Ask HN: Best way to remove CO2 in a bedroom? | Hacker News Reader

Ask HN: Best way to remove CO2 in a bedroom?

I know that CO2 sequestration/capture is imperfect and expensive; however, I was wondering if there is a doable way to remove CO2 in a bedroom at night, to improve air quality why we sleep?

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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.

> 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

> 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.)

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

Ventilation is usually the best way. Even when there is a temperature differential between the outside and inside, it's more economical to heat up / cool down in the new air than trying to scrub CO2 [1]

If you have an air quality concern, you can start by installing an indoor air quality monitor with a CO2 sensor.

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

There is no small scale human comparable in room CO2 removal bio/tech. Human breath will increase CO2 in a closed room on the order of many hundreds of ppm over hours, while even "superplants" like those in an Algen Air will decrease it at best on the order of low tens. We need improvements of two orders of magnitude to have room scale impact. So the only answer is "fresh" air ventilation.

"There is no small scale human comparable in room CO2 removal bio/tech."

Of course the tech exists, it's just not common. Some submersible tech and space tech used rebreathers or scrubbers to remove CO2 and increase O2 levels. Of course the reason it's not common is that outside of those use cases you can generally just open a window.

Didn’t Apollo have scrubbers too?

Turning on a bathroom vent fan is remarkably effective. Based on the CO2 monitor in my basement utility room, turning on a bathroom vent fan in the upstairs lowers the CO2 in the entire house within a few hours. An energy recovery ventilator would be ideal, though, as it wares much less energy while still lowering your CO2 levels.

It would be awesome if there were a way to actually capture and sequester in-home CO2, so that your in-home number could be lower than the outdoors. But I’ve never heard of such a thing.

House plants

They are practically useless in reality. You’d have to live in a greenhouse for them to absorb your CO2 basically.

What about like a tiny cactus on my nightstand?

Will that help calm my anxiety and help me accept the inevitability of death and see it as the indivisible half of the miracle of life?

It might take your mind off existential anxiety-related thoughts if you stab yourself on its spines in the dark.

You haven't seen my house.

Breathing during sleep adds only little CO2 to the environment. Here's a calculator: https://www.omnicalculator.com/ecology/co2-breathing-emissio....

I like the way you phrased that - "Breathing during sleep adds only little CO2 to the environment". It kinda sounds like you're trying to discourage them from trying not to breathe during sleep.

My CO2 sensor shows up to 2,600 ppm overnight if we don't actively vent our bedroom. 500-750 if we do, depending on which in-window fan model and how high we set it.

That's that's quite a lot more than expected. Isn't there another source of CO2?

I have two true CO2 sensors in my room (SCD30, SCD41), and I can confirm the CO2 concentration rises to 2400-2900 ppm during the night. But it's very easy to vent, a draft will reset the concentration to background levels in a minute or so.

For the kind of technology available to us, it could cost up to $100 a day to buy enough "soda lime" to filter out the CO2. You'd want the "color changing" type of product. You would also need HEPA filters to ensure the powder was not getting in the air. Temperature sensors would also be needed to ensure it wasn't releasing so much heat that it causes a fire.

The algae route is cheaper, but needs a lot of algae.

> For the kind of technology available to us, it could cost up to $100 a day to buy enough lime soda to filter out the CO2.

Doing some napkin math based on quickly googled(maybe incorrect) numbers, 100g of lime soda can absorb 10-20L of CO2 depending on the setup. Random offer off a Google search is $30 for 1kg, and I can get it cheaper locally. To go from 1200 ppm to atmospheric 421, you need to remove ~0.8L/m³. For my 50m³ room, that's 200-400g of lime soda. CO2 takes several hours with closed windows to build up that high, though, and it'd need a control loop to avoid wasting the lime soda by scrubbing below atmospheric CO2 levels...

I thought you were being hyperbolic but, damn, yeah, this doesn't sound very viable ):

Thanks for the math. The parent comment has been slightly edited.

I'd ideally want the CO2 to be the preindustrial level of under 300 ppm. The reason is that I suspect it could lead to maximum alertness and a feeling of well-being.

Note that some ventilation still is needed during the day to get O2 and remove VOCs, so the calculations are affected.

In your calculation, remember to account for the presence of people who constantly breathe out CO2. This and the need for some ventilation add to the cost.

The amount of O2 removed from the air is negligible compared to the impact of elevated CO2 on human gas exchange tbh.

I doubt you'd have much benefit going down to 300 instead of 400, and it introduces a whole other problem - aiming at sub-ambient CO2 targets makes ventilation increase the amount of scrubbing you need to do instead of decreasing it. I imagine the increase in cost would be massive.

But yeah it'd probably be better to base the calculations on co2 breathed out by people. But like I said, that's just some quick, likely incorrect math to land in the right ballpark.

