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Comment by macNchz | Hacker News Reader

After getting a (non-DIY because as much as I like hobby electronics I have a tendency to scope creep my little projects straight into incompletion) about a year ago, my main learnings have been:

- Cooking is insanely bad for the air in your house, even with an outside-venting hood. Any sort of medium-high heat cooking will spike the PM2.5 to multiple times that of the haziest days here in NYC, and with the windows shut it will stay elevated for hours after.

- The 20" box fan + MERV furnace filter hack is super effective for cutting down on the PM. A few days this July the wildfire smoke from the west coast settled on NYC and we had 40-70 ug/m3 PM2.5 outside. The box fan filter took it down to around 5 in a matter of minutes. I didn't even bother taping the filter to the fan.

- My A/C system's ability to precisely hold the temperature within fractions of a degree of the thermostat setting is impressive.

- With two people working from home in an old, unventilated-but-kinda-drafty ~700 square foot apartment, the only thing that gets the CO2 levels into "drowsy" territory (my original point of curiosity when buying the sensor) is running the gas oven for a while.

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I think the draftiness is key. In my findings, in a "modern" house (double-paned windows, built in the last 20 years), sleeping with the door closed will spike CO2 to 4 digits. The other thing is that air really needs to circulate in order to lower CO2 - opening a window in a room will not lower the hallway CO2 unless there's at least some draft / air movement.

Temperature difference helps if there's no wind. In winter time CO₂ drops from 4000+ ppm to ~450 in only a couple of minutes after opening a single window (with inside to outside temperature difference of 50°C or higher). In summer it drops to ~600-700 and stays there, and even this takes hours.

50°C difference or even higher? Damn mate, even Siberia only gets around -34°C which is less then that of you target the usual 18-21°C. Your winters sound terrifying.

Saskatchewan Canada can easily reach -40°C for several weeks during the winter.

What? Maybe in the most northern reaches.. even then "for several weeks" is a stretch.. even Uranium City doesn't hit -40°C every winter!

There is often confusion between wind chill, and actual temps.

But the parent is not inaccurate, for they did not mention "each year". Even 10 minutes North of Ottawa, I saw -42C, real, not just windchill, for a week, about a decade ago.

Winters? I thought they were referring to this last May or the previous August.

https://en.wikipedia.org/wiki/Highest_temperature_recorded_o...

He specifically said winter but having 68-74°C at minimum sounds terrifying too, honestly

That's what I've discovered over the past year! I live in NYC and my CO2 levels are fine in both the winter and on hot summer days, without even trying.

But when it's 65-75°F outside in the spring/fall... CO2 builds up in my apartment like crazy. I keep the windows open and my bathroom fan on to try to suck fresh air in... but even so can't manage to really get it below 650, and with just a window cracked it sticks around 850.

It's really frustrating.

That's why an ERV [1] is important in a modern house with good air tightness.

[1]: https://en.wikipedia.org/wiki/Energy_recovery_ventilation

In a modern home, there should be supply and return vents in every room. My home built in 1978 has 3 returns for the entire house, one on each floor. Master bedroom has a supply, but the return is located in the hallway. Over the years, the recommended door cut to allow air flow has been changed (presumably for greater privacy) - which leads to the scenario you're describing.

I could add the door cut back in, install return transfer grill, or look into adding a secondary heat recovery ventilator that acts independently of the primary HVAC system.

I know many people who put 'draft stop' devices on the bottoms of their bedroom doors in older homes (old enough to not have returns in each room, but new enough for central air), not realizing they're cutting off the one bit of circulation they need for a proper functioning central air system!

Or they buy new high efficiency windows and doors, and that 'tightens up' the house, but they don't also install an HRV (Heat Recovery Ventilator) to exchange outside air.

Realistically new windows and doors probably wont drop the homes air changes per hour below the recommended minimums, but I don't know for sure. I've gotten my crawlspace spray foamed and I'm getting my vented attic spray foamed to bring it into my envelope. Also getting new windows and doors soon, but after all of that I think enough air could leak through sill and top plates, electrical outlets, etc.

