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Community and Q&A

Poor Indoor Air Quality With MERV-13 Filters

tundracycle | Posted in General Questions on

We have two HVAC systems in our MN house that circulate air continuously. Both have MERV-13 filters. Compliments of Canadian wildfires the outdoor air is quite bad. PM2.5 = 50.3 and US AQI = 137. Inside though we are at PM2.5 = 42 and US AQI = 117 which are both quite high.

All windows and doors are closed and ERV’s haven’t run since yesterday so should be minimal outside air coming in unfiltered. MUA for range hood enters downstream of the filter so that air s/b getting filtered.

I’m not sure of the volume of house air filtered is. That would be nice to know.

Even so, I’m surprised at how bad our indoor air quality is. Thoughts?


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  1. tundracycle | | #1

    Some current readings:

  2. tundracycle | | #2

    48 hrs of history for one monitor. I'm guessing that the increase in the evening was from cooking though I don't think it should have been nearly that high. Other thoughts?

    1. Expert Member
      Akos | | #4

      Cooking is the largest source of indoor air pollution. Even something simple like making toast will spike your pm2.5.

      1. tundracycle | | #6

        Yes. I need to look at the cooking a bit closer. We have a quite good exhaust system so we shouldn't be seeing spikes like that from cooking but it is possible.

        1. this_page_left_blank | | #10

          By exhaust system, do you mean a range hood, the ERV, or both?

          I think you'll be surprised to find out how much PM2.5 cooking puts into the air, how little of it can be captured at the source, and how long it takes to clear out by dilution.

          1. tundracycle | | #14

            In this instance our range hood + MUA as well as a secondary 400 CFM ceiling exhaust above our oven stack. Our Accurex hood has a clear opening of 58"x29" IIRC plus considerable containment volume. Blower is a 1400 CFM Accurex blower that measures at 989 actual. So we should have about as good of range exhaust as possible.

            But the historical data for that period did look like it could be cooking effluent based on time.

            ERV's have been off for the past several days. They get locked out based on a number of factors including temp, dew point and PM2.5 of which both dew point and PM2.5 have exceeded maximums for several days.

          2. this_page_left_blank | | #18

            Where is your fresh air coming from when the ERVs are off? Are you also monitoring CO2?

  3. bigred | | #3

    These little sensors are known to have very large high biases. At the levels you report, I would easily estimate they may be off by 50%. Having done ambient and IA measurements for 40 years, use these on a relative not absolute basis. A quality PM2.5 sensor (either reference method or federal equivalent method (FEM) is $10,000's of dollars, so for a few hundred you are not going to be in the same ballpark. found a study where this sensor was put up against FEM sensor. The sensor correlated very well to the FEM, but was about 50% higher across a rage of concentrations from 10 to 250 ug/m3. So basically, your reading of 42, is more likely a true value of 20.

  4. tundracycle | | #5

    Eric, yes and no. Lab/Industrial quality PM measurement devices are not generally $10,000's. I borrowed a PDR-1500 (±5% accuracy) from a buddy last year to check the accuracy of our collection of devices (3 IQAir's, Foobot, Awair, uRad A3, others). I believe new the PDR-1500 is about $5k and used are readily available for under $1-2k. I believe there are others or similar accuracy for about half those costs.

    FWIW, we found Foobot and Awair (v1) to be completely unreliable. Both exhibited a similar problem of being relatively accurate in a dim room or one w/ florescent lighting but extremely (by multiple factors, not just percent) inaccurate in a room with brighter incandescent or LED lighting or sunlight. uRad is OK on PM, but does not accurately adjust for temp on CO2. IQAir AVP's were the most accurate and generally within ±10% or reading. The 42 we saw is very likely between 38 and 46.

    But yes, many home AQ monitors can be off by a significant amount.

    1. bigred | | #9

      I was referring to Reference methods or Federal Equivalent Method analyzers. Such as the BAMM 1020, TEOM, etc. These instruments for ambient work are in the upper teens to $20,000 + depending on type of options. The PDR-1500 by TECO is an IH analyzer, and a pretty good one, but doesn't come to the accuracy and precision of a reference method. Here is a study comparing the device the OP has to a FEM monitor:

      1. tundracycle | | #13

        But how much accuracy and precision is needed for home monitoring? ±0.1%? ±1%? ±10% Other? There's certainly a place for the Grimm's of the world but I don't know that that level would do me any more good in my home than what I think I have. (OTOH, what the AQMD report showed might be too inaccurate.)

