much resistance to air flow.  I've written about that, too.  Please take a look at that article, titled The Path to Low Pressure Drop Across a High-MERV Filter, to find out the best way use high-efficiency filtration in your HVAC system.

Filtration is good, but be careful

The bottom line here is that you definitely can improve your indoor air quality and filter out particles that carry viruses.  MERV-13 is the minimum rated filter I recommend.  But just popping a high-MERV filter in your system may not help because of some well-known unintended consequences.  Caveat emptor.

And speaking of caveat emptor, that applies to ultraviolet (UV) treatment as well.  We'll take a look at that topic next week.

 

Allison Bailes of Atlanta, Georgia, is a speaker, writer, building science consultant, and founder of Energy Vanguard. He is also the author of the Energy Vanguard Blog. You can follow him on Twitter at @EnergyVanguard and pre-order his upcoming book at Publishizer.

 

Related Articles

Can You Use HVAC Filters in Coronavirus Masks?

The Path to Low Pressure Drop Across a High-MERV Filter

The Unintended Consequences of High-MERV Filters

Which Indoor Air Pollutants Matter Most?

 

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Comments

If an older system, designed for the old 1" filter, can't handle a MERV 13 would a UV light in the system help kill any nasties that might be floating around?

Allison
Bailes

Dave, that's an excellent question.  I'll have an article on UV treatment next week, but the short answer here is that for residential use, I'd avoid it.

Air scrubber plus - produces hydrogen peroxide vapor coping mother nature with positive and negative ions that destroy not filter every surface area in the house , 1968 technology space certified , can't live without it

And why is this Allison?

Allison
Bailes

Ibry, I assume you're asking why I don't recommend UV lights for residential HVAC.  One of the big problems is that many of the products available produce ozone, which is a great thing when it's in the stratosphere protecting us from UV from the sun but not something you want in your house as it's a pollutant that's bad to breathe in.  Also, their effectiveness is questionable.

Allison,

What's your opinion on re-usable/washable electrostatic filters available at big box stores?

Have you ever heard of someone using a fiberglass filter sprayed with pledge?

TIA

Yet another excellent tutorial, Allison! Your informative insight is always welcome.

On a filter related topic, we use a sonic humidifier in the bed room to help offset the desperately dry air here in Northern Nevada. Unfortunately, we also suffer extremely hard water And the white powder that accumulates everywhere also ends up clogging our ceiling return filter in fairly short order. I've had to resort to the least expensive filters available and even at that, the filter begins to hiss within 2-3 weeks and is a powdery mess, ready for replacement, shortly thereafter. While distilled water would solve the problem, at 1/2 gallon a day, that gets expensive (yes, I'm penurious), so cheap filters it is.

Instead of buying distilled water (paying someone else to boil it and then condense it, bottle it, ship it...etc) Why not boil your water directly such as using a steam humidifier? Sure, whatever you boil it in will scale up, but that's much more manageable than what you are doing now.

Boiling a half gallon of water into steam requires around 5,000 Btus. Doing that via electrical resistance works out to 20-25 cents worth of electricity.

We live in southern Nevada and just recently changed from an ultrasonic humidifier to an evaporative one. The model we got is actually an air washer and humidifier in one. It is made by Venta. It does not produce any visible mist and therefore no white dust. We love it!

I've been tracking metals that can kill covid-19. Do you see filters being impregnated with copper or silver to help kill the virus?

Metals will only kill if they can touch. So, unless the filter captures the virus, and there is a metal where it's caught, it won't help. But if the filter already captured the virus, it will die long before the filter is changed, so the metal was unnecessary. Plus, at home, the virus is likely to be circulating in the house long enough before being filtered that you will catch it anyway. Commercial/retail might be another story.

The pressure drop over a filter is a function of air speed at the filter face, right? And that is a function of filter area, why don't you mention adding returns with filters, as a feasible way to enable MERV-13 if the old system has a problem. I know you know this.

Allison
Bailes

Mark, yes, you're right.  Since I've already written a whole article about that, I didn't want to reproduce it here but instead point people to that article.  Your comment me sent me back to look at the text of this article and I saw that I didn't make that connection strongly enough.  I've changed that so I hope people will go there to find out how to it properly.

For anyone who doesn't want to go back up into this article to find the link, here it is:

The Path to Low Pressure Drop Across a High-MERV Filter

What about electrostatic air filters and ultraviolet light ?

Allison
Bailes

George, you're better off with better mechanical filtration.  The effectiveness of electrostatic filters and UV lights in HVAC systems is questionable and UV lights often produce ozone, a pollutant.

I've been reading about far- UVC ligjting. It's supposed to have less serious consequences when compared to regular UV. Columbia University is doing a study on this

This is the first blog that actually makes sense that I have read amidst the virus outbreak. Great title! Also, I feel the virus cannot be filtered out because of its minute size but anyway hope for the best!

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