How do I let in fresh air without wasting energy?

October 26, 2021 10:25 AM

I live in a small flat with several other humans. If we leave the windows closed, CO2 builds up and I get headachy and stupid(*). If we open the windows, cold air comes in and we waste energy heating it up. What is the most environmentally-friendly way of ventilating a flat?

I'm happy to hear about short-term solutions I can implement right now, or medium term solutions that might require some effort, or even long-term solutions that I can only implement one future day when I remodel a home from scratch.

(*)I have actually tested this with a CO2 monitor and my headaches/loginess really do seem to correlate with higher CO2 levels.

posted by yankeefog to Health & Fitness (21 answers total)

This site from the Department of Energy describes the options. There's no way to avoid losing heat without installing some kind of a heat exchange system.
posted by jkent at 10:29 AM on October 26, 2021

You could get some indoor plants, though it would take a lot of plants to make an appreciable difference (at least, according to this article -- I haven't researched it beyond a quick Google search).
posted by akk2014 at 10:37 AM on October 26, 2021 [2 favorites]

What you would seem to need is a window-mounted air heat exchanger, in which old air is exhausted, fresh air is brought in, and the heat contained in the old air is transferred to the fresh air. This Bionaire unit looks like the cat's pajamas, but appears to be no longer in production. This Purifresh unit is temporarily unavailable but may be coming back. It claims to do energy recovery but the description lacks specifics. As an alternative, this heat/cool window air unit, or something similar, might do the trick. Basically, you would set it to run at low speed and at an equilibrium temperature (using the built in thermostat) so it doesn't particularly heat or cool the room, but would be pulling air out and pushing fresh air in at all times without heating or cooling the room.
posted by beagle at 10:45 AM on October 26, 2021 [1 favorite]

Long-term, a really airtight house, and a mechanical ventilation and heat recovery system.
posted by plonkee at 10:46 AM on October 26, 2021 [1 favorite]

And for your question about remodeling a house from scratch in the future, there are whole-house heat exchangers that would work well. [On edit, what plonkee said.]
posted by beagle at 10:46 AM on October 26, 2021

This is a real safety issue and your landlord should address it. Most heating systems consider this issue; it's rare for a house to be that tight that there's limited air exchange. I had an energy audit, they used a blower to test, and my house is quite snug; this has never been an issue. Short-term - crack a window.

Are you certain it's CO2? It could be CO- Carbon Monoxide from a poorly maintained furnace, which can kill you. Get a CO Monitor immediately, really. Your fire department might have smoke and CO alarms. Faulty furnaces kill people every winter. Carbon dioxide is no picnic, but it's slower, as far as I know.
posted by theora55 at 11:32 AM on October 26, 2021 [25 favorites]

Long-term, what you want is called a Heat Recovery Ventilator. They are quite common where I live in Canada (where winter is a serious business). Ours is a separate unit, interfaced into the return-air duct of our (gas) furnace.
posted by heatherlogan at 11:43 AM on October 26, 2021

As long as the temperature is above about 50-60F, feel free to open a window. People at a body temperature of 100F create a lot of heat, especially if it's more than just you and a partner. Below that, open for like an hour. It's much harder to cool a home than it is to heat it.
posted by The_Vegetables at 11:59 AM on October 26, 2021

Here in Austria, where it can get pretty cold in winter, where few houses or apartments have AC or forced air heating, and most tend to be sealed up tight, it’s common to open all or most windows at least a couple of times a day, for 5-10 minutes each time.

By open, I mean wide open, with doors to most rooms also propped open to facilitate a strong draft with the goal of exchanging as much of the stale air inside the dwelling with fresh air from outside in as short a period of time as possible.

If it’s very cold outside, we’ll throw on a sweater or jacket while the windows are open, but it doesn’t take long after closing the windows for the temperature to climb back up to a comfortable level.

I can’t say for sure, but I have read that this quick exchange of stale air isn’t terribly inefficient w/r/t heating costs. Your walls, floor and furniture store a lot of heat, and won’t cool significantly in such a short period, so it’s mostly just the fresh air that requires warming, and I believe heating cool air in an otherwise warm interior space doesn’t require a terrible amount of energy.
posted by syzygy at 11:59 AM on October 26, 2021 [13 favorites]

I'm mostly in line with what syzyygy said.
I'm in Denmark (dark, cold, wet 6 month/year). We just open the windows.
Stop thinking about it as "wasting energy" and start thinking "keeping healthy".
Best of luck :-)
posted by Thug at 12:28 PM on October 26, 2021 [9 favorites]

You might consider an air purifier with some sort of filter; one source I found mentions using either activated carbon or zeolite filters to specifically target CO2. (Haven't tried this particular thing myself.)
posted by saramour at 1:08 PM on October 26, 2021

or even long-term solutions that I can only implement one future day when I remodel a home from scratch.