        My AVP's are, based on comparison to the PDR, within ±10%. Greater accuracy would be nice to have but I don't think it would cause me to do anything different. Or should it?

        I wonder if IQAir made any changes after the AQMD (and LBNL) tests? My three certainly appear to be much more accurate than those they tested.

        1. bigred | | #16

          As I originally stated it is fine to use a monitor like this for relative measurement vs absolute measurement. As the AQMD study found that monitor has very good correlation to reference method, just had a large, but consistent bias. Knowing this the poster could easily assume that if values went up, the sensor was correctly predicting an increase in PM2.5. Since the study showed that the bias was about 50%, they could assume a reading of 40 was actually 20 and plan accordingly. Your question as to what precision and accuracy is needed, it absolutely depends on your purpose. For a homeowner who is just curious, the original poster can now adjust their expectations and get the level of accuracy they need. If I"m doing a study to develop new regulations for workplace exposure, my measurements better be within a know and quantifiable limit. If I'm doing a study to find Ozone hotspots within a city, I'm very happy to use a portable monitor with lower accuracy as I'm looking for relative levels, not absolute levels. If I'm going to determine if a region is non-attainment, I' better be within +/- 1%

          1. tundracycle | | #26

            In my case though I know that the bias on two (vs PMD-1500) AVP's is 4% and on another is 7%. None are anywhere close to 50%.

            Bias within 10% is, I think, good enough for me and for normal home use (50% would not be). I assume that IQAir must have made some changes/upgrades on units newer than those used for the AQMD study?

            This plus other problems we've seen with other home monitoring devices points out the need for some kind of more official and routine testing of these devices to avoid consumers being misled. We know that Foobot and Awair v1 are grossly inaccurate and there s/b a way for consumers to easily know that.

  5. brian_wiley | | #7

    This has been on my mind lately (‘tis the season…) and I found this video helpful. It was interesting to see the data and strategies people were using to mitigate wildfire smoke; in one case it was a hepa-filtered shop vac providing positive pressure to the building enclosure.

    The short of it though was that Merv-13 only gets about 50% of PM2.5, and that Merv-16 or HEPA was necessary, but even that required additional strategies.

  6. walta100 | | #8

    I have to ask is your indoor air quality a matter of live and death for someone in your home with a diagnosed medical condition?

    If so given the amount of smoke in the outdoor air recently I would be looking for some true HEPA air purifier.


  7. tundracycle | | #11

    Not a matter of life or death other than affects on general health.

  8. Deleted | | #12


  9. Expert Member
    BILL WICHERS | | #15

    >"Not a matter of life or death other than affects on general health."

    True, but within limits. Sometimes having numbers and "data" can make for consternation over things that are really too small to be an issue. I've seen confusion before with ppb and ppt, for example, when the "safe limits" are all in ppm. Basically people getting concerned over things that are literally thousands of times below the acceptable limits to be considered safe. I'm not saying that's what you're doing here, it's just an example.

    I don't think a MERV13 filter is going to get smoke particulates. The other issue is that typical residential systems cycle, and the filter is only doing anything while the system blower is running. In my usual commercial telecom facilities, the blower is ALWAYS running, it's only the air conditioner compressor that cycles, so the filter is ALWAYS filtering the air. Equipment in these facilities never shows any dust accumulation because of this. You can do the same thing on your residential system by setting the blower to be "on" all the time (expect a big bump in your electric bill though). Doing this MIGHT help.

    I would look for a higher level filter. you can get MERV16 filters now. You can also get activated carbon filters (although these are likely to clog quickly in smoke). Electrostatic filters can also help with smoke particulates, but they need frequently cleaning to stay effective.

    Another option would be one of the Honeywell HEPA filters that can be put in a room and left running all the time. I've had good luck with these before, and they're not terribly expensive. The downside is they really only help in the room they're in.

