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I'm installing a new whole-house-fan, and am expecting good results. We live at 8,000' and have ~40 degree (f) temperature swings between day and night with very little humidity.

But we also have forest fires mid summer, generally during the hottest parts of the year (when we want to use the fan the most), and the air can sometimes be very smokey, and I don't want to just keep the downstairs windows open.

I know that an air conditioner would be best because it doesn't require importing outside air, but I'm hoping there's some way to filter outside air on the way in. My fan is ~2200 CFM, as far as sizing goes.

Is there a common and preferred method to filter ~2200 CFM of air that would be drawn into a home that's using a ~2200 CFM whole house exhaust fan?

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    Filtering is easy. Smoke eating is expensive and I'm not sure how well they work on not tobacco smoke. Firefighters don't wear respirators... they wear hoods with supplied air.
    – Mazura
    May 8, 2018 at 0:12
  • I have had 4 whole house fans; never thought filtering was necessary. Aug 14, 2021 at 19:02

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I am doing this now. Using a 10"x36" carbon filter attached to a window vent (so it's kind of automatic). This is good for 1400 CFM. If you go up to 12" or 14" I'm sure it can handle 2200 CFM. About $200 on Amazon

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  • Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. And, you should probably take our tour so you'll know how best to contribute here. Jun 22, 2019 at 11:59
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A typical pleated filter should have no more than 300 CFM per square foot going through. So for 2200 CFM you should have 7.33 square feet of filter area or approximately 30”x35”. However whole house fans do not pull well against back pressure so you will drop air flow fairly significantly. An even bigger filter will of course help to minimize that loss. Typical whole house fans take air from inside your house and blow it into the attic. The makeup air comes from open windows so that is where the filter would go if you were to have a filter However it would also draw air from every crack and crevice it can find. Whole house fans while they can cool well in the right conditions are a messy proposition as far as air quality is concerned.

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  • The filter would go in the fresh air intake you'd have to install, but again, no simple filter is going to remove smoke particles. Smoke eaters for tobacco smoke (that actually work) cost thousands of dollars.
    – Mazura
    May 9, 2018 at 23:49
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I'm in the process of designing one for myself. I'm surprised this isn't a more prevalent idea.

My current plan is to have a box that drops down into the crawl space with the filters in an accordion like arrangement. It's area, and the filters I'm looking at (good for pollen and dust) do significantly cut down on the air flow.

I'm currently putting those filters into windows and it really drops the air flow down. I have 4 20x30 filters, with a fan that's rated at 2000CFM, so that's a little more than 16 square feet (16 2/3 sqft to be exact). By my calcs this is 120CFM per square foot. I'm wondering how much the leakage around the sides is, but I do try to get the filters pretty much taking up the whole window space.

With this setup, in my area:

  1. with windows just open (and much less area) the house will drop 5 degrees throughout in 3hrs, easily. This is at dusk, when the outside air isn't as cool as it is later in the night.

  2. with the 3 windows with filters (one window fits 2 of those filters) the house will take about 6 to 8 hours, and I am shutting the fans off in the morning with the house cooled only about 8 degrees or so.

I'm wondering where the 300 CFM value comes from (and these are Honeywell FPR 9 filters for allergens and dust). I'm sure a less capable filter would have a much lower pressure drop, but the idea is to get no pollen or dust in.

With all of this, the current fan solution is using way less energy than the AC did (which died, and is in the process of being replaced). A better house fan is on it's way, with a rating of over 4000CFM, but again, that's without significant backpressure.

If anyone else is going down this route, I'd love to hear your ideas. We'll see how my "filter stack" works out, if I can build it and get the house manager's approval to use it.

=== Further progress(?) as of Sept 26, 2021 ===

So, was not able to get the filter which would take air from the crawl space into the house past the house modifcation committee. But did get more info on filters, and now am working to get the air coming in through my windows filtered.

Found 5" filters (from 3M, Filtrete) which have better (i.e. lower) flow resistance and better filtraton than what I'd been using. These have (printed on the filters themselves) the pressure drop/air flow numbers which I still am not able to find online from the manufacturers. Had to build a frame that fits in the window, can seal in the window opening, and hold the filters securely and be place in the windows when it is desired to get the air cleaned as it enters the house. By standing in front of the new filter or the old filter, the 5" filter has significantly better airflow, so I need to get the other filter frames moving.

Much goofy info out there, but I did find one report from California state, where they were actually looking at filter performance, and one of my takeaways from that was that the 3M data that they were looking at seemed to be quite valid. Filter parameters printed on filter (3M, Filtrete, 5"x25"x16") With these parameters, and a 5000CFM fan, I need at least 4 to meet the "max rated airflow". From the data, you can see that the resistance goes down dramatically if you aren't right at the highest point, so that's what I'll be aiming at.

Also, from real life considerations, there are a host of other areas that will be providing airflow, and the more filters I have the less these unfiltered "volunteers" will be adding to the air being run into the house.

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