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

Filter Bypass Because Of Incorrectly Installed Filter

Indoor air quality (IAQ) researchers have established that little bitty invisible pieces of stuff that float around in the air are bad for human health.  Of course, being scientists, they don’t call it little bitty invisible pieces of stuff.  They call it particulate matter (PM) or particulates

And it turns out that the really small pieces — the stuff that’s 2.5 micrometers (0.0000025 m, or 2.5 µm) or smaller, abbreviated PM2.5 — is worse than the bigger stuff because it can penetrate deeper into the lungs and more easily find its way into your blood than the bigger particulates.  The chart I included in my last article shows that PM2.5 is probably the worst indoor air pollutant overall.

And how do you deal with it?  One of the primary methods is to filter it out of the indoor air.  In most homes, the only filtering of the air that happens is in the heating and air conditioning system.  But if you think you’re covered just because you have a forced air HVAC system and it has a filter, let me give you a few reasons why that filter that you so dutifully change may not be helping your IAQ.  (You do change your filter, don’t you?)

1.  No filter

It’s true.  If you don’t have a filter, you’re not gonna get much filtration.  And yeah, it really happens.  Sometimes someone removes the filter because it’s in a difficult spot to reach, like a crawl space.  Sometimes they take it out and forget to put it back in.  Sometimes…well, who knows!  All kinds of things happen.  But not only are you not improving your indoor air quality if you don’t have a filter, you’re also getting your duct work, blower, air conditioner coil, furnace heat exchanger, and everything else in there dirty.

I took the photo above in a nasty, damp crawl space in Atlanta.  Notice that filter slot near the middle of the photo.  Not only is the cover missing but so was the filter.  I took the picture on a hot, muggy day in August.  Imagine all the nasty stuff getting sucked in there and sent into the house.

2. Filter bypass

Take a look at that photo at the top of this article.  That’s some serious bypass.  They put a nice, deep media filter in that’s capable of filtering out a lot of nasty stuff in the air.  But they installed it incorrectly so quite a bit of air going through the system was passing right by — instead of through— the filter.  Bypass means you’re not cleaning the air nearly as well as you should.

It doesn’t take a fancy filter to get bypass, though.  Here’s one with a standard one inch fiberglass filter that was jammed into the slot incorrectly.

3.  Not enough runtime

This one may blow your mind.  Filters can clean the air only when there’s air going through them.  When the system is off, no filtering happens.  If you’ve been reading this blog for a while, you may be thinking, “Aha!  Yet another reason not to install an oversized heating and air conditioning system.”  The problem there is that when you put in a smaller HVAC system, you also cut down the air flow and the filtration volume is the product of those two quantities:

Filtration volume = Air flow  x  Runtime

You can double the runtime by cutting the system size in half, but at the same time you’re cutting the air flow in half so the filtration volume stays the same.

One way that some IAQ advocates tell people to get more runtime is to put the fan in the “On” position instead of in “Auto.”  Weeeeeell, that’s a nice idea and it will work in some places…at the cost of higher energy bills.  But in a humid climate during cooling season, you could be making your indoor air quality worse by raising the humidity.  I know.  I measured it in my home a few years ago.

The best thing to do here is size your system properly, don’t put the fan in the “On” position, and get as much filtering as you can with your system.  If you have an efficient blower motor and are in a dry climate or running your system with a dry coil, using the fan “On” setting can help.  But the best thing to do is focus on minimizing the stuff in your indoor air that needs to be filtered to begin with by doing source control, air sealing, and mechanical ventilation, both local and whole-house.  If that still doesn’t do it for you, it may be time to add a standalone fan with a filter that will run more and get you a higher filtration volume.

4.  Not enough flow

Now you know flow and runtime go together.  This other way to get low filtration volume is a common problem for a couple of reasons.  First, many return ducts and filters are sized too small.  That increases the pressure in the system, which reduces air flow.  Second — and I know this doesn’t apply to you — some people don’t change their filters often enough.  Again, the result is high pressure and low air flow.

The system in the photo above had both problems.  The pressure drop across the dirty filter and coil in this system was a super high 0.9 inches of water column (i.w.c.).  With a clean filter, it was still a too-high 0.6 i.w.c.  (Read more about this home.)  That pressure drop is about ten times too high.

5.  Low efficiency (MERV) filter

You can install a standard one inch fiberglass filter.  It’s not going to do much for your IAQ because it’s basically designed to keep dog hair, dead spiders, and lost socks from getting into the air handler.  The standard rating system for filters is Minimum Efficiency Reporting Value, or MERV.  That one inch filter is about a MERV 2.  The higher the number, the more stuff you filter out.

Here’s a chart I saw in a presentation at the North American Passive House Conference in Boston last month.  It was put together by my friend John Semmelhack of Think Little in Virginia from data he found in various places.

As you can see, you’ve got to step up your filter game to at least MERV-10 to get even half of the PM2.5.  But you really need to be at MERV-13 because it’s better to remove more than 85% of those particular little bitty invisible pieces of stuff.  And that was Dr. Brett Singer’s recommendation at his 2018 Building Science Summer Camp presentation.