    Note that if you have a particularly leaky home, the stuff coming in through the leaks could potentially overwhlem whatever kind of filtration you're running.


    1. tundracycle | | #23

      Thanks. Our HVAC blowers run continuously so are continuously filtering the air. Not sure what the volume is during recirc though.

      The system is 'designed for MERV-13' using 5" filters. They couldn't tell me what pressure drop that used so presumably a higher MERV with an equivalent pressure drop would work?

      Home is 1.36 ACH50 RESNET so about 1.9 ACH50 international.

  10. exeric | | #17

    I had personal experience with fire smoke last year. Unfortunately MERV 13 filters don't work for 2.5 or smaller particulate matter. It became very smoky in my house last year. I haven't wanted to believe it was the filter and thought it maybe a problem in my sealing of the house. After seeing the youtube video that Brian Wiley posted(comment 7) I was disabused of that notion. That was a very informative video.

    I have a 6" duct for the outside intake air at the wall cap. They discussed using an external HEPA filter plumbed to that connection. It showed via detailed measurements how a HEPA filter there completely eliminated the indoor 2.5 matter problem. I'm going to try to remove the 6" CAP and plumb a standard shop vac HEPA filter with an adapter into that location. In the video they described just using a 6 hose coming from a HEPA filter equipped shop vac running outside. That's the quick and dirty method to solve the problem. However I don't want a shop vac running outside and making noise and open to theft.

    I'll let people here know how it works out, if it works. Contrary to news accounts many of us here in California still haven't experienced smoke events this year. Crossing my fingers.

    1. brian_wiley | | #19

      I’ll be very curious to see how that works out, Eric H. Hopefully you won’t need it though.

      If you do need it though, it’d be great to know a few details about your house such as the air tightness, size, and if you have an HRV/ERV for comparison. In that video the presenter seems to suggest that he implemented it successfully on both his passive house and his neighbor’s, the latter of which was a 1950’s non-passive house without any intentional ventilation. I’m hopeful that’s the case, because that’s mirrors my own situation.

      1. exeric | | #20

        Hi Brian,
        I got around to the modifications I was thinking about. First, my house has had a deep energy retrofit that brought the 1940's house to 2.25 ACH 50. It has a Panasonic ERV and needs it to keep the CO2 levels reasonable inside. Usually the CO2 level runs around 500 PPM. The input of the ERV takes air through a 6" flexible R8 insulated duct. It is joined to a hooded wall jack on the outside near the front door under an extended roof for the entryway.

        I removed the hood over the inlet by drilling out the rivets that hold it on. I then installed a standard 6" duct union into the jack. It fits perfectly. I caulked it in place using window and door caulk. There has been some luck on this hack I've just done. It happens that a standard size shop vac filter fits perfectly over the 6" duct union that is sticking out of the wall jack. It fits very tight in the rubber gasket of the HEPA filter. In fact, I can't pull the cover off the end of the filter when the ERV is running because the vacuum is so strong. Here is the HEPA filter I bought:

        You're not going to find a cheaper HEPA filter anywhere. I thought it was going to be too restrictive since it is so small. Not so. I had to turn up the intake motor two clicks on the Panasonic but still have more clicks if I needed it. If one is in a situation where you're already at maximum airflow there is the option to get and additional filter and duct union and just stick it on the end. You would halve the air restriction then since the flow is not in series but is in parallel for the air moving through the filter material itself. It is only in series once its gotten to the center of the filters.

        Before anyone chooses to go this route I need to say that I have not yet tested it in smoky conditions. I think it should work though since HEPA filters are supposed to work for smoke. At any rate I will update this when its tested in those conditions.

        1. brian_wiley | | #24

          That’s really great work, and good to know about the happy accident of the filter mating to the 6” vent.

          Does your strategy include positively pressurizing the house at all, and if so, how are you going about it? I ask because it appeared that in that passive house video they were just disconnecting the exhaust so that it blew into the house, but I couldn’t be certain.

