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Comment by everdrive | Hacker News Reader

Not being snarky, but a window is one of the best options. Air conditioners can often be your worst option, as the air is recirculated more aggressively. We use a window fan that has multiple settings: circulate, ingress, egress. I find that no matter what setting you use, air is recirculated significantly well. On hot days, the fan is set on egress. This seems to do a really great job of removing excess CO2 while not bringing that much hot air into the house. We have a window fan running in the kitchen for the bulk of the year. We also find that simply cracking the windows in the winter makes a large and measurable difference in CO2, but not with heat.

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> a window is one of the best options.

Add a fan and it's hard to find something more optimal and cheap for this problem.

Also, don’t put the fan directly in the window, but a leave a small gap between the fan and the window to take advantage of Bernoulli’s principle

Do you have a good visual of what would be best here? I've been making sure my fan is as tightly sealed around the window as possible, but it'd be interesting to learn if there's a better solution.

This teacher helps visualize this concept pretty well

https://www.reddit.com/r/Damnthatsinteresting/comments/ul8mt...

Yes! This video covers window and fans in more detail : https://m.youtube.com/watch?v=1L2ef1CP-yw

Also if you live in an area with low humidity look into getting a swamp cooler. It won’t make the air as cold as AC but I personally prefer it

Swamp coolers can get really nasty and smelly due to being the perfect breeding ground for spores and other nasties.

I wasn’t aware of that. I haven’t had any issues and the swamp cooler at my place is old. It’s very dry here, but I guess if you were to not run the cooler for a while the water may sit and you could see stuff start to grow in it. If the water is running 20 hours a day I’d be skeptical that anything would settle there but I could be wrong. As soon as we stop using it for the season I drain the water out of the reservoir.

I wonder what would cause that. It seems intuitive that it would happen, but like the other commenter I don’t recall any bad smells when we used on growing up. But it was also a very dry climate, so maybe that is the solution?

same as humidifiers and they make bacteriostatic additives that help control this (although eventually it does need to be cleaned regardless).

Thanks, I’ll look into additives

I suspect a UV lamp over the evaporator would solve this problem. It would not need to be lit continuously, just at a regular interval short enough to quash any fledgeling fungal or bacterial growth. There may be swamp coolers which have such a sterilising function integrated, otherwise it'd be a useful hack with market potential.

Yeah that’s not a bad idea. And often swamp coolers have electric sockets inside so it wouldn’t be too hard to wire it out. If I wasn’t renting the place I’d probably give it a shot

> We also find that simply cracking the windows in the winter makes a large and measurable difference in CO2, but not with heat.

Have you maybe done measurements of what difference between the inside and outside temperature is enough to reduce CO2 without a fan?

This is a good question. Last fall I did some experimenting just trying to get the humidity out of my bathroom without a fan and it requires huge temperature differentials to do it in any reasonable amount of time. I bet CO2 is similar. A slight breeze fixes everything though.

I have. We live in the northeastern US, so our winters get quite cold. Our house is VERY well insulated, so I imagine you'll get different results depending on your situation. We have a CO2 meter, as well as a radon meter. Both work as analogues for air circulation. In the winter, you can measure temperature loss by measuring how often the heat comes on. And of course you can measure CO2 / radon just by looking at the gauges.

What we've found is that in the winter putting two windows open a crack makes a very significant different in CO2 and radon builup. CO2 without the cracked windows will be around 1200 or more, while with cracked windows can be as low as 600-800. Not perfect, but like I said, a major improvement. I don't have a precise way to measure if the heat comes off more often, but my experience is that it does not.

Temperature isn't going to be much of a factor at the temperatures and temperature differences humans encounter in their homes. The dominant factor in CO₂ exchange is the difference in CO₂ concentrations.

Imagine a completely closed room surrounded by open atmosphere with the room air having twice the number of CO₂ molecules per m^3 as the open atmosphere. The room air and the outside air are at the same temperature.

Now imagine opening a small vent in the room, and consider what happens to the air close to the vent. Molecules near the vent and moving toward it have a chance of crossing to the other side. At the vent we have a constant flow of molecules from inside to outside and a constant flow of molecules from outside to inside.

What counts as "near" depends on the mean free path, which is proportional to temperature. With the temperatures the same the mean free path will be about the same.

Since there are twice as many CO₂ molecules near the vent on the inside, twice as many CO₂ molecules will be flowing out of the room as are flowing in.

A temperature difference would increase this, by increasing the mean free path on the hot side which increases what counts as "near" on that side. But what matters is what the temperature is above absolute 0. For a room at 40℃ (104℉) and an outside at 0℃ (32℉) that's only about a 15% temperature difference.

This is a really great explanation, and matches my experience: the effects on temperature are very small, and the effects on CO2 are very large -- and the difference is enough that this approach is absolutely worth it.

Your analysis considers only diffusion of CO2 whereas my experience is that convection is also important, and running a heater is in some configurations a great way to induce convection.

I sleep in very small room -- only 45 sq feet and 45 * 8 cubic feet. I don't have a CO2 meter, but I've been sleeping in the room for decades, so I have many data points, and I am pretty sure I can tell (by noticing that my thinking is duller and stupider than usual the next day) when CO2 levels were too high overnight.

No matter how high the CO2 level are in the room, 20 minutes of having the door wide open will bring CO2 to levels I cannot distinguish from outdoor air even if there is nothing (no wind, fan or heat source) to cause convection. Here we see diffusion in action. But at night, I prefer to have the door open just an crack (about an inch). Configured that way, CO2 levels clearly get too high if I rely on just diffusion: I need convection to keep CO2 levels healthy, and a heater running inside the room provides sufficient convection (as does a fan pointed at the crack of the door).

There is one exception: when the air outside the room is hot enough (and here I guess I should mention that the door to this very small room basically opens to the outdoors) then the heater might not provide enough convection (and even if I could be assured that it did, it would make me too warm on very warm night). I.e., my system of running a heater and having the door open only a crack should not be relied on to keep CO2 levels low in very warm weather unless you want to do research I have not done (e.g., with a CO2 meter).

I should mention that this very small room is unusually airtight: even the round holes in the metal boxes inside the walls that contain the electrical outlets and light switches have been sealed. I mention this because it has made it easier for me to compare the effects of convection to the effects of diffusion because when there is only one opening (a door in my case) in an otherwise almost completely airtight room, the presence of a wind (and there is some wind or breeze most times in most places on earth) does not induce significant convection across the doorway.

I used to rely on an alarm clock to wake me up every 105 minutes to air the room (which takes 6 minutes on very cold nights and 10 minutes on the warmest nights). (When sleeping this way, the door is completely closed during the 105-minute intervals of sleep.) I no longer sleep this way because I've become less tolerant of having my sleep interrupted every 105 minutes. I mention this to underline the fact that I have plenty (decades) of experience with diffusion and convection in the context of CO2 and sleep.

(The 6 to 10 minutes of having the door wide open does not reliably reduce CO2 to levels indistinguishable from outdoor air, but I consider it good enough, and I wish to reduce the duration of the discomfort of lying in bed with the door wide open.)

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