But there’s a caveat here:  Many (most?) systems with high-MERV filters decrease the air flow because of poor design.  My next three articles will explore this issue in depth and show you how to do it right.

6.  Not filtering outdoor air for mechanical ventilation

 Some ventilation systems are designed to use the heating and cooling ducts to distribute outdoor air, too.  With the central-fan-integrated-supply type of mechanical ventilation, a duct from the outdoors is connected to the return side of the duct system.  Occasionally, a designer or installer doesn’t pay attention and connects that outdoor air duct to the return side downstream of the filter.

Oops!  When that happens, as it did below in this house with a filter grille, you’re putting unfiltered outdoor air straight into your ducts, where it can make the ducts, blower, and heating and AC components dirty as well as sending more particulate matter into your indoor air.

 By the way, I haven’t said this yet, but the biggest source of PM2.5 in most homes is outdoor air.  (If you smoke indoors or burn candles or incense regularly, those would be bigger.)  So make sure you filter that outdoor air before introducing into your home.

7.  Filter in the wrong place

 I have to think this one is anomalous but with the strange stuff I’ve seen in the wild, who knows.  The photo below was sent to me by my friend Jamie Clark in Kentucky.  The air moves through that system vertically.  There’s nothing coming in from the right side…over there where the filter is attached.  So that filter does nothing because air doesn’t move through it.


Filtration can be an important part of good indoor air quality.  Unfortunately, there are some significant obstacles to overcome if you want it clean up your air.  Professor Jeffrey Siegel has been studying the effectiveness of filtration in real homes and has found that the average home in their study has a filtration effectiveness of about 20%.  

How’s your filter doing?


Other articles on HVAC filters and IAQ

The Unintended Consequences of High-MERV Filters

Do High-MERV Filters Always Reduce Air Flow?

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

How Often Should You Change the HVAC Filter?

Can You Use HVAC Filters in Coronavirus Masks?

Can Your HVAC System Filter Out Coronavirus?

Understanding Filter Ratings:  MERV, FPR, and MPR

My Low Pressure-Drop, MERV-13 Filters

This Thermostat Setting Can Cost You Money and Make You Sick

A Few of My Favorite Filter Photos

Asthma and Poor Indoor Air Quality — The Trouble with Homes

Which Indoor Air Pollutants Matter Most?


Chart of filter particle trapping used with permission by John Semmelhack.  Photo filter installed outside the airstream used with permission by Jamie Clark.


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This Post Has 49 Comments

  1. Please allow me to suggest
    Please allow me to suggest another: Your house runs at a negative pressure relative to outdoors, and you get a serious amount of infiltration of outside air with no filtering at all.

    1. Good point, Mark.  I should

      Good point, Mark.  I should have mentioned that one.  I’d put that in the same category with number 6, not filtering outdoor air for mechanical ventilation.

  2. And how do you know if you
    And how do you know if you have a problem (or what the problem may be – like ozone where I live, or if the problem is solved)? You need to continually measure for all the issues you suspect. No measurement, just guesswork. Buy high-MERV HVAC filters every 3 months for guesswork? Install mechanical ventilation systems with their own ductwork and MERV 13 filters for guesswork? Not likely. We need fairly accurate, fairly affordable (think Nest thermostat) IAQ measuring devices that every home should have to show homeowners that there is a problem and then show problem solved when countermeasures are implemented. Otherwise – guesswork.

    1. David,
      Here is a system I have been researching that may offer some of the IAQ monitoring desired. At least they seem to be approaching the IAQ issue as a central design goal… I’m just a homeowner working on designing a future custom build for my retirement… not a rocket rocket scientist like Allison. If I am off the mark with what CERV2 is, I hope someone here will correct me… 🙂

      1. Brad, the CERV is much more

        Brad, the CERV is much more than the type of measuring device David was talking about.  It’s a device that ventilates, filters, recirculates, dehumidifies, heats, and cools.  They focus on the IAQ side of things because the CERV can solve the problem of low runtime and thus low filtration volume as well as control humidity and bring in outdoor air for ventilation.  David was talking about a monitor that’s fairly simple to use and understand and relatively inexpensive so everyone can measure what’s in their air.

        1. Allison,
          This is why I love following your Blog. Great discussion and input on a very complicated topic. As economist Thomas Sowell was famous for saying… “there are no solutions, only trade-offs”. I think that applies very well to building science. Heightened awareness of the next problem to “solve” is key.
          IAQ looks to be a major one as homes become tighter. Looking forward to your next several Blogs on this topic. 🙂

    2. David, I respectfully

      David, I respectfully disagree.  Do you need to measure everything to know that a house with an air-sealed floor or encapsulated crawl space is going to have few IAQ problems originating in the crawl space than a house with a leaky floor over a damp, musty crawl space?  Do we need to measure everything to know that we’ll have better IAQ if we use a well-designed, MERV-13 filter in our HVAC system?  Do we need to measure everything to know we’ll have better IAQ if we have a range hood with a good capture efficiency and use it regularly?

      I absolutely agree that measuring what’s in the air is important but home occupants already have enough to worry about and probably aren’t going to pay attention to it anyway.  It’s incumbent upon those of us in the design, construction, improvement, and assessment industry to see that occupants have the best possible chance for good IAQ in their homes.