          1. exeric | | #25

            Yes, it always helps when happy accidents occur. Usually my accidents aren't happy. I have the option to have slightly positive pressure in the house by turning the intake motor (there's two) up to maximum. Right now it's two clicks below that which maintains 500 PPM CO2. We'll see when a smoke event actually happens. My nose and lungs will be my detector.

            I think if your home is not that tight it may be more important to do what the person in that video you linked to did in his home. Positive pressure becomes more important then so that smoke does not come in through all the small leaks in the structure. I'm thinking that the person in the video hooked up the shop vac hose directly to his ERV fresh air wall jack. He then disconnected the input to the ERV.

            It seems to me that it would be wise to emulate that strategy in your home if you don't have an ERV. You should probably purchase a HEPA filter equipped shopvac running outside and run the hose through a hole in the structure. It's problematic cutting that hole because it will compromise the structure, so it would be best to try use an existing hole to run the hose in (if possible). That's the best I can suggest. Maybe others have a better hack for your situation. It sounds like it would only be temporary. In my location I've had to act as if this smoke situation is here to stay.

        2. tundracycle | | #27

          We have two Broan ERV-250's in our new house that was completed in summer 2020. Unfortunately the only filters on these are internal reusable washable so I'm guessing fairly low MERV which kind of surprises me given that this is to protect the ERV core. In our case that's probably fine 99% of the time as our outdoor air quality is rarely very bad.

          I'm now considering installing a 5" filter box on the intake prior to one of the ERVs (other is in a very tight closet that doesn't have room). I could probably leave it empty most of the time or a lower MERV but then put in a higher MERV when we have smoke events.

      2. exeric | | #21

        Forgot, My home is 1100 sf, single occupant.

  11. tundracycle | | #22

    "Where is your fresh air coming from when the ERVs are off? Are you also monitoring CO2?"

    Three bedrooms have IQAir AVP's in them as well as a uRAD A3 in our gym, a portable Gaslab 501 and a new CO2 measurement product we're testing in our sauna (

    Unfortunately the alternative to HRV ventilation for us is what minimal amount takes place when doors are opened or closed. If we had events like this regularly then I think I'd likely put in a filter system to allow us to continue to ventilate when outside air is poor but fortunately this is an extremely rare occurance.

  12. Expert Member
    BILL WICHERS | | #28

    Eric, I recommend a foam prefilter over that pleated HEPA filter. The prefilter will get the "big stuff", and also cobwebs and insect debris, and will likely extend the useful life of the HEPA filter it's protecting.


    1. exeric | | #29

      That sounds like a good idea. Do you mean a ready made one or foam material you secure on the outside like a rolled up newspaper?

      1. Expert Member
        BILL WICHERS | | #30

        If you can find a ready made "round" prefilter, I'd use that. If you can't find that, get some of the 1/4" "filter foam", wrap it around the filter, and secure it with string. Rubber bands will work too, but they degrade pretty quickly in the sun so string works better.


      2. exeric | | #31

        Thanks, I found the ready made foam ones on Amazon.

  13. Expert Member
    Akos | | #32

    I did a quick check of PM2.5 on my fresh air feed. Outside air is ~ 26µg/m³, after the merv 13 filter is 19µg/m³ after the HEPA filter it is around 1 µg/m³ (my meter is not that great so it is hard to tell at low concentrations).

    Air inside the house hovers around 2 to 3 µg/m³.

    So the MERV 13 does a bit but not enough if you are looking to filter fine particles.

    1. tundracycle | | #33

      I'm sort of maybe kind of perhaps missing something here. MERV-13 is 'supposed' to filter 85% of PM2.5. Above Brian referenced 50% (I've tried to watch the video three times and have yet to get more than 3 minutes in). You're getting only about 20%. Why?
      - You've a much higher face velocity?
      - MERV tests are with a specific concentration of test particles and filter efficiency varies based on this?
      - Other?

      What meter are you using?

      1. Expert Member
        Akos | | #35

        I have a Temtop 1000. The PM count for the Merv 13 filter was probably higher than reality as I was holding the sensor near a lot of air flow, so more air could have gone through the sensor than what it expects. The MERV 13 is a 10x20x1 and I'm running at 100CFM, so the FPM is pretty low. The HEPA is 12x16x2.5, significantly larger filter.

