Do High-MERV Filters Always Reduce Air Flow?

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High-MERV filters are more restrictive but do they reduce air flow?

Over the past few months, I've taken a look at some critical issues for indoor air quality.  Starting with the issue of how much time we spend indoors, I then wrote about kitchen ventilation, the panoply of indoor air pollutants, problems with filters in general and with high-MERV filters.  Now let's take the next step:  a look at what research has been done on high-MERV filters and what can be done to overcome those unintended consequences I wrote about.

Study #1

Article:  Residential AC Filters (pdf) by John Proctor, ASHRAE Journal, October 2012

In this article, John Proctor summarized some of the results he and his colleagues have found from studying California homes.  One of the concerns in their research for the California Energy Commission was homeowners changing out 1" thick standard fiberglass filters with 1" thick pleated filters. 

The Air Conditioning Contractors of America (ACCA) protocols for HVAC design assume a pressure drop of 0.10 inches of water column (i.w.c.) across the filter.  (Keep that number, 0.1 i.w.c., in mind as a reference point.  I'll be coming back to it.)  If a system is designed with a standard filter for that pressure drop, the pressure drop with a pleated filter of the same size will most likely be higher. 

In addition, poorly designed and installed duct systems already have external static pressures that are too high.  The typical furnace or air handler is rated for 0.5 i.w.c. but many run much higher pressure.  David Richardson of the National Comfort Institute says that in the testing they've done, the average system is running at about 0.82 i.w.c.

Result:  In the California study, Proctor et al. found that the pressure drop across the filter in 34 HVAC systems was 0.28 i.w.c.  That's nearly three times what ACCA protocols assume.  It's also more than half of the rated external static pressure for the whole system.

Study #2

Article:  Is There a Downside to High-MERV Filters? by David Springer, Home Energy Magazine, 2 November 2009

David Springer of the Davis Energy Group published this great article with a lot of detail about what they did and what they found, including blower energy use, compressor energy use, and the specific filters they tested.  I'll let you go there for the details because here I'm mainly going to focus on the issue of pressure drop across the filter and the resulting effect on air flow.

And speaking of filter pressure drop, here's their chart for filters with MERV ratings from 2 to 13.

Chart of pressure drop across filter for different MERV ratings  (Credit:  David Springer, Home Energy magazine;  click for article.)

They tested all the filters at the same width and height (16" x 25") but the depth varied from 1" to 4".  They used 492 feet per minute (fpm) as the face velocity because that's what the ASHRAE filter standard (52.2) calls for.  With that face velocity and those dimensions, the air flow rate is 1,367 cubic feet per minute (cfm).  (Flow rate equals area times velocity;  see my article on the continuity equation for more on this.)

Results:  As you can see, there's not a steady increase in pressure drop as the MERV rating increases.  What does happen, though, is that the pressure drop jumps up significantly as soon as you change from the standard 1" fiberglass filter (MERV-2) to the next level at MERV-6.

They also found "a definite trend toward lower air flow with higher-MERV filters for systems using PSC motors."  (PSC stands for "permanent split capacitor."  A PSC motor is the one powering most HVAC blowers.  Variable speed blowers have electronically commutated motors, ECMs.)

The final result I'll mention here is that they didn't see as much difference as they expected for filters of different depths.  For example, "the 4-inch Filtrete 1550 (MERV 12) was only marginally better than the 1-inch Filtrete 1700 (also MERV 12) and the two other [1-inch] MERV 11 filters of the same brand (1000 and 1085)."

Study #3

Article:  The Effects of Filtration on Pressure Drop and Energy Consumption in Residential HVAC Systems (pdf) by Brent Stephens, Atila Novoselac, PhD, and Jeffrey A. Siegel, PhD, HVAC&R Research, Vol. 16, #3, May 2010

This one is an academic paper so if you want all the details, equations, and references to other works, click the link above and download the paper.  (Brent was a student at the time he wrote this paper and has a page on his filter research on his website at the Illinois Institute of Technology, if you want to dive even deeper.)  They looked at filter pressure drop and energy consumption, both theoretically and using four months of data from two air conditioning systems in a test house in Austin, Texas.

Results:  What they found is what you would expect, and in their limited range of MERV ratings tested, they did see an increase with each step up the MERV scale.  Here are their results for filter pressure drop:

Low-MERV  (<4)     0.10 i.w.c.

Mid-MERV    (8)      0.19 i.w.c.

High-MERV (11)     0.32 i.w.c.

So the low-MERV filter is hitting the ACCA design pressure drop.  The mid-MERV is twice as much and the high-MERV is three times as much.

They also found that air flow in the high-MERV filters dropped by 7% and 11% in the two HVAC systems compared to the low-MERV filters.  Likewise, the mid-MERV filters also showed decreased air flow relative to the low-MERV fitlers, this time 3% and 8% lower in the two systems.

Summarizing the results

As you can see above, the research shows that in general, HVAC systems with high-MERV filters have a higher pressure drop across the filter.  This part is common to all three studies above.

What happens with the air flow depends on what kind of blower the HVAC system uses.  In a system with a PSC blower, the air flow drops and energy use doesn't change much.  The Stephens paper cites a 2002 study showing that 90% of all residential HVAC systems had PSC blowers.  Certainly that number has fallen in the past 16 years, as high-performance homes and high-performance HVAC systems have become more popular.  But I'm sure the vast majority of homes still have PSC motors running the blower.