Having the foam crew do a blower door test for free if we go with them, so I guess I'll see. I live in climate zone 5a (cool, humid) so historically natural ventilation was used via induced pressure differentials generated by mechanical means (dryer, furnace, bathroom and kitchen vents). When we get new furnace and water heater that pull combustion air from outside, we're left with mostly manual means for generating negative pressure that drives infiltration. (But also wind, and pressure differentials caused by temperatures in winter)

In our case, we're definitely considering either an HRV or ERV to supply fresh outdoor air that we control (input and output CFM, MERV #). Our indoor air quality is poor enough to the point where we live with the tangible symptoms (Rhinitis), not just numbers on a sensor.

For those with interest in these topics, I suggest skimming a copy of one of the ASHRAE fundamentals handbooks[1]. It reads as a textbook but has incredible depth into the physical sciences that relate to building science, heating, cooling, and ventilation.

[1] https://archive.org/details/ashraehandbook2017hvacfundamenta...

these things are almost never mentioned in the "money saving tips" blogs , "lifehack articles" , r/frugal advice, etc places.

High efficiency windows and doors don't tighten up a house when it comes to air leakage. They bump up your thermal performance/energy efficiency.

And even if the installer air sealed your doors and windows, there are plenty more places for air to leak in.

You really need to run a blower door test to see if you need an HRV/ERV.

> In a modern home, there should be supply and return vents in every room.

That must vary by region. I live in the PNW, and I have not seen a recently built house with a return in each room. One or more supply vents per room depending on size, one return for each floor seems pretty much standard.

Interesting. I live in the upper midwest and remember my first home I lived in (parents built it new in 1980) had returns in every room. My current home is primarily forced air and there are returns in every room with multiples in the larger gathering spaces. Was built in the late 90s and I think building code may require a specific number of returns per square foot.

Could be regional -- I would bet that in most cases houses in the upper midwest have to deal with much colder temperatures in the winter than houses in the PNW. Maybe that influences the HVAC design.

But I wouldn't be the least bit surprised to find that there isn't really a specific reason, it's just "how we do things in this area." A surprising amount of building choices seem to be made that way.

Over here in Finland, nobody has returns in each room. It's a cold climate, so houses are heavily insulated, and moisture condensation is a concern during the winters.

We typically have returns in toilets, showers, etc. with one or more supply vents in each room; a large room may have several. Air flows in via supply vents, flows out under mandatory gaps in doors (if closed), through to the toilets and then outside. Moisture, CO2, and smells are constantly flushed in the right direction.

When the hood is on for cooking, air supply is typically enhanced so it doesn't mess up the overall pressure too much.

Incoming air is filtered, of course. The ventilation unit transfers heat from outgoing air to the incoming air to save energy and avoid problems with condensation.

In my home office, TVOC as measured by an Eve Room rarely goes above 100 (mostly happens when I pour an occasional whisky in the evening).

Old houses don't have ventilation machines at all, but rely on natural ventilation instead: warmer air flows upwards via chimneys and such due to gravity, pulling in fresh air from whatever gaps the structure has (accidental or purpose-built).

A 1" door undercut is only about equivalent to a 6" duct in cross section and that size should max out at 120 ft^3/min with well installed duct. If you have more supply then there will be airflow issues in that room/house.

In the last decade I built 2 houses in the same neighborhood over 6 years. The first didn't require a return per room and the second one did - they had changed their code. A few years later and they had reversed their decision.

I guess the point is that it depends on when and where something was built. My most recent home is only a few years old and doesn't have a return per room and no code to say it should shouldn't.

Pretty sure this is mostly American. Assuming your bath has a window it's not unlikely there are zero returns in central Europe.

