The Unintended Consequences of High-MERV Filters

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Aprilaire high-MERV filter with carbon

What is a hair's breadth?  A strand of the finest human hair is about 20 micrometers (also called microns) in diameter.  Take a piece and slice it in half.  Then take one of those halves and slice it in half again...and then one more time.  Now you've got a sliver of hair that's 2.5 microns wide.  Officially, 2.5 microns is 0.0000025 m, or 2.5 µm.  If you must, that would be about 0.0001 inch.  Whatever units you use, though, it's a really small size.

Fine particulate matter and filters

That also happens to be the size of particulate matter that causes a lot of indoor air quality problems.  Last month I wrote that fine particulate matter is the really bad stuff.  Fine particulate matter is also called PM2.5 because it's the same size as that superfine hair sliced in half three times.  It's 2.5 µm or smaller. 

The size is important because really small things can get into places that bigger things can't.  Like deep into your lung tissue, where it can find its way into the bloodstream.  And that was the point of my article on which indoor air pollutants are of most concern.  

The size of PM2.5  (Image from US Environmental Protection Agency; click to visit page.)

But you're in luck.  As it turns out, the PM2.5 particles can be filtered out of the air with the right type of filter.  After my article on indoor air pollutants, I wrote about filters and even included a nice chart from my friend John Semmelhack.  It shows the percent of different size particles trapped by filters with different MERV ratings.  The chart shows that to remove at least 90% of the PM2.5 from the airstream, you have to use a MERV-13 filter.  Of course, you should read that whole article because I also talked about a lot of reasons why you may not be getting the filtration you think you're getting.

Today, though, we're going to home in on just this one issue of upgrading to a filter with a higher MERV rating, a filter that can remove more of the small stuff.  First, we'll take a quick look at what MERV is and then some of the potential problems with using high-MERV filters in your heating and air conditioning system.

What is a MERV?

MERV stands for Minimum Efficiency Reporting Value.  It's an acronym created by ASHRAE to provide a scale showing the effectiveness of filters at removing particles of different sizes.  The range of sizes in the MERV scale is 0.3 µm to 10 µm, subdivided into smaller ranges.

For example, to achieve MERV-13, a filter has to catch 90% of particles in the 3-10 µm range, 90% of particles in the 1-3 µm range (where PM2.5 is), and 50% of particles in the range 0.3-1 µm range (the really, really small stuff).

In general, filters with higher MERV ratings catch higher percentages of particles as well as smaller particles.  And MERV-13 is pretty much where you want to be.

What could go wrong?

When you put a filter in the airstream, the air has to go through it.  (Unless you have bypass, but you’re reading this so I'm guessing you already know to use a good filter cabinet and install the filter in it properly.)  Going through the filter material, the air encounters resistance

A high-MERV filter uses material with higher resistance to air flow

How much resistance depends on the type of filter material and the amount of area.  The standard fiberglass filter doesn’t resist the air flow much, at least not when the filter is clean.  As the filter loads up with the stuff it catches, the resistance increases.

If you want to remove more stuff from the air, you can use a filter with a different material.  The pores in these filters are much smaller, allowing the filter to catch more pollutants from the airstream but also adding more resistance.  The higher MERV filters have higher resistance.

When you add more resistance, you can get a bigger pressure drop across the filter.  And that’s where the problems begin.  The blower in the furnace or air handler is rated for a certain amount of pressure drop.  If you use a lot of it up just to get the air through the filter, there’s not much left to move the air through the rest of the system.

When the pressure in the duct system goes up, it's possible that the blower will use more energy.  Paradoxically, that happens with the more efficient blower motors (ECMs), and not so much with the less efficient PSC motors.  (OK, OK.  Here's what those two things stand for:  ECM = electronically commutated motor;  PSC = permanent split capacitor.)

And that brings up the next problem:  reduced air flow.  If your system is rated for a total pressure drop of 0.5 inches of water column (i.w.c.) and you use up half of it just at the filter, you’re not likely to get the amount of air flow the system was designed to deliver.

One consequence of low air flow is reduced comfort.  In the hottest and coldest weather, your system may not be able to deliver enough heating or cooling to the house.  Or it may just be one or two rooms that get uncomfortable.

Another consequence of low air flow is that the air conditioner coil may get too cold and even freeze up.  That starts a vicious cycle of lower and lower air flow and a colder and colder coil until the coil is just a block of ice.  Then the refrigerant doesn’t evaporate and goes back to the compressor in the liquid state, which can damage the compressor.