For those with the other type, the electronically commutated motor (ECM), the controls on those motors typically ramp up the motor speed as the pressure increases so relatively constant air flow is maintained.  But there's a penalty.  Blowers with ECMs can be more efficient than those with PSC motors when they're operating against the pressure they're designed for.  But when the pressure is higher, they can end up using more energy than the PSC blower.

So what can you do to be able to use a high-MERV filter and not suffer a high pressure drop across the filter and the resulting loss of air flow (PSC blower) or increased energy use (ECM blower)?  It's actually pretty simple.  You just have to make the filter large enough to have a low face velocity.  And that's where I'm going next with this series.  You can get a head start by reviewing the other articles I've written on this topic in the past few months, especially the one on the continuity equation.


Related Articles

What Percent of Time Do You Spend Indoors?

Which Indoor Air Pollutants Matter Most?

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

The Unintended Consequences of High-MERV Filters


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Great article and studies! Can't wait to read what you have coming next in the series.

Thanks for your indepth article in IAQ. Are you familiar with the new PBS series on PROOF IS POSSIBLE?
Did you notice that Greenbuild is now looking at the chemistry of IAQ?

You're welcome, Dixie. Yes, I know of Corbett's show and have known Corbett for nearly a decade now. I haven't seen the show yet but I'm sure he does a good job with it. I hadn't heard about Greenbuild & IAQ but that's always been part of the LEED rating system for green buildings.

It would seem the increased filter size will only slightly help and physically might be hard to retrofit in a widespread manner. And then you still have the excessive static in existing duct systems which is also difficult to resolve. So as Tim the Toolman would say, it may be time for "More Power" with widespread use of ECMs in retrofits. Of course ECMs cost more upfront but an appropriately sized unit generally solves the problem. Plus they run at reduced cfm as much as 90% of the time when paired with two stage or better equipment. But to gain ECM reliability, a known concern, hvac manufacturers need to push whole house surge protection and incrementally increase specified ECM horsepower to reflect the reality of ductwork installations in the USA. Finally better consumer education to the state of the residential hvac industry might be the ultimate solution, allowing market forces to act.

I agree that variable speed ECM motors (not X-13) that can compensate for restrictions can generally solve the problem but, I don't see it happening.

As a full time HVAC service provider (NCI) trained, I have seen a direct correlation between failed ECM modules and subsequently tested high return static pressure.

Due to a focus on S.E.E.R. (One measure of efficiency) instead of tested efficient, manufacturers provide the smallest motors possible in the equipment. The 2019 FER standard is only going to compound the problem. The amount of ECM motors out there is increasing substantially. I see it everyday!

The only solution I see is proper filter sizing which often requires dual filters. Yet, homeowners and certainly builders and not embracing due to space issues.

I am not sure what Dan means that "manufacturers provide the smallest motors possible in the equipment". Most manufacturers that I know provide indoor air handlers and furnaces that can go up to 1" ESP, even though we are only required to rate at lower ESP's. We know that many installers will under size the ductwork and homeowners will use higher pressure drop filters.

Most people don't realize this, but if you do have a high system ESP requirement due to undersized ductwork or high pressure drop filters, you are better off with a lower airflow rate (perhaps 350 or even 300 cfm/ton rather than 400 cfm/ton). Why? The reduction in blower power is greater than the increase in compressor power so you have higher system efficiency, the lower static pressure results in less duct leakage, it will be quieter, and you will get more latent capacity which can be a benefit in humid climates. So if you have a constant-cfm ECM blower, turn down the airflow rate. "X-motors", which are ECM motors without the constant airflow rate feature behave more like PSC motors in this respect.


I am referring to residential equipment; per nameplate specifications the majority of variable speed motors are limited to a total ESP of .5" W.C. Yes, there are some exceptions, I have seen some rated for a max of .8" W.C. But, the risk is failure of the controller and reduction in efficiency as the maximum static is reached or exceeded. I agree that setting a reduced CFM may solve that problem. If you ever see a residential data plate with total external static pressure of 1" W.C. please post it.

Dan, I have worked for 3 different residential furnace manufacturers so far, and all of them have furnaces that are able to go up to 1" ESP. I don't want to mention brand names on this web site. I guess that I was unaware that we even put ESP limits on a nameplate, so I will have to check into that. I know that we would all like our customers to design systems at 0.5" ESP or below, but we also know that many do not. So maybe the nameplate is a CYA thing. As for why we limit the maximum ESP, it is not because of risk of controller or motor failure. PSC motors automatically unload at high ESP's, and ECM motors have electronic protection that will limit their speed at high loads when necessary. So you can abuse them with poor duct design, but they will protect themselves. We do not like unnecessary warranty replacements.

I appreciate the insightful comments. I plan on gathering some data on future failures I encounter. Be well.

Hello, I wonder if any of the studies measured dirty filters? That just might have an effect on things!

Yours, Larry

In my opinion, air filters should be at the return air grills, not at the equipment. This location has several advantages. It helps keep the return air ducts clean. It is usually a simpler location for replacement (avoid high ceiling returns). In a properly designed system with my return grills, there will be more return grill area, thus lower face velocities and pressure drop. The only downside that I see is that this will lower the pressure in the return ducts even further, but if they are properly sealed this should not be a problem.

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