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Community and Q&A

Choosing HVAC Filters for Wildfire-Prone Region

aunsafe2015 | Posted in General Questions on

This summer will be my second summer living in a “wildfire season” region of the country.  Last summer I wasn’t too prepared but made it through OK.

My HVAC setup is a supply-only fresh air intake that has it’s own 12x20x2 filter box.  The air from that feeds into my furnace return plenum and then the furnace fan distributes it through the house.  The furnace has an Aprilaire 20×25 4″ filter box.

For the 12x20x2 fresh air intake filter box, it seems the best I can do at a reasonable price is a 12x20x2 MERV 14 + carbon filter at $23.41 per filter (  I did locate a 12x20x2 HEPA filter, but it’s $79 and has no carbon (

For the Aprilaire 4″ filter box, my choices are between a MERV 13 + carbon filter ( and a MERV 16 (without carbon) filter (

My inclination is to go with the MERV 14+Carbon for the 12x20x2 air intake filter, and with the MERV 16 (without carbon) for the Aprilaire filter box.  Seems like it’s probably the best combination of price + carbon + ultra-fine filtration. (Total cost = $23.41 + $107.99 = $131.4)

My second choice would probably be to spring for the 12x20x2 HEPA filter and match it with the Aprilaire MERV13+ Carbon.  Might do a slightly better job overall?  Total cost = $79.00 + $107.99 = $186.99.

Any thoughts on which would be best to maintain healthy air during wildfire season?  Any votes for 12x20x2 MERV14+ Carbon for the air intake + Aprilaire MERV13+Carbon for the furnace?  I’m inclined to think this option is too much carbon and not enough ultra-fine filtration.

Thanks for any input!

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  1. DennisWood | | #1

    We're in the same boat so I've been experimenting a bit.

    If you want deal with smoke VOC (smell), you really can't beat a carbon canister type filter on your intake. A MERV 13 may grab 95% of the particulate, but it won't help with the VOC issue. A paper filter with a very thin carbon layer will work to some degree, but it does not perform anywhere close to a canister like the 8" I've been testing, which has 15lbs of carbon inside. These can be restrictive in the 4" sizes (tested, and drops flow 15-20 CFM from the 90 CFM testing baseline), but the 8" flows a lot better:

    The combination of velocity and very thin carbon layer does not provide enough contact time in a typical filter, which is why the commercial offerings usually have 3-4 filters inline. If you have room in your setup, the ideal setup is have the carbon filter last in line after filtration, not mounted on your intake. By all means try the paper/carbon filters you spec'd but don't expect an "effective" life of more than 2-4 weeks in heavy smoke, and don't expect them to completely address the issue. The larger 8" canister will have much longer life (and you won't need it all year) and will work much, much better for VOCs as air flow exceeds 30 CFM.

    I tried a slide on hood (just 30 CFM of flow) with a few of the off the shelf carbon filters that you'd purchase for a kitchen hood and it did make a difference, but I could still smell some smoke inside.

    1. aunsafe2015 | | #3

      Thanks for the response. How exactly would you even use one of those Terra Bloom fans with an air intake? They are not inline fans for an ERV; they suck air in from the entire cylindrical shape (i.e., a duct is only connected to one end) according to the picture at step 4 of the installation instructions:

      1. DennisWood | | #7

        The easiest method is to sort out an 8" 90 degree fitting over your existing HRV/ERV intake and attach to the carbon canister outside your house. You just need to keep rain off it. In my case, the HRV intake is under an eave in a corner of the house that is pretty much hidden, so I'm installing an exterior box to cover the carbon canister and ERV intake. I'll post pics once done.

  2. Expert Member
    Akos | | #2

    My experience with a Merv13 filter is that it won't remove enough of the fine pm2.5 particulates. On paper it is supposed to be about 70% efficient, when I checked the outlet of my Merv 13 pre-filter it only dropped the count by about 20%. The HEPA post filter drops it bellow measurable levels.

    Somewhere in your system you need a merv 16+ filter.

    1. aunsafe2015 | | #4

      Thanks for the post. I've also been thinking I need a Merv 16+ filter somewhere in the system. Which is why I'm inclined to go with the Merv14+carbon at the air intake, and the merv16 in the furnace. My filter box actually has enough room that I could stuff additional carbon material above and below the 12x20x2 filter if I need a little extra carbon boost.

      If I can remember I'll report back in August or September with updates.