      That doesn’t mean I’m against all measurement.  I regularly tell people to get a low-level carbon monoxide monitor and to measure humidity, too.  But to say that we’re offering blind guesswork in giving advice about filtration or ventilation is a bit extreme.

      1. Allison – I respectfully
        Allison – I respectfully disagree with you. Every house has a thermostat to measure air temperature; many houses have relative humidity gauges; many houses have CO detectors (although there is quite a bit of opinion as to whether or not these detectors are worth anything). The environment within a house is constantly changing. IAQ depends on a lot of factors that are in flux (amount of cooking, amount of time doors/windows are open, amount and duration of occupants, type/amount of outdoor pollutants or humidity, etc.) to say that you don’t have to measure (the most important aspects, like PM2.5, ozone, CO, CO2, RH – not “everything”) in order to get a good handle on IAQ is akin to saying all you need to do to maintain comfort is to turn the HVAC unit on with a wall switch whenever you are uncomfortable. How do you know you have “fewer IAQ problems”? You may have different problems that occur as an unintended consequence or is driven by outside forces your countermeasures were not engineered to combat. You may have the same problems, but just slightly reduced so you need to add more countermeasures. How do you know that all you really need is just a recycling air purifier instead of a full-blown mechanical ventilation system (so you don’t introduce outside contaminants but still achieve clean air with interior air temp destratification)? Or whole-house dehumidification instead of just relying on the HVAC unit? Is the HVAC (or mechanical ventilation unit) air filter(s) doing a good job? How do you know when to replace them? Every situation is unique enough that it deserves unique monitoring. You don’t manage what you don’t measure.

        1. Measuring temperature is easy

          Measuring temperature is easy.  We can do it with thermal expansion of liquids, the bending of bimetallic strips, thermistors, thermocouples, optical pyrometers, and more.  Measuring humidity and carbon monoxide are relatively easy, too.  We have so many ways to measure these things that ordinary people can feel confident in the results.

          But when we move to indoor air pollutants, which ones of the hundreds or thousands are you going to measure?  And how reliable will the results be?  I think you’re a bit too confident in our ability to measure the concentrations of hundreds of different chemicals reliably and inexpensively.  And I think you’re too dubious about what we can do without such measurements.

          1. There is a point in
            There is a point in technology where the measurement accuracy becomes “good enough” to recommend to a homeowner. Lots of different thermometers/thermostats on the market/recommended by building scientists but none truly measures human comfort (see Mean Radiant Temperature and the related MRT thermometer). Lots of RH meters recommended too, as well as CO detectors (even though their accuracy does not meet the safety levels of OSHA or BPI). The only research I’ve seen on IAQ monitoring devices for residences was by the brave work 2-years ago by Nata Adams and the recent work by Lawrence Berkeley Labs ( where the test results were far less than stellar and not recommendable. No outcry from any authoritative public or private entities (DOE, HUD, BSC, PHIUS, Taunton Press) on the dearth of such devices as a precursor for recommending/implementing mitigation technology; no sponsorship to develop better/more affordable devices by the building science community. Just more emphasis on “ventilating right” (whatever that means since we’re not measuring anything). All the discussion on ventilation schemas, filtration media, central vs distributed, EPA outside air quality measures, etc. become academic exercises – not solutions that you can recommend to a client because pre/post testing showed there was a an issue in their specific case but the implemented technical solution mitigated it. This is a paramount cause the whole building science community needs to get behind as it will be a proof/driver to the non-scientific clients of the world that changes need to be made (purchased) – like more extreme air tightness, better air filtration, better bath/kitchen fan technology, different heating/DHW/cooking technology – NOW. Otherwise there is no proof/incentive.

      2. I tend to agree with Allison
        I tend to agree with Allison on this one- not because I don’t think measuring is worthwhile, but because I measure for a living and I can tell you that the cost of equipment to adequately measure these parameters exceeds the cost of the entire HVAC system. There are a few attempts at consumer grade devices (Foobot, for example) but the accuracy is so poor that I don’t consider them measuring devices. They are indicators, like having a battery light on the dash to tell when your car’s electrical system isn’t right. But everyone has access to the EPA Airdata website ( ) or your state or county website to look at what the air quality is like in your region, and those monitors likely cost more than your house. So my recommendation is look at ozone and PM2.5 levels from EPA and get a CO detector and call it good.

  3. Unfortunately equipment air
    Unfortunately equipment air filters really have one duty, to protect the equipment. In the past 20 years the push was on to design and add IAQ devices to the equipment but little done to enable the equipment to handle the additional restricion to airflow of the filtering media or other air “cleaners”.

    1. Robert, yes, a whole lot of

      Robert, yes, a whole lot of installations have problem with excessive pressure drop from restrictive filters.  The solution isn’t in the equipment, though.  It’s in the duct design.  Stay tuned.  I’ll cover that in my next three articles.

    2. You are quite correct, Bob.
      You are quite correct, Bob. For this reason, I only recommend higher efficiency filtration if the filter surface area can be increased significantly or if a standard filter is kept in place to protect the equipment while a self contained bypass HEPA filter system is added to the return ducting.