Our multi-room apartment in Redmond, WA, has an always running intake fan in the master bathroom advertised as a way to keep air fresh. I always wonder how effective that really is (at apartment scale) and if it's really worth the 24/7 power draw.

Probably more for preventing mold damage from lazy tenants I imagine.

It's to keep positive pressure in your unit for safety purposes. If there is a fire the fresh air will help keep smoke out of your unit and as a bonus it keeps other building odors out. Generally it is a large fan on the roof that just sucks air down a stack pumping it into every unit. You could put a filter over the register and filter that air.

Sorry, by intake I meant it's sucking air in. That is, air is leaving my apartment. I think you're on the right track with the safety angle here though. Will do more research.

Air is leaving, thus, it is being sucked out of the apartment.

Its probably the opposite, it sucks out air. It does stop smells from this apartment going into neighboring spaces.

Almost certainly designed to ASHRAE 62.2's specs so that you get enough ventilation through a combination of infiltration (leakage) and additional mechanical ventilation. It's certainly worth it and if designed as a continuous ventilation fan should be one of the lower (~15W) draw units. Not sure how to really measure effectiveness other than by trusting in the experts who came up with the spec and air quality as an indirect measure.

https://homes.lbl.gov/ventilate-right/step-3-whole-building-...

I bought my parents a Netatmo weather setup. The one in the house measures temp, CO2 and the one outside temp, humidity.

During Christmas Day visits with only a half dozen people the room CO2 spiked to over 1,000ppm. Older house from the 1970s no HVAC. Opening a window (cross draft) brought it down to 300ppm. The room was stuffy before and great after so it shows how sensitive we humans are to air quality.

The box fan and high-merv filter hack is a good idea, but proper ventilation of filtered outdoor air is the best solution. Not always practical, but historically buildings breath via air infiltration due to pressure differentials, stack effects, wind, etc - might as well bring in the air on your own terms rather than it leak through your walls and insulation.

I'm not actually sure it's always the best. My rooms face the street and I can't tell if the air I get on that side is actually better than just having air from the other side +purifier.

Quality Energy/Heat Recovery Ventilators (which are basically mandated by code [in some jurisdictions] in modern construction due to air tightness requirements) should allow you to select the filter with the MERV value that works best for your situation.

But to be fair, you may have a good point. Vehicle emissions and tire wear generates particulates consider to be ultrafine (0.1 max nanometer), which is even difficult for HEPA filters to deal with.

On my leafy Brooklyn street the particulate pollution outside is well within the recommended levels nearly all the time, so thankfully there's not really a pressing need to retrofit modern ventilation into a 120 year old house. That is, assuming the presence of wildfire smoke from 3000 miles away remains a rare event, which...who knows.

> - Cooking is insanely bad for the air in your house

Hold on! Not all particulates are equally harmful - and by far!

A particulate sensor detects water vapor or particles of (non-burnt) cooking oil that are harmless to humans.

In the same way, it detects very dangerous (carcinogenic) compounds from combustion engines or burning coal or wood.

Cooking is in no way "insanely bad" (unless you are grilling your food black or cooking on an open wood/coal fire)

It does not measure water vapor, at least not the one I'm using (PMS5003). I always have windows tightly shut because of the extreme pollution outside, so 100% humidity is easy to reach simply by mopping the floors (which stay wet for hours after). It does not change PM measurements one bit. Fry a couple of eggs though, and PM 2.5 quickly goes over 100 µg/m³. Burn a piece of meat just a little bit and see it rise over 300 µg/m³.

What about suspended water droplets?

The thing that sends my PM2.5 skyrocketing the most is, interestingly, my toaster oven. Like from 50 to 1500 in the space of 10 min. No food needed within -- just the heating itself.

I have no idea what the particles are though -- metal? Paint? Carbon from burnt residue? Or just whatever the heating element itself is made of?

Separately, cooking with gas (oven+stove) also sends CO2 way way up -- in my kitchen, from 500 to 1300 or so within 20 minutes. And that's with my range hood on.

