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Ask HN: How to filter incoming air to a building

4 points by jxramos ↗ HN
We're in the middle of another California wildfire season and I'm trying to figure out what the options are for getting fresh air into the home under these circumstances while working from home.

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?

7 comments

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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

The minimum standard is 0.35 exchanges/hr:

https://www.epa.gov/indoor-air-quality-iaq/how-much-ventilat...

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...

MAKE instructional: https://makezine.com/2018/11/16/build-an-affordable-air-puri...

Also possible Covid-19 benefit:

https://www.wired.com/story/could-a-janky-jury-rigged-air-pu...

jxramos
One use case we're facing is dealing with the negative pressure created by exhaust fans and an exhaust-only air conditioner. Acting together they pull air in from the outside. I was hoping there'd be some means to have some filter media handling that intake air thriugh a controlled portal so we can still exchange air. Afterall even with multiple circulations filtered or otherwise we still need some fresh air for humidity, oxygen, and stuffiness sake.

You make a good point about the house being leaky in the end though. It's a real effort to have a completely sealed house for sure, companies specialize in that work. We bought three Honeywell air filters last year running continuously in the meantime.

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.

jxramos
we have filters, that solves the recirculating problem, it's the intake side that I don't know how to handle. I posted an answer where I'm growing a list of ideas and products out from. There's some interesting stuff in this space. The biggest unknown is how they perform in smoke conditions. They all assume standard pollen dust etc irritants in the air.
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...

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. 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-...

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