  3. DennisWood | | #5

    What I see on the larger commercial filtration systems is that the carbon/VOC portion of the filter system is essentially a wire mesh box full of activated carbon. If wildfire smoke continues to be an issue, I suspect you may see some offerings like this so that for your application a 12x20x1 bulk carbon filter box could be paired with a MERV 12x20x1 pre or post filter.

    These folks look to be selling these refillable carbon trays already in various sizes specifically for wildfire smoke:

    Based on my shop experiments, that box would work miles better than your typical "carbon impregnated" paper filter.

  4. Nathan_Iltis | | #6

    A few additional thoughts on this topic, to note some caveats here for readers that may not realize the following:

    1) This is a common (although not the best) way for homes to receive fresh air from outdoors. The best way is to have a dedicated HRV/ERV, because it runs constantly and fresh air supply flows to the rooms served are more dependable, and because it recovers heat/cooling/ and humidity (ERV). An HRV or ERV could have an in-line filter box such as the CFB or CFB-HP from if the unit installed doesn't have HEPA or carbon filter options.

    2) in the arrangement described in the original post, the furnace (or A/C) fan must be running in order for filtration to be happening. The thermostat would need to be in fan-only mode when no cooling or heating is needed.

    3) An alternative option for health during wildfire season is to buy a portable room air filter and run it locally in the room you are currently spending time in. If you work at home, run it in your home office, and move it to your bedroom at night. This option doesn't get you fresh air , so your indoor CO2 levels may be a bit high (can cause drowsiness or slightly worse sleep quality). Turn off the furnace/AC fan-only function , or run it intermittently if you want some fresh air and don't need any heating/cooling.

  5. DennisWood | | #8

    We had our first wildfire smoke incident this week with smoke from Alberta fires (over 2000 kms away) reaching us for ~2 days.

    Using the Terrabloom 8" carbon canister on the HRV intake works extremely well, although this week I had it inside, running a fan through it in our main living area (hidden in a corner) where the HRV fresh air dumps. I'm sorting a permanent outside install via a wall mounted box that covers the HRV intake. The inside setup works, but is obviously not as effective as having it on the HRV intake itself.

    During the worst of the smoke outside (hazy with a noticeable smoke smell) the MERV 13 filter installed on the HRV intake (14x14x4) reduced PM2.5 from an outside peak of 20 ug/m3 to about 9 ug/m3 inside our home. Our home's PM2.5 levels are typically around 2 ug/m3. I suspect once I have the carbon canister installed properly on the HRV intake these levels will drop inside further as the carbon sequesters some of these fine smoke particles. The 14x14x4 carbon impregnated filter I tested in the HRV filter box was not effective at reducing the smoke smell inside the house, pretty much as expected. We had an outside skunk spray incident that again the 14x14x4 "carbon" filter did not handle.

    1. aunsafe2015 | | #9

      I also have been getting some smoke from the Alberta fires, though my PurpleAir sensor has not been reading more than about 50. During peak wildfire season we will sometimes see 150-ish for days straight and 300+ for hours here and there.

      How exactly do you have the Terrabloom hooked up to a fan? Do you have a fan that has a duct coming off of it, with the duct running to the Terrabloom? Mind sharing a link to the fan or posting a picture? This sounds like it would be better than a box fan with a filter taped on it.

  6. DennisWood | | #10

    I’m at camp this weekend, but will post a pic later. The temporary setup is with the Terrabloom sitting on its base in a corner behind a recliner. This is the same room as the HRV dumps fresh air. Sitting on top of the carbon filter is an a AC infinity 6” EC fan…using a 6” to 8” reducer. It just sits in the 8” duct collar on the Terrabloom carbon filter and blows upwards. The fan is set at about the mid point for speed, so likely 250 CFM.

    The permanent setup will be in an exterior box that covers the HRV intake, suspended under an eave, using the HRV fans to pull air in.

    1. aunsafe2015 | | #13

      Thanks for the info

  7. walta100 | | #11

    I would be surprised if your fans are able to move enough air thru that small of a MERV 13 filter you may need 2X-4X the surface area.


  8. DennisWood | | #12

    Walt, the fan is directly attached to a freestanding circular carbon canister, 24” long, about 14” across with 15lbs of carbon contained. I chose it as the smaller 4” had too little surface area…

    From Terra-Blooms site:

    This 8 x 24 inch model has a gross weight of 23 lbs and contains 15 lbs of carbon inside. It can handle airflow rates up to 750 CFM. For seamless operation pair this filter with TerraBloom 8” EC Fan, model ECMF-200.