  4. Allison — you are getting
    Allison — you are getting there step by step. I am afraid I am going to jump ahead. The VAST majority of filters we see in our studies are: Too restrictive. They are too restrictive because they are too small. I would never again specify a system with less than a 4 inch deep filter, even then the amount of filter face area would have to be more than twice the size that is regularly installed. The trade off between airflow and filtration is a difficult one.
    On another note — I might be wrong, but I think that the effectiveness of filtration increases with lower airflow. If I am correct then we need to change the equation above to
    air velocity X area X run time.

    1. Perhaps I should elaborate:
      Perhaps I should elaborate: Air Velocity * Filter Surface Area * Run Time * Capture Effectiveness (CE) where CE is a function of the filter material (eg. MERV) and Air Velocity

    2. John, you have indeed

      John, you have indeed anticipated where I’m going with this series.  My next article will cover the continuity equation and face velocity. 

      Thanks for the link to your article.  I don’t recall having seen that before.  For those of you who didn’t notice it since he hotlinked his name with it, here it is:

      1. There is a mathematical
        There is a mathematical (physics) mistake within that article. Kudos to anyone who can find it

  5. After decades of doing
    After decades of doing maintenance on systems my company installed, I found that the cleanest coils after years of use were coils that had hogs hair type of frameless filters installed. Being frameless, if sized a 1/16″ over the size of the filter rack, there was no air bypass around the perimeter of the filter. Because these washable/vacuumable filters are plastic fibered, they have an electrostatic quality that attracts airborne particulate to them. Because of their continuous filtering through the depth of the filter, they do not get coated with particulate, affecting the air volume.
    The major downside to these filters is that they tend to be flimsy when dirty. I always threaded steel insulation rod used to hold up insulation batts through the filters at 8″ spacing and bent over the rods at the ends of the filter. This kept them nice and stiff. The other main factor is to make sure the size of the filter allows for a low velocity of air per square inch of filter to prevent particulate from being pushed through the filter.

    1. Robin, the other downside to

      Robin, the other downside to filters that rely on electrostatic charge is that they filter less and less as they get loaded.  Non-electrostatic filters, on the other hand, filter more and more as they load.  Of course, if you really want to filter everything, a piece of plywood works really well.  ;~)

      1. Allison, I agree with the
        Allison, I agree with the electrostatic factor. One of the reasons I like the hogs hair type is that they have enough space between the fibers that they tend to collect the most particulate on the surface before fibers below the surface actively collect particulate. After the surface fibers are coated, each layer of fibers below the surface pick up the slack. I have pulled very dirty hogs hair filters that were dirty to 75% of their thickness that had no signs of dust on the remaining 25% of fibers. The coils on systems with hogs hair filters were consistently cleaner than coils with well maintained electronic air cleaners. I do believe that the air having no way to bypass a frameless filter sized properly is a major factor for those cleaner coils.

  6. Interesting article and I
    Interesting article and I read some of your previous articles on dehumidicication. Over here in the UK we have mainly mini split systems over central air systems. You call them ductless systems. On these units unless they are ducted you cannot set the fan not to run when the compressor is off, if you put the fan to auto the fan modulates based on the temp difference between return and set temp. When it reaches the set temp only in heating does the fan shut off, otherwise it runs slow.

    On the back of this, I have just returned from Florida where its been about 105F some days, humid as well! Never experienced anything like it! We went to Disney and Universal studios and I am suprised that they air conditioned the outdoor spaces such as where you board the rides which have no roof on but air conditioning ducts blowing conditioned air down to where the boarding area is for the rides.

    Also all the stores there have the doors open and because they use Central air, the building ends up pressurised, their units must be so oversized because all the doors to the shops/rides are open yet the interior is very cold and you can feel a cold area outside at the front of the doors! Would be great to see an article specifically on this type of thing as this differs to the UK where we have different systems in use and use door curtains or keep doors closed when A/C is on.

  7. I lived next to (150 ft away)
    I lived next to (150 ft away) from one of the largest urban freeways in the country for 7 years. Second year in, I developed strange symptoms in the wintertime. I had become sensitized to the constant barrage of pm2.5 and even smaller stuff and drink coffee without feeling completely on edge. Airports did the same thing a couple of times in those two pre-filtration years. Since I had no duct system, I built my own dedicated filterbox with an inline blower and moved approx 250CFM in a 1600 sq ft house all the time and kept the windows closed. It cost about 15 dollars per month, but the DYLOS counter LOVED the results–an approximately 75 percent reduction in particulates, and a 99 percent reduction in the positive pressurized room I set up. I also FELT MUCH BETTER. Research shows that the UFP (smaller than pm2.5) near a highway and downwind is very high–much higher than background. Even though much of the crap was below the threshold of visibility to a Dylos Pro, I am confident that the Microguard LR did a great job because HEPA and near-HEPA do even better at the smallest particles than the .5 micron stuff. Now that we moved away, I am back to a standard 1 inch filter that can’t do anywhere near the same air-scrubbing, but seems sufficient in this cleaner environment.