Dust in the toaster?

A lot of range hoods don't actually exhaust outside, and just suck stovetop air over some steel wool to condense the grease particles somewhere other than your walls.

Not saying that's the case here, but several times I thought the range fan was exhausting the air, only to learn upon examination that it was just shooting it back out into the same room.

The last place I lived in - the cooker hood did exactly this, without even the steel wool. We noticed a growing stain on the ceiling and found out there was no outlet, no filter, just a fan blowing the greasy air onto the ceiling. This was a 1 bedroom apartment with one of the ubiquitous "kitchenette"s that are fitted these days, so that landlords don't lose a whole room to a kitchen that exhausted commuting professionals will never use.

Our current place has a proper kitchen, but the cooker hood vents outside directly next to the central heating boiler's intake.

Carbon from burnt residue, most likely, and that could be quite unhealthy. Clean the toaster!

Thing is you can't really deep-clean it... it's not particularly dirty, I keep the bottom free of crumbs etc., but when oil droplets from food collect against the back surface and polymerize there, what can you do...?

You can't exactly throw the toaster oven in the sink and scrub the inside surfaces with steel wool and Barkeeper's Friend under running water like you can with pots and pans. Nor do toaster ovens have a "self-cleaning" cycle where you try to just vaporize it all... they simply don't get hot enough.

You're leaving out the most important part: cooking with gas.

"Natural gas and propane stoves can release carbon monoxide, formaldehyde and other harmful pollutants into the air, which can be toxic to people and pets."

https://ww2.arb.ca.gov/resources/documents/indoor-air-pollut...

"Gas stoves can generate unsafe levels of indoor air pollution"

https://www.vox.com/energy-and-environment/2020/5/7/21247602...

No, I'm responding to the statement about particulate. Gasses are a different topic.

Also, the dangers of open flames are well known, unlike the ones of particulate.

I am able to find many journal articles confirming my observations that cooking produces significantly elevated indoor PM2.5 levels, many of which suggest it to be harmful, but I'm not finding anything about the relative impacts of the different substances that make up fine/ultrafine particles. People involved in this research seem to think it will require more study:

>“We know that inhaling particles, regardless of what they’re made of, is detrimental to health. Is it equally bad as inhaling exhaust from vehicle emissions? That we don’t know that yet,” [1]

Regardless, my sensor, at least, doesn't register water vapor as particulate. I can run an ultrasonic humidifier with distilled water to no effect on the measured PM2.5, or with tap water which will spike the particulates from the dissolved minerals.

1) https://www.theguardian.com/environment/2019/feb/17/cooking-...

This is one of the selling points of induction stoves, but you will have to balance air quality with exposure to high EM fields.

Are there known health impacts from EM fields generated by induction cooktops? The only plausible risk I can find is to people with pacemakers.

i mean, i'm on an electric stove - and i have a cloud of oil/smoke when i properly sear stuff, given i have no ventilation outside of a small window. can't imagine that's too good.

and before you ask...100 year old house and no money

If someone knows the answer to this I’d be interested to know :

Do different PM2.5 components affect the body differently, even for the same overall count?

Eg, are the PM2.5 particulates from burning toast comparable in bodily health impact to a similar PM2.5 count coming from a heavy Diesel engine’s exhaust (which I presume would have a different particulate makeup).

My understanding is that PM2.5 with heavy metals or toxic payloads is more dangerous, but both are dangerous.

Still an active field of study but your intuition seems to be right. One study is here [1]. Combustible “types” of PM are usually worse (think smoke / carbon) vs. vaporous types of PM. The method of action is hypothesized to be the carbon particulate getting lodged in your lungs (similar problems from smoking, carbon nanotube inhalation, or charred foods).

[1] https://ehp.niehs.nih.gov/doi/full/10.1289/ehp.0800185?url_v...

