
Ask HN: How to filter incoming air to a building - jxramos
We&#x27;re in the middle of another California wildfire season and I&#x27;m trying to figure out what the options are for getting fresh air into the home under these circumstances while working from home.<p>How effective would furnace filters be at filtering wildfire smoke air? Are there any products designed to both exchange AND filter air from the outside?
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Xcelerate
I wouldn’t worry about trying to filter the air on the way in. Your house
won’t be sealed up well enough to prevent smoky air from getting in. Instead,
you can set the air to recirculate and augment your home filter with an air
purifier. You want one that filters a large volume of air per hour. There’s a
lot of snake oil with regard to air purifiers out there; I generally try to
stick to high airflow HEPA filter units to remove the smoke particulates. Some
units also have an activated charcoal filter on top of the HEPA filter. These
help with removing some of the VOCs that cause the smoky smell, but a lot of
these filters are thin and don’t last very long. (I’m not actually sure if the
smell of smoke is bad for you or just the smoke particulates, but I prefer to
get rid of the smell if I can.)

There’s also some DIY solutions out there involving box fans and air filters.
I don’t think most box fans are actually powerful enough to move large volumes
of air when combined with an air filter, but I did see one guy on Reddit who
put a high MERV filter and a squirrel cage fan inside a wooden box enclosure
and that seemed to work pretty well.

Also, if you’re going to order an air purifier, do it soon. I suspect they are
about to sell out very quickly.

~~~
dredmorbius
This.

Your home should be seeing multiple air exchanges per hour. It doesn't much
matter if particles are filtered on entry or after, but (paradoxically) an
_indoor_ filter which accomplishes multiple passes of air through the filter
membrane is likely more effective than an _intake_ filter which only
conditions the airstream once.

Most homes or bedrooms should see 5-6 exchanges per hour:

[https://en.m.wikipedia.org/wiki/Air_changes_per_hour](https://en.m.wikipedia.org/wiki/Air_changes_per_hour)

The minimum standard is 0.35 exchanges/hr:

[https://www.epa.gov/indoor-air-quality-iaq/how-much-
ventilat...](https://www.epa.gov/indoor-air-quality-iaq/how-much-ventilation-
do-i-need-my-home-improve-indoor-air-quality)

DIY box-fan + furnace filter methods _can be_ effective at reducing indoor
particulate levels significantly. Not quite as effective as manufactured HEPA
filters, but at a fraction of the cost.

PM2.5 reduction @ 50 minutes: 71% box fan, 89% HEPA filter.

[https://healthybuildingscience.com/2018/11/18/diy-box-fan-
me...](https://healthybuildingscience.com/2018/11/18/diy-box-fan-
merv-13-versus-hepa-air-purifier/)

MAKE instructional: [https://makezine.com/2018/11/16/build-an-affordable-air-
puri...](https://makezine.com/2018/11/16/build-an-affordable-air-purifier-
with-a-box-fan-and-hepa-filter/)

Also possible Covid-19 benefit:

[https://www.wired.com/story/could-a-janky-jury-rigged-air-
pu...](https://www.wired.com/story/could-a-janky-jury-rigged-air-purifier-
help-fight-covid-19/)

------
jxramos
I just called an HVAC company and there's a product known as a _Heating
Recovery Ventilator_ that pulls fresh air in, exchanges heat with the outside
air coming in, and passes the incoming air through a filter. The filters
however were not designed with the expectation that the air be full of smoke.
They do offer some amount of filtration however. The store apparently stocks
Fantech products.

There's also these grow room products that have fan and filter combinations
such as [https://www.homedepot.com/p/Viagrow-6-in-Carbon-Air-
Filter-w...](https://www.homedepot.com/p/Viagrow-6-in-Carbon-Air-Filter-with-
Inline-Fan-Combo-V6CF-KIT/203554119)

This fella creates a DIY window based intake filter from a furnace filter, an
air conditioner prefilter, and some other charcoal prefilter mesh
[http://www.air-purifier-power.com/windowfanfilter080515.html](http://www.air-
purifier-power.com/windowfanfilter080515.html). It apparently works well for
everyday dust but is unknown how it stacks up against smoke.

There's also this product much in line with the above DIY project _Activated
Carbon Passive Window Air Purifier_ [https://www.homedepot.com/p/Activated-
Carbon-Passive-Window-...](https://www.homedepot.com/p/Activated-Carbon-
Passive-Window-Air-Purifier-AC-736/206173822)

There's some airbox inline filter design too such as this unit
[https://airboxfilter.com/Airboxfilters/air-
box1](https://airboxfilter.com/Airboxfilters/air-box1)

------
verdverm
2 filters, box fan, painters tape

~~~
dredmorbius
Just one filter is very nearly as effective (if not more so by increased
airflow) at reduced cost and more supplies for others.

Taping the filter to the intake side helps form a better seal -> more
effective filtration.