    1. I risk being shunned for
      I risk being shunned for advocating energy-consuming devices, but portable (or installed) indoor air fan/filter air purifiers are (in my experience) the single highest intervention that produces results for people with IAQ complaints. Sometimes adding a MERV-13 filter to a system works, but adding an independent fan/filter unit always works. The downsides are white noise and energy consumption. Whether the energy consumption penalty is offset by the difficulty or cost of designing (or redesigning) an installed HVAC system to handle improved filtration, I don’t know. I look forward to learning more about that in the upcoming articles!

      1. Bobby — I for one agree with
        Bobby — I for one agree with you. You get continuous filtration at what can be a relatively low cost if the unit has an efficient fan. When you try to filter with your HVAC system you are moving maybe 1000 or more CFM through a usually junk duct system with an often inefficient fan motor.

    2. Painting homes near the
      Painting homes near the freeways always required extra attention. Though houses were a thousand feet away, there was still a lot of black from rubber and oil on the paint.

  8. Allison, MERV is defined in
    Allison, MERV is defined in ASHRAE Standard 52.2 and the particle size efficiency values in your table are somewhat different from the values in the standard. As usual, I am just nitpicking you.

  9. I am taking exception to some
    I am taking exception to some of the statements above. Clean Air Delivery Rate (CADR) is the important metric for air cleaning and it is equal to the airflow rate times the filter efficiency. Thus, a HEPA filter is overkill since its efficiency is 99.98% when compared to high MERV filters that have efficiencies greater than 90%. The difference in efficiencies is negligible, but the airflow rate makes a huge difference in CADR when you compare a portable air cleaner or bypass HEPA to a high MERV filter on the return air for the whole house. Of course, you have to run the high MERV filter and blower to get this benefit, and there are other issues in humid climates as identified by others.

    1. This is why I only recommend
      This is why I only recommend HEPA filtration when it is used in a self contained bypass system attached to the return ducting.

    2. Roy – I agree with you about
      Roy – I agree with you about CADR being the important metric for filtration. However, there is another important factor and that is where the filtered air is drawn from and delivered to. A flimsy fiberglass filter in my portable air purifier will deliver more clean air to my room than a HEPA filter in a unit that is not running, or in a different HVAC zone. The main advantage of portable air purifiers is that they deliver the air where the occupant needs it. The second advantage, as discussed above, is that some systems can’t handle a MERV-13 filter, much less a higher MERV filter.

      As to sufficiency, MERV 13 still only filters <75% of particulate less than 1.0 micron. Since PM2.5 is everything less than 2.5 microns, 90% of greater than 1 micron is not the same as 90% of PM2.5. To hit that benchmark, you need MERV 15 or better. In HVAC terms, MERV 16 is HEPA. (In technical terms, it has a different, specific definition of 99.97% of particles at 0.3 micron, but for all practical purposes they are the same. Filters built to the "HEPA" definition typically hit the ASHRAE 52 rating at MERV 16-17).

      Personally, for general IAQ purposes, I am not a strict "True HEPA" advocate - the "HEPA-type" filters that claim 95-99% of allergens are nearly as good and half the price of true HEPA. The difference between MERV 13-16 isn't nearly as important as getting filtered air where you need it.

      This article highlights the main issues with relying on HVAC systems for filtration. If the system isn't designed for MERV-13 or better filters, or if the duct system isn't tight enough for the HVAC filter to make enough of a difference, I recommend to go with the standard filter (ASHRAE recommends minimum MERV 6) to protect the equipment and add a portable air purifier to capture the small stuff in the breathing zone. I agree with Robin, a bypass HEPA in a good duct is great for capturing PM2.5 since fine particulate remains suspended in the air stream. A portable filter is as good as the filter and fan you purchase. Depending on how you use it, a portable filter can help control particulate at the source before it reaches the HVAC system, and can achieve a higher standard of filtration than a typical residential HVAC system is designed to achieve.

    3. Roy — Don’t forget what
      Roy — Don’t forget what happens with a duct system in the attic, R very little duct insulation, Duct surface area very high and continuous fan == Massive conduction loss (or gain)

      1. That’s a good point, John,
        That’s a good point, John, about thermal conduction losses/gains in ducts in unconditioned spaces causing additional heating/cooling loads when running fan only for filtration. Throw duct air leakage in the mix and you can even have additional contaminant loading. This is why I now live in a house with a sealed (conditioned?) attic and these things are no longer issues. I just can’t understand why anyone would put ducts and air handlers in spaces that are vented to the outdoors.

  10. My competence in the world of
    My competence in the world of HVAC and such is at the level of – woodworker. In short, my world is dust masks and dust collectors.

    That aside, I started looking into MERV ratings to improve air quality. Sadly, the unit had nothing to offer on which filter we needed. In fact, ours takes a 5″, but the PDF on the unit only talks of 1″.

    After our blower motor went paws up, nearly fifteen in, I looked even closer at filter ideals. A lot of experience with the aforementioned collectors warned me cleaning the air too well could kill off the new motor too.

    Still, the more I read, the more I knew I wanted to increase air scrubbing. “Plan A” involves a roll-around, home made unit with a couple filters (twice the air input area) and at least MERV 11 filters, protected by pre-filters. A simple wafer switch and I could switch between low, medium and high. A manometer and I could monitor the condition of the filters.