> Cooking is insanely bad for the air in your house, even with an outside-venting hood. Any sort of medium-high heat cooking will spike the PM2.5 to multiple times that of the haziest days here in NYC, and with the windows shut it will stay elevated for hours after.

I’ve found this varies quite a bit. Though, I don’t do a ton of high heat cooking, so maybe there’s no way to do that without pollution.

But at medium to low temperatures, you can get crazy variance in AQI output:

* Lack of enough oil/butter in the pan will send levels soaring

* Cooking one thing in the pan then another after is very bad. Eg I used to cook sweet potatoes then add meat after and not clean in between to save waiting. Boy was that bad for the air. I think it’s because tiny bits of food you may not even see stick to bottom of pan and burn

* Browning is when it gets worse. Any kind of browning on foods spikes pollution.

* Any food particle that gets on the burner will invariably lead to hazardous AQI levels. Like even a single fleck of meat left cooking will fill the house. You may not necessarily smell it as much as other things

* Turning heat off at end of cooking works wonders. I do this with bacon and it takes it from a hazard to not even spiking aqi. Finished product looks and tastes the same

"Turning heat off at end of cooking"

What is the alternative?

I think he means that you turn the heat off before cooking is finished and let the residual heat of the pan do the final bit of cooking. As opposed to turning the heat off when cooking is completely done.

Precisely that

Funny that the Maillard reaction causing browning and using the resulting fond is exactly what you want to see and eat from a culinary, tastes-good perspective.

Totally. It is possible that a better cook than me can thread the needle of producing such a reaction without producing much pollution.

I haven’t tested AQI with steak for example. And I can say that with bacon I have certainly managed to have it turn brown without pollution. But, once things are turning brown you are on thin ice and it is easy to go from “none” to “hazardous” in a flash.

> I didn't even bother taping the filter to the fan.

I found bungee cord with J hooks works really well. As you correctly point out, you don't need to secure it at all, but it is annoying to tap the box fan and have the filter fall over.

That seems odd to me. I've never tried this, but it seems to me that you have to put the filter on the intake side of the fan, other the fan would just blow it off. And on the intake side, would not the pressure differential hold it in place?

I feel like you could place it on either side of the fan as long as air is directed to go through it. My personal intuition is filter then fan but I don't see a reason it couldn't go the other way. I wonder why most devices place it on the intake though. Maybe to prevent dust from getting inside a device, but even then it'd only protect the fan from dust. Hmmm...

Fan and then filter could lead to the fan blades getting dirtier faster. Other than that, I'm not sure if it makes a difference as far as effectiveness.

Good on you, master of the obvious!

Agreed with all these observations, especially gas oven or stove usage! Our former third floor ~1200 sq ft condo would spike CO2 up well over 1400 ppm when we’d run it, and almost instantly.

Even just with two people working from home, much of the space would quickly get up to 800 or 900 ppm. I’d usually open windows as I saw it creeping up, which would quickly reduce CO2 to under 600, and then it would slowly creep up over the day.

>The 20" box fan + MERV furnace filter hack

Living here in the west with hyper toxic AQI for the better part of two months now, I decided that the low CFM and leakage of a DIY box fan are not adequate to keep up. I went with a far more industrial solution: I went to an indoor grow shop and bought an activated carbon filter and high CFM inline fan. Cost $150 and is a bit noisy, but has been well worth it.

Link?

I wouldn't recommend this specific setup, but these are the type of filters/fans I am talking about:

https://www.amazon.com/VIVOSUN-Inline-Controller-Ducting-Ven...

You can buy carbon activated MERV filters. I got some from Nordic Pure.

what MERV rating is that. I do that now but I thought it's MERV 8 which really won't remove much at all

I’m surprised to hear this - a standard box fan from ace has something like ~600 cfm, right ?

That’s fairly high - what is the size and cfm of the fan you bought?

Not when you strap an HVAC filter on it. My setup is ~600cfm with much more filtering. Not sure what the MERV is but the cylindrical layer is much thicker and heavy duty than an HVAC filter.

