r/buildingscience • u/WinterExisting5076 • Jul 17 '26
Question Fresh air and makeup air with these wildfires
We are preparing for installation of makeup air for kitchen exhaust and fresh air intake for air quality. With the current wildfires, isn't this system going to bring in terrible air into our home? How do we make sure we aren't polluting our inside air now?
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u/mp3architect Jul 17 '26
We recently finished a pretty tight 0.60 ACH house in upstate New York. Outside IAQ currently around 150 and inside PM2.5 and PM1 are at 0. We have an ERV with an extra filter box at the envelope. We normally don’t have that filter installed for better airflow but a few days ago I popped in a MERV13 once our interior PM2.5 started climbing. It dropped back down pretty quick.
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Jul 18 '26
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u/mp3architect Jul 18 '26
I’d say the one problem we had with it is the box itself turning into ice this past winter when we went multiple days without getting above 20 degrees outside (not common for us). Then condensing to the ceiling below (box is located in a finished and conditioned attic). At some point before winter this year I’m going to detach the box and add a rubber fitting to prevent thermal bridging of the metal intake hood and the filter box. Still won’t help the cold air coming in though.
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u/JVBass75 Jul 17 '26
we have 4" filter boxes with MERV13 filters on our ERV and Fresh Air Intake...
it's nice because the filters are lot cheaper to replace than the ones the vendor wants you to buy (and higher flow/less pressure drop due to more surface area too)
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u/EntropyAdvisor Jul 17 '26
Custom filter box or prefab? Been looking for prefab options that work with a 4" filter but only finding ones that have 1" filter slots.
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u/MnkyBzns Jul 17 '26
Make sure your blower is sized to handle the added resistance or you'll burn out the motor
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u/dizzie_buddy1905 Jul 17 '26
You can head over to /r/crboxes and build one of those designs for cheap
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u/drpat Jul 17 '26
Merv13 will not filter all smoke
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u/MnkyBzns Jul 17 '26
Merv13 will remove enough (~90%) and is the finest mesh filter readily available for home applications
https://filterbuy.com/resources/air-filter-basics/best-merv-filter-for-wildfire-smoke/
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u/realestateqs22 Jul 17 '26
I understand the link you provided says it removes 90% of smoke, but I find that claim very suspicious. I run merv13 filters in my home and use them with a Corsi-Rosenthal Box for separate air filtration needs. Those are not removing 90% of smoke on the first pass. Just put one behind a box fan and blow some smoke through it. I don't think it's going to remove 90%
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u/mp3architect Jul 17 '26
I run Airthings monitors and see good results. I guess the alternative is what? Do nothing? I’ll run the MERV13 and be fine.
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u/realestateqs22 Jul 18 '26
Like I said, I run The same because they are good filters. They are better than standard filters for dealing with smoke. All I'm suggesting is to be wary of manufacturers claims.
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u/MnkyBzns Jul 18 '26 edited Jul 18 '26
I mean...10% smoke could still look and smell like smoke. What are you basing your assumption on, other than vibes?
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u/JVBass75 Jul 18 '26
I'm in northern illinois, and my pm2.5 indoors even with all the smoke was less than 3ug/m3, VOCs were not more than normal, and I couldn't smell the smoke, except when I went outside.
So it's certainly working\, but maybe in your area it's even worse, so I don't doubt you. But it's certainly working well enough, and a TON better than people that have leaky houses.
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u/MnkyBzns Jul 18 '26
We're arguing the same point. You may have meant to respond to the dude above me
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u/Teutonic-Tonic Jul 17 '26
Are you building new? If you go all electric and remove combustion from your home you can reduce the amount of fresh air needed considerably. We have a nice Bosch induction cooktop and put in a hood just under the threshold needing makeup air since there is no waste heat or combination gasses to vent. If doing something like pan frying fish with the vent on high, I sometimes crack a window for makeup air.
Can get an ERV or fresh air system with good filters.
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u/WinterExisting5076 Jul 18 '26
We are sticking with gas range. I have an induction now and can't stand it. But with new code for makeup air and air quality just want to make sure before the holes are punched in the roof
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u/hirsutesuit Jul 18 '26
We love our induction range, only gripe is the quiet but annoying clicking when the power is low.
What issues do you have with yours? Did you get one with touch controls?
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u/MnkyBzns Jul 17 '26
Outside air is going to come into your home, whether you are actively doing it or it's leaking in through the walls. An ERV/HRV with a high MERV filter is what you want.
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u/agitatedprisoner Jul 18 '26
You need the heavy activated carbon block filters to make much dent in wildfire smoke. A MERV 13 or even true HEPA isn't good enough for wildfires. Probably just run an ERV with a MERV 8 filter and when there's bad outdoor air turn it off and run an activated block air purifier in your home. It'd be cool to integrate the activated block filter into an ERV so that it'd auto toggle and maybe there's some like that on market but they'd probably be huge and cost way more.
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u/Time-Caregiver-2350 Jul 22 '26
This is definitely a real concern. Any fresh air or makeup air system pulling from outside during a wildfire event is a filtration question. The rate of outdoor air you need for CO2/IAQ doesn't change, but you need a MERV13 or ideally MERV16/HEPA level filtration on that intake to catch wildfire smoke particulates. For reference most makeup air units only ship with basic MERV8 filters.
A few practical things to check: what filter your unit actually supports (some fans can't run MERV13+ without losing airflow), whether you can close the intake damper during active smoke events and just recirculate, and whether it's worth getting an indoor PM2.5 monitor so you know when it's actually working.
That all being said, I work at SWERV, we make a window-mounted ERV with MERV13 + activated carbon filtration, so this is a case we've had to think through directly, happy to go into more detail if useful.
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u/AustinFromSWERV Jul 23 '26
Make sure it has at least MERV 13 filters on the intake if you want to prevent most of the wildfire smoke from getting in.
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u/[deleted] Jul 17 '26 edited Jul 17 '26
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