    Meanwhile, it ocurred to me the home system intake leading to the 5″ filter at the bottom, just under the blower, is wholly accessible. In other words, the top could be capped and two or even three 5″x20″x20″ filters could be installed for two or three times the opening (1,600 sq inches of opening vs 400).

    Simple frames could be made to hold the filters (one above the other), including pre-filters.


    1. Filtration is a matter of
      Filtration is a matter of physical values based on the surface area and free area of the filter media, the amount of leakage around the filter and between the filter and the blower motor and how these three affect the air velocity across the filter. There is also the added factor of the filter material’s ability to attract and hold particulate. With the media material being equivalent, the more surface area is required for the less free area of the filtering media material. Otherwise, we raise the velocity of the air, degrading the effectiveness of the filtration. If we cannot increase the surface area of the filter, we should not decrease the free area by using a more “effective” (restrictive) type of filter. No filter is going to be effective in preventing particulate to pass as long as there are ways for the air to flow without going through the filter media. Perimeter gapping around the filter, leaking filter access openings, unsealed connections, screw holes etc. between the filter and the blower are all areas where unfiltered air will enter the system.
      Air filters are one of the few things in our industry that I will support that bigger is better.

  11. Kelly I guess I am a little
    Kelly I guess I am a little confused, you have a 5 inch deep filter? Your inclination to increase the surface area of filtration (deeper pleats or more filter size) is correct. York is now advertising a 5 inch deep MERV 13 Filter that looks pretty nice I have not found the specs on the pressure drop through it.

    1. Yes, John, my filter is 5″
      Yes, John, my filter is 5″ deep.

      That said, my want is to increase scrubbing efficiency without compromising the blower motor and it’s ability to move air.

      INTERESTINGLY, the HVAC folks had to swap the motor out a couple months ago. They converted it from the ECG to the capacitor system. The left with the blower wired for high, but which was wired for medium speed before. It moved a lot of air, but was notably louder. I bought and installed a 20 amp, 240 VAC wafer switch to allow me to switch between speeds. All is well, but…..

      Since my initial post, we note the system is louder than it was initially, even on medium speed. I’d installed a manometer and noted the beast was a half inch up from where I’d marked it for some “MERV 11” filters I bought, even though the one in it was a fresh MERV 8. That goes, I’m GUESSING, hand in hand with why we were getting a little rumble in the ducts.

      I dug through my filter collection and found one more of the fresh “MERV 11” filters and swapped it for the fresh “MERV 8″ filter that was in the system. The manometer dropped back 1/2”, and the rumbling subsided.

      Of course, the foregoing breeds the question of if the “MERV 8″ was mislabeled and is actually higher than the MERV 11’s, or if the MERV 11 over rated and is less than a MERV 8.

      Anyway, the original intent, again, was to increase filtration without compromising the components of the HVAC. The [maybe/maybe not] MERV 8 does put a drag on the system. As such, I was hoping to increase air flow, through the filter(s) by removing the single filter under the blower and moving it to the side of the return duct.

      The return duct is 10″ x 22″ and runs from the floor to near the ceiling. That would allow me to install the 5″x20″x20″ in the return stack, then go above that and install either a 10″x20″ on top, or a second 20″x20” above the first, to allow the air to more easily flow into the return, when using a MERV 11 or 13 (probably wishful thinking).

      1. Kelly I don’t think I have
        Kelly I don’t think I have heard the size of the unit (tons) that you are putting the filter on. However if that 0.50″ of water column is across the filter, then you have used up all the properly available static pressure of most residential units. When they switched from an ECM motor they switched from a motor that attempts to achieve airflow by amping up (and potentially burning out). On the other hand the motor they installed PSC (permanent split capacitor) when faced with excessive static pressure just slows the airflow and stays “calm”.
        You do not want to put filters in series … that just adds more static pressure. You want to put filters in parallel (either side by side or in different branches). There is an ASHRAE Journal here:
        that talks in detail about these filters. You will note that it suggests a 0.05 inch water column design with clean filters and a face area (not filter material area) of about 350 sq. in. per ton for a MERV 13 4″ pleated filter.
        PS to the technical people here there is a moderate size error in that article, but it is good advice anyway.

        1. I was worried you’d think I
          I was worried you’d think I was putting the filters in series, but didn’t think of using that term.

          Staying with what I know as an electrical circuit description, it has always been my intent to set the system up for filters “in parallel” to, as I said earlier, increase the surface area air could be drawn into the system (e.g., from 400 sq inches (ignoring pleat depth) to 600 or 800 sq inches).

          As I noted, the meter dropped back 1/2″ (to the normal run position) when I went to the higher rated, but obviously less restricting filter from another company. As such, I stayed with that filter, pending improvements that MIGHT allow the more restricted filter (more surface area).

          In life, my experience has been, repeatedly, that merely that something is not routinely done does not mean it’s wrong or even not an improvement. On that, as some point out, more complex systems are more costly and may not be practical for the average house. I’ve seen filters that had a half inch of build up on them because renters (college kids) didn’t know you had to change them. (I wasn’t there for the furnace, but the kid complained about lack of heat and I asked about the filter – she didn’t know the system used them).