Yeah the cooking indeed. My parents have a Philips air filter, when they cook or turn on their AirFryer or even when lighting the bbq outside, the thing's led-ring goes from blue to red and it starts to work very hard. It thought that was pretty striking. I usually do the cooking at home, now I wonder if I'm going to have a shorter life compared to my wife :s

What sensor are you using? Just curious.

For DIY, there was a pretty good discussion on suitable sensors in the AirGradient post from three months ago that Jeff linked to:

https://news.ycombinator.com/item?id=27125060

tl;dr: seems like you want an NDIR sensor (https://en.wikipedia.org/wiki/Nondispersive_infrared_sensor).

Some options, with digikey @1 prices:

Senseair S8 (Sweden): $48

Sensirion SCD30 (Switzerland): $63

Amphenol Telaire T6713 (US): $96

If I were to buy an expensive black box iot solution, I'd at least verify that it had an NDIR sensor.

I'm using an SCD41 with an OLED+microcontroller (using Sensirion's example code hacked a bit) and it seems to work well, cheaper than the NDIR using some photoacoustic magic.

$37 @1, but not in stock at digikey, and an 11 week lead time.

It's pretty tiny... I wonder if/when it will be possible to put a high quality co2 sensor in an IP68 iPhone...

I had purchased the dev kit sek-scd41 but its out of stock as well. The SCD41 also does RH and T so you don't need a separate sensor for that.

I'm using an Awair Element, I didn't do a ton of research on alternatives, but the app is pretty good and it "just works".

I've been thinking of getting it what are your thoughts on longevity of it? Will it last at least a few years? I also see in some Amazon reviews that measurements get "stuck" to some extreme. Have you noticed that too?

I've had an Awair Element for about 16 months. The only time I've experienced what you may mean by "stuck" measurements was about a month ago, when PM2.5 was puzzlingly and worryingly at a consistently higher than usual level for a couple of days. Trial and error eventually suggested that there was a piece of dust stuck in the sensor or something like that. Upon blowing on the sensor, the measurement returned to normal.

After a year I don’t see any issues with it, the sensors don’t seem to have drifted or changed in their responsiveness at all. It has crashed once or twice and stopped responding but was fine after a power cycle.

With any IoT device I’m wary about longevity re app support, I can’t say how long Awair will support this device or whether they’ll provide access to it after they stop, but I’m less concerned for a device like this vs, say, a major appliance.

Like you, and for the same reasons, I recently bought an Awair. At the risk of a derail, one thing I have been thinking about ever since: when I played D&D as a kid, they said the difference between Intelligence and Wisdom was, "Intelligence is knowing that it's raining, Wisdom is knowing to go inside." How much of these things are adding to our intelligence without adding to our wisdom? I am now constantly aware of particulates and everything else it cares to send me alerts about (which I will soon shut off) but how much better off am I once I established my air quality is basically Very Good.

The problem with all this "intelligence" is the Internet is very good at showing you search results for what you want to see, so Googling "Should I be worried about fine particulates" is a search destined to turn your anxiety into a full-blown panic attack.

No idea what the answer is, and I am impressed with the level of effort that went into the Awair app, but it would be nice to have more reasonable context in all IoT devices. Yeah, I know.

I definitely see where you’re coming from. When I first got it I was freaked out by the frequent spikes of VOCs, before determining that it’s actually hypersensitive to farts. I’ve come to ignore most transient fluctuations and just use VOCs/CO2/PM as guides for when to open or close the windows.

I’ve also found that regularly glancing at the 5 minute resolution charts of just temp and humidity has helped me get a better sense of my own comfort range…for example I like to run my AC as little as possible in the summer, but I’ve learned that playing chicken with the heat is foolish beyond 81F as I only wind up sitting at my desk grumpy and unproductive.