        No matter the reading, it definitely looks like a fair bit of PM2.5 makes it through the MERV 13 and squat through the HEPA filter.

        This correlates well with my ragweed allergy symptoms. The MERV13 reduced symptoms, with the HEPA in-line, they are gone.

  14. brian_wiley | | #34

    I can’t speak to Akos’ particular setup, but in that video they talked about the falloff of a filters efficacy. Thats probably not groundbreaking news or anything, but could be a contributing factor depending on the age of the filter. They were seeing merv-13 filters slide into the 11s (and lower in extreme smoke events) pretty quickly. That and the lower accuracy of the monitor at low concentrations might explain that 20% number.

    Excuse the image quality of the screen grab from the aforementioned video, but it should give an idea of the efficiencies they were seeing.

    1. Expert Member
      BILL WICHERS | | #36

      It's probably worth mentioning that filters work better as they age, assuming they don't get holes in them. As the filter "plugs up", it gets better and better at filtering stuff out of the air. The tradeoff is that the backpressure -- the resistance to airflow through the filter -- goes up as the filter clogs up.

      Filters do NOT get worse about filtering particulates out of the air as they age, assuming the filter media stays intact (no tears or holes).


      1. brian_wiley | | #37

        That’s true, and good to point out. Raw filter media doesn’t get less efficient in trapping particles in and of itself, and like you said, there is a bump in efficiency.

        I should have been more clear on the reason why they were seeing that slide in performance. Many filter manufacturers apply an electrostatic charge to the filter media to enhance the capacity to attract and capture particles, but the US Ashrae testing doesn’t really account for what happens when that charge wears off. Some manufactures do a secondary test (Appendix J to the 52.2 standard) that simulates the loss of efficiency when that charge wears off, and that’s where the ‘slide’ in efficiency that I mentioned earlier comes from. The manufactures that do usually report that somewhere as the Merv-A rating if you dig deep enough.

        And from what I gather, filters tested to the ISO standard do two tests: one fully charged and one totally discharged and then reports an average of the two. However, I haven’t dug deep enough to know if they weight the average at all, but either way it seems more representative of the true performance that you’d see in the field.

      2. tundracycle | | #39

        "As the filter "plugs up", it gets better and better at filtering stuff out of the air."

        Is that proven or theory? Seemingly, as portions get clogged the face velocity of unclogged portions would increase and so filter efficiency actually decline?

        1. Expert Member
          BILL WICHERS | | #49

          It’s real, but it only applies to the filter media itself. If your filter media has a poor seal, then you’re correct that the increased back pressure through the filter media will mean any air leaks will get “more leaky”. How much those leaks cancel out the increase in the clogged filter’s ability to filter stuff depends on the ratio of “through filter” airflow to “through leak” air flow.

          I would recommend a properly sealed filter housing though regardless. A simple slot with a slide-in filter is probably pretty leaky all the time, which will always reduce the overall effectiveness of any filtration system.


          1. exeric | | #50

            Bill, this may be an implied critique of my non filter housing installation of a HEPA filter (comment 20). Maybe that wasn't your intent. That installation happens to be just as airtight as a filter housing. It is an unusual case and it cannot be expected that most hacks will work so effectively. That referenced filter over a 6" diameter duct union does fit very airtight. I would not have recommended it if that wasn't the case.

            Also, I found out that the foam filters I ordered that you recommended are for use INSTEAD of the regular cartridge filter media, and are to be installed directly over the cage that the cartridge filters normally go on. The foam filter are to be used instead of the cartridge filters when used to vacuum liquids. So they are not appropriate as a cartridge pre filter as you were recommending.

  15. tundracycle | | #38

    Yesterday was the highest PM2.5 recorded here of 206 µg/m³.

    Today it's currently 98 µg/m³ outside and 73 µg/m³ inside. Both HVAC system fans are running continuously with 25x20x5" MERV-13 filters. Each fan is about 800 CFM so we s/b getting about 1600 CFM of filtering.