          Anyway, I play with air in my little woodshop, which has a two bag, a two cartridge and a single bag dust collector. Experience has shown me the value of better AND bigger filters. The better (.5 micron) filter for are needed for improved toxic dust collection, and they need to be larger in size (either a bigger bag for the one double system or the pleated canisters for the other double system, and a finer and larger bag for the single bag beast) to allow the system to move air at or near the rate for which it’s designed.

          While much of what I come across, including your PDF, is over my head, I note some of the details seem to suggest I am not far amiss looking at improving air flow to allow the system to use a higher MERV filter, if I’m interpreting things right:

          “The face areas and filter location depths will have to be increased to allow the use of these filters without harming the airflows through the furnace, air con-ditioner, or heat pump.”


          Thanks for your time and effort responding.

        2. Apologies, John, I neglected
          Apologies, John, I neglected to mention it’s a 2.5 ton Lennox Lennox Air Handler, CBX27UH-30-230-6-02

          Original motor: 5SME39HX LO15A X13 FM 19 (230VAC 1/2hp 1050 RPM)

  12. I have a large 16x25x4 space
    I have a large 16x25x4 space for my furance/HVAC filter. The standard 4″ filters at Lowes won’t fit but I could fit two 2″ filters. I was thinking about buying a Merv 8 and a Merv 13 and putting them in series. The Merv 8 would catch the large particles and the Merv 13 would catch the smaller particles. Is this a good idea?

  13. George No that would not be
    George No that would not be a good idea, the pressure drop would be too high and the airflow too low. Can’t figure out why a “standard” 4″ 16X25 filter would not fit. Perhaps you need to go on line to order one. Does your furnace also have AC? and how many tons is it or how many btuh is the furnace rated for? Where are you located?

  14. I’ve just moved out west and
    I’ve just moved out west and the air quality has been quite poor, still only getting worse. I’m somewhat sympathetic to those advocating for better “layman” AQ sensors, although I appreciate the candor about accuracy since I was on the search for something. Trying to do *something* to hyper-locally mitigate all the persistent ambient pollution has been a chicken and egg conundrum trying to decide where to allocate limited resources. Baseline question: “Is the PM2.5 (for starters) worse indoors or outdoors?” Answer: no way of knowing. Regardless, if indoors can be well-scrubbed then I know it’s likely safer indoors, I thought. This blog has helped illuminate all the factors involved with “well-scrubbed”… It seems well-working HVAC system/filtration is preferable to air-purifiers for overall residential IAQ; however, where does a lay(wo)man begin? When I asked a local HVAC company guy who came to tune up the AC about filter quality, I was told the “system couldn’t handle” a 4″ filter (no mention of face area or pressure drop or anything else), and I wasn’t informed enough at the time to press further. So now I have a 1″ 16×25 (ostensibly) MERV 13, but from reading the blog, this 1″ may be doing more harm than a 4″. (?) And then there’s trying to find someone competent to assess all the duct work, pressure dropping, air flow, bypass, etc. to see what the system’s actual operational efficiency is like. And that all sounds like cruisin for a cash bruisin. So then I think, well for now if I get an air-purifier at least I know there will be one room where the air is non-toxic. But trying to make an educated air-purifier purchase with its own set of co-depending efficiency factors is proving just as fraught, again with no way of ultimately detecting/verifying any improved air quality results. It’s becoming clear that bang for your buck will fizzle unless there are enough bucks to create a big enough bang that all components involved in losing efficiency can be mitigated… IAQ is only as strong as the weakest link. And so we can’t afford with health to pay for placebo, but also can’t afford to ante up for an overhauled HVAC and it’s very hard to know which air purifier (if any) will actually yield results. It’s all very discouraging, and in the meantime knowing you’re still breathing day in/day out a good batch of toxins no matter where you are. I wonder if some kind of IAQ consultant would be the best place to invest… are there gurus out there for hire, no snake oil? At any rate (and rant), many thanks for this blog and helping some blind eyes to squint open.

    As an aside, my mother is a schoolteacher and began school this week. They were told the schools had been updated with “hospital grade filters,” which sounded very comforting to all of us ignorants since teachers there have scant-to-none PPE for their rooms. I pressed her to inquire further of the custodian, who told her they would be “upgrading hopefully soon” to a MERV 10. Ironically, the kids aren’t allowed outside for recess right now due to bad air quality.

  15. Sarah Well let me start with
    Sarah Well let me start with the easy part MERV 10 is not a “hospital grade filter” Not sure there is such an animal. HEPA comes to mind but that is not really an option for your HVAC system. Note that the V stands for ventilation WHICH MEANS OUTDOOR AIR. So now we have the problems of the fires in the West. That then means the outdoor air is good because no virus, but bad because of PM2.5.
    So unfortunately you are now stuck with filtration to mitigate the problem.
    1) Unless you have had your duct system tested for leakage, do not run the fan continually to “filter” the air. Since ducts often leak and often quite a lot, they end up bringing in outside air with the wildfire generated PM2.5.
    2) if you do have a “leakless” duct system have it modified to have more returns with much larger amount of filter area with 4″ to 5″ filter grilles using MERV13 or higher filters.
    3) A low cost alternative if you do not know about your duct system is to buy a few cheap box fans, Duct tape MERV13 or higher filters to them and run them inside.
    Here is an item I sent to contractors yesterday:
    Ventilation, Covid19 and Filtration

    There are often recommendations for dealing with the Covid virus that suggest, “increase ventilation”. Unfortunately for many of our customers they don’t know how to apply that suggestion since they are not real clear what ventilation is.