Thinking ahead, as the atmospheric outdoor level keeps climbing, ventilation will become insufficient to achieve a desired target indoor level. Air conditioning suffers from the extreme version of the same problem. Yes, the cost is substantial if using Soda Lime.

Amines are traditionally used instead in a closed loop system, but they are corrosive and unsafe for use at home, although refrigerators and ACs too make use of similar loops. GPT says that some non-corrosive common amino acid salts can be used instead, but I find it difficult to believe.

Energy recovery ventilator. It will exchange indoor air with outdoor air while maintaining indoor heat and humidity. Unfortunately small window mounted models are hard to come by. Most are whole-home systems.

This is the way. As others mention opening window is your cheapest option, but you lose a lot of your heating and cooling efficiency. We installed a whole house ERV for about $10K and saw a significant drop in CO2 readings.

Whole home ones are only real ones having a chance to work.

I’ve installed mine in about 6hours (but I do have moderately accessible attic).

They are somewhat cheap too - at $1k or so.

I used to have a window mounted unit from a small company. It worked wonderfully in my bedroom.

Panasonic ones go down to 50cfm, which is fine for a bedroom. That being said, everything in construction comes down to “it depends” because there is so much variability including just in execution. In California for instance there are mechanical whole home ventilation requirements, but those requirements are quite low and are commonly ineffective for the OPs goal depending on how the system is arranged. Same thing goes for the ERV.

Often times the best way to solve the OPs problem is just to run the central fan (assuming there is central air) as that will exchange bedroom air with the rest of the house, which assuming the house is significantly larger than the bedroom, will basically solve the problem.

If you are going to do an ERV I would recommend adding a filter box for allergens/dust/etc assuming the ERV you select doesn’t have one built in. Not expensive, it’s just a sheet metal box you put filters in, HVAC guys buy and make them all the time.

If you want to spend even less money, and don’t have central air, you can install a bathroom fan in the ceiling/wall of the bedroom, and have it vent to the rest of the house but this takes a bit of thought with regards to airflow and how the air will mix. But even if all you are doing is effectively doubling the volume of your bedroom, that makes a big difference.

Agree with all these suggestions. Except I would caution against simply having a fan that sucks air out of the bedroom without also having a low-resistance path for air to enter the bedroom. If there's no easy way for air to get in, it will get sucked up through the walls and floors taking odors and mold spores with it.

Do you have a CO2 monitor? I bought one during Covid to monitor the classroom I teach in. I was surprised by how bad it got: well over 1000 ppm.

When I brought the monitor home, I was surprised all over again by 1000+ ppm levels with just a couple of people and a cat. All the efforts to seal the house for energy efficiency were in direct opposition to advice to lower CO2 levels. The only reasonably effective solution that isn't absurdly expensive is to open the windows a little. All the time. You pay more on heating and cooling, but.. there is no alternative.

The good news is that it gives you a reason to leave the drafty window air conditioners installed all winter long.

And house plants have negligible impact on the levels. Keep plants for the aesthetic, but they aren't going to solve the problem.

Another point in the "leaky" house's favor is that it doesn't get as dusty nearly as fast as one with an open window. Granted the severity of this issue depend on where you live :)

If you have an airtight home, you need a separate ventilator for the reasons you state. New construction building code often requires these.

I've found an in-window fan unit (blowing outward) is very effective at keeping CO2 levels down. Noisy, though.

Before that, overnight CO2 levels exceeded 2,600ppm. Now around 700 or so. Can go lower if I crank the fan up.

Now I have the new problem of poor thermal regulation. My window is directly above my HVAC duct, so cool air goes straight from my air conditioner through the fan and out the window.

I plan to wire up a microcontroller to turn the window fan on and off based on both CO2 levels and A/C activity, but I'm not really happy with the the setup. Too hacky, too much noise, and I need to seal the window screen better to keep bugs out.

I've tried just keeping a window open, with no fan, but my CO2 sensor indicated it wasn't super effective. Plus you have to make sure you close the window when it rains otherwise muggy air fills the bedroom.

It’s typical for bedrooms to have a ACH (Air Changes per Hour) rate of between 0.35 and 1

So the CO2 is removed regularly.

This.

I studied for like 12 years in the biomedical field and I don’t really understand why OP is worried about this.

He could have an unusually tight bedroom. Also, ACH on windless nights will be manyfold lower than on nights with even a 2-MPH breeze.

Doesn't this assume forced air with the fan running for some percentage of the time? It's possible their fan doesn't run enough. It seems many people have that issue at least during some seasons.

Is there evidence that CO2 levels in the air matter to human health? Humans can tolerate a fairly wide range of blood CO2 levels. We have a buffer system in our blood, and we can use our kidneys to pick up the slack when ventilation (via the lungs) is suboptimal.

Depends on your climate/weather, local noise/crime, and construction of your house.

Opening a window can be an option. So can an open bedroom door.

