DIY ventilation heat exchanger (2010)
ecorenovator.org
ecorenovator.org
For those with more room, an exhaust duct can transfer heat to cold intake air, https://www.loudawson.com/17884/how-to-build-air-cross-flow-.... Commercial units start around $1K+, https://www.sylvane.com/search/results/?keywords=hrv.
One can also combine a DIY enclosure with commercial parts, e.g. crossflow, https://www.aliexpress.us/item/3256801499168685.html or ductless, https://www.aliexpress.us/item/3256805773352155.html.
ERV pros/cons and review of $500 Panasonic FV-04VE1 ventilation fan: https://doctorpapadopoulos.com/in-depth-advantages-and-disad... & https://www.amazon.com/Panasonic-FV-04VE1-WhisperComfortTM-V....
The CO2 cognition studies show huge results. It’s virtual impossible that none of the military CO2 studies would have picked this up with much higher CO2 levels.
Also navies the world over have been running submarines for decades with astronomically high CO2 levels with no reported cognitive deficits.
Whatever these recent CO2 cognitive decline studies are measuring it’s not coming from CO2.
One of the commonly citied cognitive decline studies shows a 50% reduction in cognitive ability at 1,400ppm. Submarines routinely operate at much higher levels and there was a study done where they exposed subjects to 40,000ppm for 2 weeks with no measurable cognitive decline.
Here’s a good summary from an old HN comment https://news.ycombinator.com/item?id=19863319
> The CO2 cognition studies are almost definitely wrong. The military has done numerous studies over the decades with much much higher CO2 levels with none of the reported cognitive results reported more recently.
Thank you.
2. There aren’t dozens of positive studies. There are a handful by 1 or 2 research groups.
There was a meta analysis in 2020 looking at only the academic literature (so excluding military studies). It found that for every positive result there was at least 1 negative one.
Here’s a good walkthrough of that paper.
https://www.construction-physics.com/p/we-need-more-research...
Are they published in reputable journals?
The real kicker for me is how absurdly high the effect is in the recent studies. You should basically see A students failing tests when the teacher closes the window.
You should see huge swings in standardized test scores based on building ventilation.
Companies with good ventilation, should absolutely dominate their markets.
And nuclear submariners operating at 10,000 ppm should basically be non functioning if there is any kind of dose dependent effect (which the studies in question say there is).
If you’re saying they suggested soldiers only need 4 hours of sleep long term, do you have any citations for this? Links to the studies they used to justify it.
Because you can read the studies on CO2 and judge the methodology yourself.
Hell, if I have a day where I am quasi-forced to spend most of it in musty rooms, I get a guaranteed headache.
ISS astronauts are routinely living in a 5000ppm environment. If the size of the effects of these studies were real, they’d be noticeable.
The most commonly cited study shows that 1400ppm cuts basic decision making ability by 25% and complex decision making ability by 50%. The study also shows a dose dependent response.
So how are people routinely operating at nearly 5 times that level for weeks at a time with no noticeable decline?
Results this large should by very easy to detect by simply measuring the CO2 levels at multiple SAT test taking sites and comparing scores.
Even without looking, an effect this large is the kind of thing that would jump out at the College Board or any other standardized test creator anytime they’re looking through their data to normalize scores or detect cheating.
Like I said I’m usually a scientific man but this is such a basic thing that anyone with any daily life experience would immediately recognize this 4000ppm (or whatever) as wrong.
Humans are also quite sensitive to noticing when a room is “stuffy”, so, yeah.
The problem with these casual discussions about CO2 levels is they don’t control for other factors. Opening a window definitely wakes you up, but is it the cold breeze or the CO2?
So yeah, my gut feeling was right. It’s probably good to not consider one negative study disproving a body of positive studies.
Here’s a good walkthrough of a meta study attempting to figure out what’s going on.
https://www.construction-physics.com/p/we-need-more-research...
This is why we do randomized, placebo controlled studies.
Saying “you can feel the mental fog clear” is likely a placebo effect.
https://www.smallplanetsupply.com/small-planet-blog/2020/6/6...
$300 option from Alibaba, http://mistersustainable.blogspot.com/2021/08/another-mini-e...
$700+ options, https://old.reddit.com/r/hvacadvice/comments/wdl0mn/do_windo...
OpenERV design (writeup pending), https://x.com/open_erv
I start with a board and cut/slot it so it fits neatly into the top and bottom tracks, then I move the latch receiver to the board, zip a few screws through the board where the holes won't be noticed.
The glass door opening becomes a bit narrower, and I can cut as many holes as I want in that little bit of wall.
If it's painted white it usually ends up looking better than most of the corporate maintenance work on the building.