    To clarify. Ventilation is outside air entering the space. THINK OUTSIDE!

    Circulation on the other hand is moving air around within the space. THINK A FAN

    So we want ventilation. We want to move air from outside into the space. There are standard and simple ways to move outside air into the space. 1) Open up windows. 2) Turn on bath and kitchen exhaust fans – moving air out of the house means outside air comes in to replace it. 3) Install and operate a window fan that helps make windows more effective. 4) There are more sophisticated systems that intentionally move outside air into and out of the house, often with heat exchangers between the two airflows.

    OK so what about our customers ceiling fans, air conditioners, floor fans, furnace fans? These devices circulate indoor air, they do not dilute the concentration of the pollutant (the Covid virus). So, why do people use them? Because they make us feel cooler. Air moving across out skin causes evaporation, which makes us feel cooler.

    Outside air is what we want to dilute the viral load in the building space. Sometimes customers do not have enough ways of obtaining adequate ventilation and we have to supplement the ventilation with Filtration. Filtration is definitely a second tier response to the problem. But not just any filter. HEPA or MERV 13 or higher.

    Please note that these “high efficiency” filters are better at removing particulates, but they are also much more restrictive to airflow. To compensate for that we need to add additional return filter grills and revise the existing ones to accept 4” to 5” deep filters.

  16. Sarah – some of the very
    Sarah – some of the very points I’ve tried to bring up w/o any definitive answers. One of the tenets to management is “you don’t manage what you don’t measure” and it seems like very few HVAC gurs/designers/technicians incorporate any reasonable cost/accuracy measurement plan. The Passive House movement seems to advocate the “outside air is always better” philosophy (but it isn’t where I and many others live). The “code regulated” group just says “whatever meets the local codes for either ventilation or ERV/HRVs” (still no measurement nor filter prescription). If you knew what was “bad” (i.e., is PM 2.5 really the target or PM .25)? What about ozone? And how much?) you could create an action plan for HVAC filters, air purifiers, ERV/HRV equipment. You could even build your own room air filter units. But you have to know what’s “bad”, and how “bad” YOU are first; then you will know if your efforts made things “better”.

  17. What is your take on 1″
    What is your take on 1″ washable electrostatic filters that I wash and rotate (I have 2) every month. Mfg claims MERV 6. I have a washable MERV 13 that I put in occasionally on bad air days.

  18. The The “code regulated”
    The The “code regulated” group just says “whatever meets the local codes for either ventilation or ERV/HRVs” claim would be a red flag for me, as to the competence of the expertise of the individual.

    Take electrical, for example, many electricians hired to wire home hobby shops, because the owner recognizes his or her limited knowledge and ability, are more than happy to wire outlets with fourteen gauge wire, because that is all code requires in many places.

    An expert who cares about his customer would know the customer needs 12 gauge for the extra five amps it can carry for tools and equipment.

    Though I’m not an HVAC guy, I always went by the “I’m the expert and it’s up to me to educate my customers, as to what’s best for them, what their options are any why some options are better than others” approach.

    Sadly, many so called pro’s know just enough to do the least required of them.

    We don’t have to try to give customers enough information to make them our equals, but we can explain enough to plant valuable seeds [of thought] to help them help themselves in making decisions.

    To the end of helping customers know they have options, it might be worthwhile to make a simple brochure explaining things like manometers and Magnehelic Differential Pressure Gauges, increasing air flow via pleated filters, pitfalls of higher MERV rating filters and so on.

    From my experience, customers remember those who educated them. Even if the education could make you replaceable, they still wanted you to do the work, because you did take the time to educate them.

  19. At one point you mention, “My
    At one point you mention, “My next three articles will explore this issue in depth and show you how to do it right.” on how high Merv filters decrease airflow.

    I’ve quoted the articles that show up below. Are these them?

    ” Related Articles

    This Thermostat Setting Can Cost You Money and Make You Sick

    A Few of My Favorite Filter Photos

    Asthma and Poor Indoor Air Quality — The Trouble with Homes

    Which Indoor Air Pollutants Matter Most?”

      1. Thank you for clarifying. I
        Thank you for clarifying. I had a feeling the ones showing up below after you mentioned them, weren’t the ones you wanted to connect with your information.
        Now that I’m reading more of the comments here common I’m thinking of Putting efforts in a good air purifier.
        If anyone can help with good choices of models I would appreciate it. I’m getting ads for them now that I am searching… so far the certified Bissell 320, the REME Halo (which fits on your existing furnace or air system, also whole home air purification services.
        This is only my 1st couple of days exploring the stuff because of the smoke from wildfires but I will try and look through your threads or blog to find my own information on purifiers if you have them.

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