The best option to me is to just leave your air handler fan set to "on" rather than "auto". Of course you need forced air for this. My house is relatively newer and moderately sized. It still seems to keep the whole house CO2 levels like 50-100ppm over outdoor levels. The bonus here if you're looking for "air quality" and nor just CO2 is that your air handler filter should catch many particulates (although most aren't capable of high merv ratings, it still seems to help).

From this post, https://news.ycombinator.com/item?id=40688403 use the excess CO2 for your new hobby in welding

We leave our bedroom door open whilst we sleep which pretty much solves the issue. Leaving the ensuite bathroom door open also helps but not quite as much as the door to the hallway.

In the UK, 'trickle vents' are mandated in new build houses. They're little vents placed above external facing windows and doors that can be opened and somewhat closed. The goal is to allow moisture to escape to prevent mould but they also obviously allow constant airflow. They don't really allow bugs to get in either.

video: https://www.youtube.com/watch?v=6_2SdLEbHcM

In the UK, trickle vents above windows and doors were historically used for passive ventilation to prevent mould by allowing airflow. However, modern building practices and regulations, including stringent air tightness requirements mandated by Building Control, have shifted focus towards more efficient ventilation solutions like Mechanical Ventilation with Heat Recovery (MVHR) systems. While trickle vents are still used in some cases, their prevalence has decreased compared to earlier practices

Possibly, my flat was built 3 years ago and has trickle vents all over.

Cheaper option than MVHR but notably worse. Missed opportunity.

We're in the process of getting windows replaced, and the window company said they are now mandatory on all new and replacement windows. Apparently, it became law in June 2022.

Seems they're mandatory unless you're deliberately fitting leaky windows!

> When windows are replaced, a background ventilator or ‘trickle vent’ should be placed in the new window. This will replace any ventilation you lost, when installing a new window, because your previous window was leaky. If replacing your window did not reduce the amount of useful ventilation, this should be proven, and more ventilation is not needed.

[0] https://www.gov.uk/government/publications/home-user-guide-t...

Yes in retrofit, OP was suggesting that they are mandatory in new build houses and that is not the case. If they were mandatory nobody would be fitting MVHRs as trickle vents make them not function.

Buy a CO2 monitor.

Then keep a window open enough to keep the air fresh.

I find a few centimeters of gap is all that's needed.

Periodically open your windows wide. If the CO2 quickly builds up though, you could perhaps just leave some windows cracked open. Or you should get some proper ventilation installed. If CO2 is high then other pollutants are likely also high.

I have spent a considerable about of time on this question: and it really seems there is no practical way to remove it. You can let it out by ventilating, but that’s about it.

This realization has made me quite anxious about the Keeling curve.

It is possible to remove it. If it weren't, then submarines full of people would not be able to stay submerged for months at a time like US Navy submarines routinely do. Right now it is pretty expensive like you point out, but if millions of people were to start wanting CO2-removal equipment, surely our civilization would find a way to get the price way down.

Mechanical ventilation with heat recovery is a common term (MVHR). Basically a fan but it tries to recover the heat lost in winter. Usually comes as a whole house system but there are some that do single rooms.

I may be missing something (perhaps OP doesn't have windows?) but I open my windows in all the rooms a couple of times per day (in the morning and usually in the afternoon-evening). Isn't that enough?

Yep, I assumed that either you don't have windows, or you can't open them during the night.

VMC in general, for energy usage reasons (meaning opening the windows cause too {cold,hot} air inside, open windows (with eventual mosquito net if needed) when external climate allow that.

Opening the window should help, unless the outside air is highly polluted. One can also install a fan that either vents air out of the room to the outside or draws fresh inwatds.

While not directly related to CO2 removal, for air quality, especially if you live in an urban area, adding an air purifier is very useful.

Sleep with the door open, if opening a window isn't an option. At least you won't be hotboxing yourself with CO2 in a tiny area

When using a CO2 sensor in my room, opening a window was very noticeable as a drop in CO2 on the graph.

You could try to use plants and a growing lamp for them to do photosynthesis.

Have a fresh air intake installed on your HVAC system.

What’s the best device to measure CO2 in a bedroom?

Fan in the doorway

and blowing to the outside

I have one that blows air in from the next room which has an open window.

Depends I think (that is, if you care about cooling down the room too)

If it's closer to the floor, it will be moving colder air, so you may want to point it inwards

Closer to the ceiling and you'll be moving warmer air, so you want it to go out

I want to experience pre-industrial co2 levels

You won't experience a difference

This is a bit of a nonsense question. The CO2 diffuses away rather quickly and there is no reason to worry about this.

You clearly don’t have a CO2 monitor.

Plant

Not sure that’s effective enough to make a practical difference.

Doubly so at night.

It's pretty hard. Recently I saw https://www.youtube.com/watch?v=xWRkzvcb9FQ which has gigantic barrels of algae and direct light input and air being blown through them and still the scope of what you need to make a serious dent in your CO2 output is pretty impressive.

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