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Community and Q&A

Filtering Smoke Through HRV With Activated-Carbon Felt Pads

Kathryn Oseen-Senda | Posted in Mechanicals on

As we’ve been suffering through bad air on the West Coast, I thought I’d share some impromptu testing of a jury rigged air filter that seems to be working pretty well.

We have a Venmar HEPA HRV that I got specifically for fire season (though I’m quite happy to have fresh air from a balanced system the rest of the year). Unfortunately, when the smoke hit, the house still smelled like a camp fire because of all of the VOCs that come through the HEPA filter in gas form. While these HRVs have core filters (to protect the exchanger core from bugs and leaves), a pre-filter and a HEPA filter, there isn’t a design slot for an activated charcoal scrubber.

Since I didn’t want to add too much additional pressure drop, I cut pads of activated carbon felt and put them snugly behind the registers. I don’t have quantitative data, but the smell immediately got better in the house and our eyes don’t sting anymore.

On the next round, I may try for a thinner filter (for less pressure drop) and try and retrofit it into the slot for the core filter, but I’ll first have to find a metal frame to put it in.

Good luck to those in the thick of things. I hope this was helpful.

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  1. Mark_Rob | | #1

    Fantech makes a 3 filter air filter that has a course, carbon filter and a HEPA filter in it, the CM3000, that could be used for filtering either incoming air or recirculating inside air.

  2. Kathryn Oseen-Senda | | #2

    Thanks! That looks pretty nice. I'd still like to find a way to modify the HRV rather than adding on a whole new fan system in series, but that would work well for some people.

  3. Eric Habegger | | #3

    Here's a fantech inline filter box that is typically used for HRV/ERV. It comes with a Merv 13 filter but looks like an activated carbon filter might be added fairly easily. It would be plumbed to incoming supply side air duct coming from the outside.

    I'm getting something similar:

    I'm taking a bit of a risk because I think it will create a larger pressure drop than the Fantech. I like it though because there's a big selection of filters you can get including carbonized ones.

  4. Expert Member
    Zephyr7 | | #4

    Note that activated carbon filters don’t last very long when filtering a heavy load of contaminants, and from the pictures I’ve seen, the smoke out west would certainly qualify as a “heavy load”. Thicker filters with more media will last longer before a change is needed. Activated carbon works by trapping contaminants in the many, many very fine pores in the grains of filter media. Once this pores fill up, the filter media stops working.

    You’ll probably find that your activated carbon media will gradually loose effectiveness over a period of a week or two, so make sure you have spare media on hand.


  5. CarsonB | | #5

    I got a 4" merv 12 filter (the highest available at home depot) for my forced air system, however we could smell smoke pretty strongly coming in through the registers. It's hard to tell if this is from leaks in the ducts through the crawlspace, or possibly air being pulled in through the gas vent for the gas furnace. I then just took out the filter and duct taped it to a box fan. That seemed to work pretty well as a poor man's air purifier and I could see the coloration of the filter within hours. Adding carbon filters to it would have likely helped further, but the house does not smell of smoke.

    1. Expert Member
      Zephyr7 | | #6

      The big difference is any of the MERV-rated filters filter out primarily particulates, bits of dirt in the air, basically. Activated carbon filters can actually filter out some types of chemical compounds, and tend to be particularly effective at “stuff that smells”.

      The downside to activated carbon is that the very, very tiny pores in the filter media are easily plugged by “big stuff”. This is why you ideally want a fine particulate filter — like your MERV 12 filter — ahead of an activated carbon filter. Combining those two will make a very effective filter to deal with smoke.


      1. CarsonB | | #7

        thanks Bill, I guess I should invest in some carbon fabric. For the box fan, does it matter which side of the fan the filter is on? I've seen it both ways. I guess on the "return" side of the fan the filter at least gets sucked against the fan. I bet those $700 air purifier companies must be doing quite well this year.

        1. Expert Member
          Zephyr7 | | #8

          It doesn’t really make any difference which side of the fan the filter goes on in terms of filtering the air, but it might make a difference in terms of how well you can seal the unit. Be careful you don’t restrict the air so much that the fan motor overheats too.

          I’d probably put the filters on the sucking side to make sealing easier. If you put the filters on the blowing side, you’re likely to find the fan blows around the edge of the filters and leaks a lot of stuff.

          It might be worth noting that electrostatic air filters can also filter out very fine particulates like smoke/ash particulates, and their filter “media” generally washable, often in a dishwasher. That might be useful if you find you are doing lots of filter changes.

          I hope all you people out in the fire zones make it through ok.


  6. Eric Habegger | | #9

    I was inspired by the question of the OP to do some work on my own home. I installed the filter box that I mentioned earlier in this thread and plumbed it into my Panasonic FV-10ve1 ERV. I've included a photo of the installation. It holds a 14 x 14 x 4 inch merv 13 filter. It replaces a much smaller merv 8 filter that came with it. Again, a photo is included with the two filters next to each other.

    To my surprise the new filter is much more free flowing than the original filter even though the merv 13 filter filters smaller particles. Wonderful surprise. I was able to turn down the intake and exhaust settings from 70 cfm to 60 cfm. I used my requirement for getting to a least below 500 parts per million of CO2 in my house to make that assessment. The original smaller filter only managed slightly above 500 p/m at the higher setting while the new filter is running around 450 at the lower power.

    I can't speak for other ERVs but the Panasonic unit definitely benefits from the new larger filters. They are also much cheaper at about $15 per filter vs $40 for the much smaller Panasonic proprietary filter. It definitely pays to use filters that are generic and widely used. I also checked the power needed to run the ERV for equivalent CO2 readings in the house. The ERV now consumes 34 watts whereas it used to consume 54 watts. Its not working as hard now and is operating much quieter. This is actually more important than it might seem. I have a small battery back up system that runs off 4 solar panels and it needs all the help it can get when there are fire safety power shutoffs by PG&E.

    1. Eric Habegger | | #13

      I know it has come up that consumer grade CO2 monitors are notoriously inaccurate. This is the one I have and it seems pretty accurate.

      I use it as a "relative" indicator and it seems very accurate in that capacity. For instance, I had the ERV turned off for several days while I installed the new filter housing. The CO2 monitor indicated 1200 parts/million. Once it was hooked up and allowed to stabilize after 8 hours the CO2 level indicated 450/million. So I think this particular CO2 monitor is one of the better ones and relatively accurate. For instance, you take it outside and the CO2 monitor indicates close to 400/million. This is about the actual known CO2 level in the year 2020. So that's a good indication that 450/million in the house is a good number.

  7. Eric Habegger | | #10

    I guess I should add that the new filter housing came with that one merv 13 filter. I've ordered 4 more at $15/each and I also ordered 4 activated carbon filters at around $20 each. I'm only going to use the more expensive and less durable carbon filters during smoke events.

  8. Expert Member
    Zephyr7 | | #11

    The back pressure presented to the fan by the filter is dependent on both the MERV rating (higher number filters finer particulates, but more back pressure) AND the area of the filter media. If you have twice the flow restriction with a higher MERV filter, for example, a filter twice as large gets you back to the same flow restriction as the original. Four times the area gets you down to half the flow restriction. Less backpressure from less flow restriction allows more airflow with less energy.


    1. Eric Habegger | | #12

      Yes, that is an excellent way to describe it. It also makes intuitive sense, which is a plus.

      1. Expert Member
        Zephyr7 | | #14

        Much of physics is fairly intuitive if you take the time to explain it right :-) The Feynman lectures on physics are good examples of this. I’ve always liked his way of presenting things.


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