    1. tundracycle | | #41

      Our uRad A3 was boxed up to go back for a new CO2 sensor so I unboxed it and plugged it in for a bit more data. It's showing considerably lower PM2.5 inside than the IQAir's. When I'd tested the A3 against the PDR-1500 it'd looked quite good as had the IQAir's but that was at much lower concentrations. Currently the A3 is showing 51 µg/m³ and the IQAir's about 66 µg/m³.

      1. tundracycle | | #44

        I moved the A3 to outside for a bit (there's about a 30 minute gap when it couldn't communicate with an AP). It's indicating mid 70's for PM2.5 while the local EPA/MNPCA station 3 miles away is indicating 119 µg/m³. Perhaps most importantly though is that it's indicating about a 25-30 pt difference between inside and outside so according to it the inside air is definitely better than outside.

  16. walta100 | | #40

    Seem to me trying to cleaning smoke from the air MERV 13 filter is not going to do much.

    More or less shooting marbles thru a chain link fence.

    Unless you can quintuple the surface area of your current filter the HVAC blower motor will not be able to move enough air thru the HEPA filter you need to clean the smoke from the air.

    I think you should look at getting a portable HEPA air filter.

    Or 4 HEPA filters and a box fan


    1. tundracycle | | #45

      I have one from last summer built w/ MERV-13's based on Allison Bailes design. It works well in a single room but had no measurable effect house-wide so I'm not sure how well it'd do w/ MERV-16's and given the cost of MERV-16's it'd be an expensive experiment. But I agree that MERV-13's are minimally effective.

  17. Expert Member
    Akos | | #42

    I think you can get a Merv 16 filter (Lennox makes one) that size. If your blower can handle the extra bit of pressure drop, that would be pretty close to HEPA performance.

    Another option is to improve the filtering on the ERVs. An inline multi stage filter box on the intake is not too expensive plus it keeps the core much cleaner. I find that as long as I filter the incoming fresh air enough and run the ERV unbalanced to pressurize the house, I don't need any extra filtering inside.

    1. tundracycle | | #43

      Thanks for the tip on Lennox making these filters. Got 'em on order. Trying to find filters is surprisingly difficult. The system can handle them according to notes from our HVAC contractor. I'm assuming lower airflow though so potentially won't cool as quickly or possibly as well.

      ERV's are currently not running because of the OAQ. CO2 last night hit about 900 in a couple of bedrooms which is higher than I'd prefer but kind of the lessor of two evils right now.

      1. Expert Member
        Akos | | #46

        Keep us posted how the new filter works out. If you have a way to measure pressure drop across it VS the Merv 13 would also be good to know as there doesn't seem to be much information out there for these.

        1. tundracycle | | #51

          Yep. The final excuse to finally get a manometer. :-)

        2. tundracycle | | #55

          20x25x4.2 filters @ 823 CFM's:
          MERV-13 (Filterbuy) = 0.23
          MERV-16 (Lennox) = 0.28

        3. Expert Member
          Akos | | #57

          Looks like Lennox did a good job with the filter design keep the pressure that low for a high MERV filter.

          How is it on removing PM 2.5?

  18. walta100 | | #47

    Please understand restricting the air flow to your HVAC system is a dangerous game putting expensive parts of your system at risk.

    When the filter slows the air flowing thru the system the same number of BTUs is removed from a smaller quantity of air. When a system is working correctly the return air temp is lower more than 15° but less than 25°. Let’s say the return air is 78° it is cooling the air by 25°to 53° to do this the coil needs to be about 20° colder than the air at 33° this is very close to freezing. If the air is somehow restricted the coils could get below freezing. If the coil freezes the water on its surface will turn to ice and further restrict the flow of air and the coils will become an ice ball. The ball of ice cannot absorb many BTUs from the air that is not passing over the coil. When the coil does not absorb enough heat to boil the liquid refrigerant inside the coil liquid will pass thru the coil. Should liquid refrigerant makes it back to the compressor the compressor will try to do the impossible feat of compressing a liquid. The compressor will make some very sad sounds and self destruct given enough time.

    If you are intent on play this game carefully monitor the discharge air temps any number under 55° has you on at risk.


    1. tundracycle | | #48

      Agree. According to our HVAC contractor our system s/b able to handle these specific filters. Good point about keeping an eye on the discharge air temps. I'll prob stick a logger on one to monitor it.