To add to the list of sensors: I was most curious in CO2 levels, so I got an Aranet4: https://www.amazon.com/dp/B07YY7BH2W/ref=cm_sw_r_cp_api_glt_...

It’s got an eink screen, and I haven’t had to recharge it since I got it 8 months ago. I really like it.

Are there any affordable CO2 sensors that don’t use outside air for calibration of ppm? I want to know how outside air CO2 levels change as well. Would be interesting to see day to day changes and how big they are in absolute terms. (Also to show people about how CO2 levels are changing globally… without just pointing to a website of someone else’s measurements.)

When the air is good outside, I typically run our whole-house fan pulling in outside air while I'm baking or making dinner - didn't consciously decide on it, but it just seemed to feel better to do those together.

Now I really need to investigate more details - like does induction cooking (vs. gas) improve the quality? What about the type of food (ie, cooking vegetarian vs. grilling sausages)?

Investigate fume hoods and implement a good one with enough make-up air to take everything coming off of your range out of your kitchen. Air quality's negatively impacted by heating pots/pans/etc. and their contents up and less about how they're being heated. With the exception of h2o just about everything you cook has a negative impact on the air quality and the pans themselves are doing all sorts of interesting things to the air when they get hot.

I agree on PM2.5 but gas is clearly worse from a CO2 (and other combustion byproducts) point of view compared to induction which is limited to 'just' the food items and the containers they're in.

Not an expert but from what I've read, gas stoves are by far the worst. Electric (including induction) are much healthier.

- Cooking is insanely bad for the air in your house, even with an outside-venting hood.

weeps in searing steaks with no hood at all :'(

One thing I found is our furnace fan and its filter is surprisingly quick and effective at cleaning up wildfire smoke (and cooking smoke...)

https://twitter.com/bfirsh/status/1434677458171678721

What sensor do you have?

And what is the boxfan hack? I live near very bad wild fires and I need a much better filtering then my current grow filter which doesn't seem to work that well.

> And what is the boxfan hack?

Literally just a box fan w/ a hvac filter taped to it. There are a few articles/youtube videos where people claim it works a lot better than a fancy air filter+fan device that costs $100s.

If you try it, just be careful it doesn't burn the fan motor and/or cause a fire.

https://www.youtube.com/watch?v=kH5APw_SLUU

you just put a fan filter on the back, you can increase the amount of filters to not hurt the fan motor as much by making a triangle config or fan on top of 4 filters in a cube. fans are for air throughput not static pressure so real air purifiers have squirrel cage blowers

The gas stove thing was surprising, but of course it makes a ton of sense!

There are better setups than the 20" box fan + MERV furnace filter. For example check out this similar DIY design that reduces risk of an electrical fire by making it easier for the fan to draw air: https://encycla.com/Corsi-Rosenthal_Cube

From that page,

>The EPA funded a study by Underwriters Laboratories (UL) which tested box fans with MERV-13 filters attached, finding them to not present a fire hazard even in extreme, modified conditions.

Then it links to, https://chemicalinsights.org/wp-content/uploads/DIY-Box-Fan-...

which says,

>Two-Sided Obstruction

>Figure 6 shows the time series of monitored temperatures under extreme condition where both sides of the fans were blocked. Despite the severe blockage of air flow, temperatures of all five box fans peaked within the first 20 minutes and then plateaued, remaining steady for the remaining 7 hours. Winding temperatures had the largest relative increase of 30 to 57 °C above ambient temperature, with peak absolute winding temperatures remaining at or below 80 °C. Peak temperatures on other fan parts did not exceed 50 °C.

> Temperatures at the power cord, guard, and switch, remained close to ambient room temperature (±6 °C) during all filter test scenarios as well as during the one-sided blocked tests. For the two-sided blocked tests, temperatures at the power cord, guard, and switch increased initially before plateauing to a steady temperature for the remainder of the 7-hour test duration. All fans remained operational for the 7-hour test duration and did not show any signs of potential hazard or damage.

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