- ERV - Dehumidifier - HVAC
And basically no regular installer does it right. Lots of efficiency nerds over at Green Home Builders, subscription required.
Also to watch for: Some solutions produce irritants like ozone. Not to say they aren’t appropriate, just that they don’t come risk free.
It would be a good question for that forum for sure.
Condensation and all that.
Basically, as long as the outside air is drier than the inside air, an HRV will work well. The moment the outside air contains more moisture, ventilating will just drive up the humidity of your space - causing many issues.
In the UK, the outside air dew point in the depths of winter is likely to be between -5 and 15 °C, and in summer likely between 10 and 25 °C. That might be a little dry at the driest, and a little sticky on a couple of days in the summer, but it's perfectly reasonable to just bring that air into the house without messing with the humidity. Most UK houses are too humid and don't have enough ventilation.
However, as soon as you start wanting to cool a house because the outside air is too hot, then that outside air will basically have 100% relative humidity when it is cooled to your house temperature, and it will probably have condensed out a load of its moisture onto whatever was cooling it, so you absolutely do need to mess with the humidity.
Outside air is normal humidity 50-80% RH at -10C. Inside it will be 20% and way too dry.
Ventilation, heat recovery and moisture recovery would be ideal.
During summer we rarely cook with and AC and windows are open most of the time.
Curious if anybody has any more insight.
Moisture builds on the heat exchange surfaces of the heat exchanger, particulates, spores, etc. in the air deposit nutrients and viable life in the crud, it thrives and multiples right into the incoming airstream for your residence.
When the window is opened the air mixes without a surface for condensation and there's much less immediate deposition so there's not an enriched medium to sustain accelerated growth entirely in the air stream, but it still probably occurs to some extent in a more diffuse manner that produces a significantly lower ambient spore/toxin/particulate load
Edit: looks like somehow it moves moisture between the exhaust/intake somehow... not just heat.
If you know what you're doing (not hard to these days with so much information online), and depending on where you live, DIY HVAC can be far more reliable than getting a "licensed professional" who will attempt to overcharge for mediocre work or even scam you.
The only counter-argument I have is that if all the heating were done using ventilation, you might have to live in a very windy house, but I feel like entirely decoupling the two is not the best solution to that problem…
But otherwise I think there is a lot of value in other forms of heating. Specifically in radiative heating. Floor heating is great for that, but other water-based systems are also possible.
Not an issue at all. A properly installed ceiling vent will blow air along the ceiling, where Bernoulli's principle makes the fast moving air stick to the ceiling until the speed drops enough for the higher density to make it sink gently.
https://blog.swegon.com/hs-fs/hubfs/Bild4-1.png?width=1100&n...
Now, most HVAC contractors go by gut feel/rules of thumb, but if you want to pay for it, a well designed system would never be 'windy' or noisy.
Look up Passivehouse standard, modern construction can make extremely effocient buildings, but it relies on the building being nesrly airtight, so it must have ventillation.
'Natural' ventillation naturally steals your heat, it's incompatiable with efficiency
I have pubished my STL files for an earlier version of the TW4 energy recovery ventilator. It's totally practical to make your own with a printer, but it would take you a few days of labor time for sure to get everything working nicely.
Reliability is very much on offer even with diy. In reality I have looked at many commercial units and they have serious reliability problems for long term like 5 years or more, and we should be planning for 50 years wherever building tech is concerned, imo.
Making your own HRV may or may not be worthwhile for everyone who does so, but more people learning how to make one is doubtless a good thing.
https://www.nilan.fi/tuotteet/poistoilmalampopumput/vgu/vgu-...
But the adsorbent only starts adsorbing at about 60% RH. So in some cases that actually leads to efficiency loss, compared to some kind of ideal adsorbent that adsorbed at any differential.
GOLogic homes[1], IIRC, use a ducted ERV with return from bath, laundry, and kitchen, and supply to bedrooms.
and the goog has a few more. https://www.google.com/search?q=DIY%2Brecuperator&rlz=1C1CHB...
In this one the heat exchanger construction is closer to OP, with coroplast sheets glued together. However in a follow-up video, he says this was a bad idea for his humid climate.
i have a couple of big unknowns about ventilation heat exchangers for houses, and i'm wondering if anybody knows the answers to these questions:
- why is everybody using recuperators when a regenerator would be mechanically much simpler? i'd understand if preventing mass flow was important, but in fact this thread says that in some cases they build their recuperators with water-permeable heat exchange membranes in order to facilitate mass flow of water, which is the main mass flow that a regenerator would introduce here. why don't people use regenerators?