Same thing can happen with a furnace.  Low air flow means the furnace heat exchanger gets hotter.  It could even crack.  If that happens, your duct system can become a carbon monoxide distribution system.  You’ll sleep well…but you may not wake up.

But there is a positive consequence that comes with low air flow, too.  In a humid climate, the air conditioner will dehumidify better.  That’s only a benefit as long as the air flow isn’t so low that it turns the condensate to ice.

To summarize, a high-MERV filter can:

  • Add resistance to the air flow
  • Increase the pressure in the duct system
  • Increase the energy use of the system
  • Reduce the air flow
  • Cause comfort problems
  • Freeze the air conditioner coil
  • Damage the compressor
  • Crack the heat exchanger
  • Put carbon monoxide in your home
  • Dehumidify better

These are things that can happen, but what really does happen?  And how do you avoid those pitfalls if you do use a high-MERV filter?  We'll answer those questions in the next two articles with a look at the research and at what works.

 

Related Articles

7 Reasons Your Filter Isn't Improving Your Indoor Air Quality

Which Indoor Air Pollutants Matter Most?

What Happens to Air Flow in Ducts When Size Changes?

 

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Comments

Not all filters have a Merv Rating & many of those are purchased at the big box stores.

Allison
Bailes

Yep.  Unfortunately some companies, like the big box home improvement stores, have decided to create their own rating scales, thus confusing everyone.

Wouldn't a greater pressure drop across the filter decrease the pressure in the duct system not increase it?

Allison
Bailes

Andy, yes, you're right.  A big drop across the filter means the pressure in the return duct is lower, not higher.  But the pressure difference between the return duct and the air outside it will be higher as a result.  Also, the total external static pressure in the system will increase.

I just realized we have two different pictures in our head. I'm thinking of a typical Michigan basement furnace with the filter 4 inches from the blower motor and all of the outside of that.

If you're using VRF mini-splits in a tight house, what kind of system would you install for filtration?

Allison
Bailes

Good question, Lee.  If those mini-splits are ducted, it's easy to put in a standard filter.  That's what we're doing in our office.  With ductless mini-splits, you'll have to get your high-MERV filtration elsewhere.  It could be in a standalone filtration system with its own fan.  It could be part of the ventilation system.

My impression is that ductless heads, being ductless, don't have the capacity to mount much pressure head, and therefore you would look elsewhere for high-efficiency filtration, whether plug-in HEPA cleaners or a ducted dedicated ventilator designed for use with a HEPA or MERV-13 filter. Having filtration powers independent from heating and cooling demand is a good thing anyway.

One option, particularly for the South, is a Ventilating Dehumidifier (e.g., UltraAir 98H). Comes with a MERV 13 filter. They have external housing for a MERV 14 filter as well, kind of wish I'd gotten that installed as well.

http://mattrisinger.com/dehumidifier-necessity-for-texas-homes/

Probably useful even up north, but perhaps there are ERVs with enough ummph to handle MERV filtration as well?

Allison
Bailes

Avery, you are correct.  Ductless mini-split heat pumps come with a little filter whose job is to keep the coil clean.  You have to clean it regularly to keep the air flow up.

Allison
Bailes

James, yes, a ventilating dehumidifier is a good choice for mixed-humid and hot-humid climates and it can definitely help with the filtration.  Depending on the size of the house and the static pressure, though, you may or may not get enough filtration volume.  Most energy recovery ventilators (ERVs) and heat recovery ventilators (HRVs) don't give you MERV-13 or better but there's at least one that does:  the Zehnder CA200.

The Broan (used to be Venmar units) ERV200TE has a HEPA option. Also has my favorite controls in the Broan line because you can independently select the high and low speeds, each from the full range, and balances by reducing motor speed instead of dampers.

Hi Allison. I just want to point out that MERV-13 filters are available In ALL Zehnder ERVs and HRVs.

Thanks Allison! This is a really big deal. We may be able to help our clients get a super-insulated and amazingly strong building envelope up quickly and sealed tightly with low-skilled labor, but THEN that's where the next challenge starts - proper mechanical ventilation. And without skilled professionals knowledgeable of proper filtration and its appropriate motor sizing you can be creating problems instead of solving them.

Allison
Bailes

All too true, Charles.  As my friend Kristof Irwin says, we need to fight the "hegemony of enclosurism."  Mechanical systems matter!

Brent Stephens' Built Environment Research Group at Illinois Institute of Technology did some measurements of filters that can be found here: http://built-envi.com/portfolio/filter-testing/ In terms of pressure drop, the data appears to show that the main way to get good filtration while minimizing pressure drop is to use larger filter depths. For example, the 5" deep Lennox Healthy Climate MERV 16 has about 40% less pressure drop @0.12" wc compared to the 1" 3M MPR 600 (equivalent to MERV 8) @0.21" wc.