I'd go for heated floors, personally. I'm also a window-opener, highly-insulated houses always have me asking "but what about fresh air?!", and I've been focusing more on localized or contact heating, using things like heating blankets, or heating only small spaces (using room dividers, kotatsu-style arrangements, etc). Heated floors would be the logical extension of that.
posted by trig at 1:16 PM on October 26, 2021

A heat recovery ventilator is of course the answer (they can be 90%+ efficient, so you keep 90% of the heat in the air you are exhausting inside the house). HRVs are great and make your indoor environment much more pleasant. It may not be intuitive, but this time of year is actually the worst for ventilation. In the coldest part of winter, there is more natural air leakage through your walls and all the cracks in your home, because there is a greater temperature differential from inside to outside. Around now and in the spring, when temperatures are not that different inside and out but your windows are generally closed, you get a lot less natural ventilation. This can mean that a home that has acceptable natural ventilation in the coldest part of winter might not have enough ventilation in the fall and spring.

But if an HRV isn't possible, there is little you can do other than opening windows or turning on exhaust fans if you have them. It doesn't really matter if you open them a little for a while or a lot for a short period. The amount of heat that escapes is directly proportional to the amount of fresh air that comes in, so you just want to aim for the minimum amount of cold air that makes you happy with the indoor air quality. It probably makes more sense to open windows in the warmer parts of the day and to turn off heating near windows if you're opening them wide open, but otherwise it doesn't really matter what you do.
posted by ssg at 1:24 PM on October 26, 2021 [1 favorite]

This is something I have commented about many times here on ask and on the main site: everyone should cross-ventilate their home for at least ten minutes a day every day, year round. It is not only important for your health, it is very important for your building's health. Don't even begin to think of the relatively small energy cost it may or may not cause. A moldy construction is a very expensive problem.
And looking ahead: when we all have carbon zero energy sources, hopefully in a not too far future, the way we talk about healthy buildings and heating/cooling will be a completely different beast. I have carbon zero energy (which you can choose here) and I am really relieved that I can just open the windows at all times all year round.
As serendipidously, we actually had a discussion about this at work today -- at a technical university in a building science department. My young colleague who has all the time to read all the current research agrees that it is a question of time before we all think buildings in a new/old way.
posted by mumimor at 2:30 PM on October 26, 2021 [3 favorites]

Nthing syszygy and thug - that is the recommendation in Switzerland as well.
posted by koahiatamadl at 4:18 PM on October 26, 2021 [1 favorite]

I haven't run the maths on this, but I don't think that the energy required to heat up the air in a house is very significant, most of the heat is stored in physical objects (walls, floor etc.). My German host family had me open my bedroom window every day when I went for a shower in the morning. I admit that initially I was sceptical, but even at -20C, it didn't seem to change the temperature much once the window was shut again.
posted by kjs4 at 4:41 PM on October 26, 2021

Response by poster: Thanks, everybody! Based on your very helpful answers, I have concluded that:

• the short-term solution is to open the windows as needed and not worry about the small amount of energy involved;
• the medium-term solution is to investigate a window-mounted unit;
• the long-term solution is to switch to electrical heat from a sustainable source and also consider a whole-house ventilation system.

Also, just so you don't worry about us: we have a CO monitor as well as a CO2 monitor. The CO monitor was rated highly by Which?(*) magazine so we're confident it's accurate. We don't have a furnace, but our gas boiler is checked and serviced annually, with the most recent gas safety check just a few weeks ago. I take this very seriously, and I'm about as confident as I can be that we do not have a carbon monoxide problem.

(*) For you Americans, that's the UK equivalent of Consumer Reports. Not sure what the equivalent is in other countries.
posted by yankeefog at 2:51 AM on October 27, 2021 [2 favorites]

I remember reading this article a while back and liked that there are German words for opening the windows daily:

Impact ventilation, or Stosslüften, which needs explanation for most people unfamiliar with Germany except for experts in air hygiene, involves widely opening a window in the morning and evening for at least five minutes to allow the air to circulate. Even more efficient is Querlüften, or cross ventilation, whereby all the windows in a house or apartment are opened letting stale air flow out and fresh air come in.

posted by fabius at 5:24 AM on October 27, 2021 [2 favorites]

Whew, thanks for updating.
posted by theora55 at 12:33 PM on October 27, 2021

Finally, an excuse to bring up the neatest thing: Japanese apartment air vents.

Yes, Japanese apartments also have windows, but sometimes you just want a quick venting. Apparently these were mandated by law to help avoid sick building syndrome. Also helpful during a typhoon, where you obviously don’t want to open the windows.

No idea how you’d get them installed outside of Japan, but we thought they were great when staying in 2018.
posted by ec2y at 3:39 PM on October 30, 2021

Oops, upon closer reading I got the typhoon part wrong: close the 換気口 (kankikō) in that case.
posted by ec2y at 3:43 PM on October 30, 2021