    To be clear, once set up on the HRV intake, it will only see from 50 to 90 CFM depending on the automation bits and sensor readings on the system.

  9. DennisWood | | #14

    Here is the temporary setup which actually works extremely well as an interior air cleaner for smoke, skunk odor, VOC reduction etc. It sits in a corner behind a chair for now. The total height is 41”. Fan is an AC Infinity 6” using an EC motor. ( These filters come with a basic filter sock which pre filters dust before it reaches the carbon itself. Filter is the Terrabloom 8" ( )

    The carbon portion of this setup will end up on the HRV intake, outside the house, housed in a box with diverter damper (so carbon filter can be bypassed when not needed) and filter.

    1. andyfrog | | #15

      How loud is this setup? Would you say it's more than adequate for a single room (120 sf)?

    2. aunsafe2015 | | #17

      Very cool, thanks for posting. I might have to give it a try.

  10. DennisWood | | #16

    Loudness is pretty much a function of the fan speed, but around mid point in the speed range the 6" fan sounds likes a small floor fan and beyond that is quite noticeable. However, for a smaller room, you'd only need to run it at speeds 2-3 (of 11). The 8" carbon canister and 6" fan is enough to manage a small home..which is why I'll be using the 8" carbon canister (minus the fan) on my HRV intake shortly.

    I've tested a smaller version of this setup (4" carbon canister, 4" EC fan) and that is more than enough for a single room. It certainly worked (as in 100% effective) for a small live cricket habitat (daughter had a chameleon) that was stinking up the entire top floor of our home, running at a very low speed. My daughter actually preferred the white noise from that setup, (again running at lower speeds), in her room as a sleep aid. It was that setup that got me thinking that a larger canister would be the most cost effective way to deal with wild fire smoke and a whole house intake system. The Terrabloom 8" specifically has that extra surface size (cylinder is ~24" long, ~12" in diameter) and has a 1.8" thick carbon layer (15lbs total) that correspond to effective VOC treatment with low resistance at 100 CFM. At $129 USD, they are pretty cost effective as well. They can be refilled with activated carbon, although it is a messy job.

    1. andyfrog | | #18

      Thanks Dennis!

  11. DennisWood | | #19

    Here's a trial setup of the carbon filter on the outside of the house. For now, I just removed the 4" hood and connected things up with 4" duct. This will be replaced shortly by 6" ducting. My intention is to build a box to shield the filter from weather and incorporate a 16x25 filter at the base as well so I can use the intake system minus the carbon filter when not needed. Right now the carbon filter is wearing a sock provided by Terrabloom to filter out dust etc. before it gets to the activated carbon filter. I removed the HRV internal filter (on fresh air supply only) and tested the flow rates to find that at the HRV's 50, 65, 75 and 95 CFM profiles, flow has actually increased about 5%. The large carbon filter and sock is less restrictive than the internal (small) HRV filter.

    This corner of the house is hidden from view by a tall hedge, so the aesthetic is not an issue.

    1. Expert Member
      BILL WICHERS | | #20

      Ideally you want a pretty good particulate filter ahead of the carbon filter. I would use a bag filter if I had a choice, otherwise a good pleated filter. The more the filter ahead of the carbon filter can filter out, the longer the carbon filter will last.

      Are you seeing improvement with your system so far? I'm outside of the Detroit area, which is apparently (according to the news today) the second worst air quality in the world after New York due to the smoke from the wild fires in Canada in Quebec and Ontario. I have sinus issues, and the smoke has been bothering me the past 2-3 days. I hope we get some rain in the next few days as predicted, since the rain is nature's particulate "filter" for the air.


  12. DennisWood | | #21

    Bill, the final setup will see the filter sleeve over the carbon canister removed (it’s fairly restrictive) and the canister in a box with a 16x25 filter in the MERV11 range as a pre filter. We have not seen serious smoke since I installed the carbon canister outside, although fire models our area with light smoke over the last 8 hours, with PM2.5 in the 11 ug/m3 range currently reported outside. Levels inside our home are at 1.7 ug/m3 currently.

    I made the mistake of having a few windows open during a smoke event a few days back and pm2.5 quickly spiked inside to 59 ug/m3. It took about 12 hours to get levels back under 2 ug/m3 with windows closed and the HRV running in its demand driven profiles…so varying between 50 and 100 CFM.

    Right now the filtration from outside in:
    1. fabric filter sleeve (sleeve over carbon filter)
    2. 8” Terrabloom carbon filter (1100 square inches surface area, 15 lbs carbon)
    3. 16x16x4 MERV13 filter.

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