Allison
Bailes

Brian, you seem to know where I'm going.  Brent's paper from his grad student days at UT Austin with Jeff Siegel is one of the studies I'm going to discuss in the next article.  And I'll be talking about the issue of filter depth.

This is why a deep filter media cabinet and a semi-annual preventive maintenance program comes with every heat pump system we install.

Be a little careful specifying high MERV filters with ducted mini splits - their air handler "static budgets" are often much lower (1/4 - 1/2) than that of conventional air handlers...depending on individual system circumstances, there nay not be enough static to spare. Proceed with caution.

Allison
Bailes

Curt, yes, you have to know what you're doing to do high-MERV filters on ducted mini-splits.  But I'd argue that you should be just as careful with doing it on any ducted system.  John Semmelhack gave a talk at the North American Passive House Conference in Boston this year and showed how they're putting MERV-13 filters on Fujitsu ducted mini-splits and doing so with a pressure drop of only 0.03 i.w.c.  We're in the process of installing a Mitsubishi ducted mini-split rated for only 0.2 i.w.c. in our office and are aiming for 0.03 i.w.c. or lower.  I'll let you know how low we get it after it's installed.

Hi Allison,

Great topic and discussion. Home Energy published an article by Dave Springer of Davis Energy Group at the time, of pressure drop and energy use of several MERV filters. It was from 2009 but still quite relevant I think. Here is a link to the free article:

http://homeenergy.org/show/article/nav/indoorairquality/id/667

Allison
Bailes

Jim, now you've gone and spilled the beans on one of the other research studies that's going to be part of my next article.  And yes, it is absolutely still relevant.

An HVAC instructor I know used to say the most efficient air filter is a piece of plywood. No particles would get through it, but no air would get through it either.

I was fairly comfortable looking at filtration for PM2.5 (even thought it becomes obvious that the best way to address that is with a stand-alone air filtration unit) until I saw the HomeChem scientists' video series on YouTube. PM0.5 particles? Ozone causing new chemical combinations? Now I'm much more alarmed and see that all this discussion about IAQ looks to be too little, too late.

Great article. Improper filter design extremely common, and is one of those "little things" that can kill system performance, in more ways than one.

Increasing filter surface area (by increasing depth and/or length & width dimensions) solves the pressure drop issue. In new construction this is easy to achieve. My specs require filter face velocity not to exceed 200 FPM, or roughly 2 sq.ft. per ton. Even less for ducted mini's. Problem solved.

Existing construction is more challenging. Simply replacing a 1 inch filter with a 4 inch may not be enough, especially if the filter cabinet is mounted at immediately before the blower (sized to match the return opening). In that case, it's sometimes easier to get rid of the filter at the air handler, add another return and install filters at the return grilles. But 'ya gotta make sure those return ducts are tight. Otherwise bypass will eat your lunch.

New homes utilizing ductless mini-splits shouldn't have a significant need for additional fine particle filtration, unless they are affected by outside sources such as high ambient pollution levels. For many homes, a HEPA filtered vacuum provides enough control of particle pollution to maintain good IAQ.

David Eakin, I wouldn't stress too much about the research on ultrafine particles and ozone reactions. That research is for ambient (outdoor) air or specialized research or manufacturing environments. Unless you have a local ozone source, there is seldom enough ozone indoors to cause those issues. A metallic coil is an excellent ozone filter, so nearly all residential HVAC systems are effective ozone filters. And residential HVAC system design is the wrong place to address ultrafine particles or filtering to clean room standards. Let's focus on the benefits to the most people, not the special cases that might need additional particle controls.

Right, but I happen to live in an area where frequent ozone "action days" occur - SouthCentral PA. Many hundred thousand of us, mostly in old or "code" houses so I'd suspect lots of indoor ozone due to leakage. The truth is "most people" do not live in Passive House air tightness, nor in new "code" houses (look at new home building stats) so the largest benefit will be to people living in older homes. And in much of the mid-Atlantic - without forced-air HVAC units (maybe add-on AC units with air handlers and ducts run in all the wrong places). So if you really want to "focus on the benefits to the most people" you will not be focusing on the 1% (or less) that can/choose to afford a new construction, highly-engineered house.

I am a builder/designer, not an HVAC guy. But D. Butler's comment above echos a design solution I have already incorporated. Build the return so that it can house 2 18x18 high Merv filters side-by-side giving you 648 SI of surface area for filtration.

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