  19. exeric | | #52

    Referring to comment 20,
    I showed a modification to the filtering system on my ERV. Previously I used a MERV 13 filter that was not filtering smoke. I then showed a ShopVac Hepa filter I plumbed into the fresh air inlet for my ERV. I said I'd update information when we actually had smoky conditions.

    Well, the wind changed and today I woke up to very smoky conditions. It's now at 160 PM 2.5 as measured by the Purple Air site. I can't smell any smoke inside my home. That's a new experience for me when it has previously been this smoky.

    So I would say this has been a successful experiment and that a ShopVac Hepa filter can be made to work with the addition of 6" diameter duct unions. Be aware that the filter can only be used for ERVs/HRVs and not HVAC systems because HVAC systems require more air than a ShopVac filter can flow.

    1. exeric | | #53

      I should add one more encouragement. Many people already have a shop vac that has a filter of the size of the HEPA filter as specified in comment 20. If you have a shopvac filter with the same dimensions (it's a prevalent standard dimension filter) then go to your local lumberyard and buy a $6.00 6" dia duct union as shown in my photograph and test fit it into the shop vac filter. You might be surprised at how secure and airtight it is. There is no need to commit to doing such an alteration before you have satisfied yourself that it will work and be airtight using standard duct fitting practices.

    2. brian_wiley | | #54

      That’s super encouraging to hear, Eric. I’m glad to hear that it worked so well. Thanks for the update.

      1. exeric | | #56

        Your welcome Brian. Today the air quality has gone down to 290 PM 2.5. Full disclosure, I was starting to smell smoke then. I have a self contained 4 filter MERV 13 with fan setup that Allison Bailes described in one of his blog posts used for slowing COVID transmission. I turned that on and and it has stopped the smoky smell and throat irritation I was getting. Multiple stages of filtration seems to be the way to go, especially in really heavy smoke.

        1. Expert Member
          BILL WICHERS | | #58

          Multiple stages is how most large systems work. I would recommend coarser filters towards the intake, and the finest filters on the exhaust. This helps to keep the finer filters lasting as long as possible before they clog.

          You might try an activated carbon filter to help with smoke too, just be sure to put a particulate filter ahead of it.

          I hope you guys in the fire zones get some rain soon for some relief.


  20. brian_wiley | | #59

    Hi all, thought I would post a quick update on my situation. After relatively clean air for the past couple weeks, we woke up to significant smoke this morning. Our IQAir showed an outdoor measurement of 151 and at one point an indoor measurement of 97. I ran out to pick up a HEPA for my shop vac to test out a more low-brow solution to add positive pressure to the house.

    I hooked up the shop vac as a blower, and routed the hose through a piece of rigid foam so that I could tape the window shut and fired it up.

    20 minutes later, the AQI was 33 indoor/139 outdoor, and about 2 hours later it is bouncing between 12-17 indoor/145 outdoor. It’s also helped quite a bit with CO2, which had been between 850–1000. Our house is 1185 square feet, and while I’ve never had a blower door test, my assumption is that its very leaky as it was build in 1955.

    The two downsides so far are the noise, and the fact that I’m blowing hot air into my house. I’m hoping that a variable speed controller will help with both of those things and that I can dial in the lowest CFM setting to maintain positive pressure.

    Hope that helps someone that doesn’t have an HRV/ERV in the short term. I’ll post back with any updates or changes.

  21. tundracycle | | #60

    Follow-up on this. Per @Akos recommendation we got the Lennox MERV-16's. Indoor PM (1.0, 2.5 and 10) all dropped to below 5 within a few hours while outside continued to remain quite high.

    After about 10 days outside had returned to normal so we pulled the 16's and put the 13's back in. This was purely a cost of filters thing. Pressure drop going from 13 to 16 was minimal.

    In mid 2022 we had a similar incident. Swapping in the 16's took care of IAQ for the 4 days that this one lasted.

    We've got the 16's stored in bags for any future needs.

    Thanks all!

    1. charlie_sullivan | | #61

      Thanks for the follow-up. Great to hear that that worked so well.

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