- now that the price of solar energy has fallen so much, and in particular solar panels cost half what they did even a year ago, will house ventilation heat exchangers remain economical?
In Austin, texas, I think you got quite a bit more energy and money saved per year from avoiding cooling costs, but in Ottawa, Canada, you save about 100x as much in winter as you ever could in summer.
the main difference is that you can heat up a house (inefficiently) with a nichrome wire or a gas burner, while cooling it requires considerably more elaborate equipment
but if electricity costs 2¢ per kilowatt hour and daytime electricity is free, will the savings of a heat exchanger be enough to pay for the device?
Using a regenerator to get 5°C of cooling back with some 30% of efficiency probably won't be cost-effectice. But if you have to heat by some 30°C, the numbers change a lot.
>why is everybody using recuperators when a regenerator would be mechanically much simpler?
They're simply a small fan that reverses every few minutes and a heat storage material. They also have wireless ones that coordinate the fan direction between them.
Too early to see energy benefits yet, but one thing that /really/ helps already is kicking both units into intake when cooking on the hob with the hood fan extracting. I should probably finish that hood sensor (looks at sensor sitting on desk).
In trickle/night mode the rated performance is "up to 90%" (at 1.2W and 15m³/h in unspecified test conditions). These ones require a 165mm diameter hole, so the heat exchanger cross section is ~2.7 times the 100mm version, and presumably more efficient.
I also installed (refurb, old house) a recuperative MHVR upstairs. This type has two fans for separated permanent intake and exhaust channels. The alternative in my country is knocking 7000mm² holes in the walls (aka passive ventilation). I have to meet various building requirements as a refurb to qualify for energy upgrade grants.
> one thing that /really/ helps already is kicking both units into intake when cooking on the hob with the hood fan extracting
Do these standalone units recover heat only (HRV) or heat and humidity (ERV)?
The house also has new external wall insulation and other upgrades, so it's rather more airtight than before (though not formally tested just yet). Otherwise they run in synchronised cycle mode for heat recovery and air movement.
I'm not well versed in all the subtleties of theory and current technology, but I don't think regenerative and true ERV go together. If condensation forms in the unit on the exhaust cycle I'd guess does contribute a little to the recovered heat?
These units are BSK Zephyrs, with one heat mass component (ceramic honeycomb) and one fan inline in a telescopic duct. To prevent condensation issues they are fitted with a 2° slope outwards, and a humidity sensor with settable thresholds to purge high humidity internal air at full rate (equivalent to about 1.2 air changes/h for me).
However if you want maximal efficiency you want to add some zeolite 3A adsorbent, which you can sort of do yourself I am pretty sure. You just put some through a coffee grinder, put the regenerator in a garbage bag with it and shake it to get the powder inside the regenerator. It's not idea but it'll certainly help. I tried this with a thermal wheel of comparable size with silica gel and measured the result with AHT10 sensors and it worked pretty good actually.
Zeolite lasts longer but you need more. Obviously you gotta try not to get the powder everywhere by getting excess off etc.
Once the thermal mass of the "batch" of air is relatively small compared to the thermal mass of the regenerator, adding more thermal mass doesn't help.
The reason you can't go to town with high surface area is viscous drag. The fans make a lot of noise when they have to work at high speed, which they have to do to produce higher pressures. Fan blade noise increases in proportion to the eigth power of blade velocity. So really really fast. So if you make the holes in the regenerator smaller and the walls thinner to increase surface area, you don't get enough flow.
The usual units really hobble at the best of times. If you calculate how much energy (which increases linearly with flow*efficiency) they save a year, they usually actually cost more in amortized total cost of ownership than they save.
The best thing is to redesign both the fan and regenerator, that's what I did.
There is a nice fan that's produced by a german company that I'm trying to get samples of but they are super slow.
obviously this structure is not feasible to manufacture by conventional processes, but you can 3-d print it
cf. https://dercuano.github.io/notes/capillary-heat-exchanger.ht...
it seems crazy to me that people would use alumina as a regenerative medium for house ventilation; it's a pain to get into the shape you want, which makes it expensive in practice even though it's an abundant natural mineral. it does have an immense virtue as a regenerative medium, which is that it can withstand like 1750°, but if part of your house is over 1000° you have bigger problems to worry about than the efficiency of your hvac system. if you're trying to maximize the thermal mass of a stable solid per dollar, something like unfired clay is probably optimal, though it expands and contracts with humidity; lime mortar (quicklime and sand) avoids that problem. those are low thermal mass per kg; to optimize that, you'd probably want polypropylene instead (or paraffin wax, but it might melt)
unfired clay has significant moisture-adsorbent ability too, which is why it expands and contracts with humidity. muriate of lime is commonly used as both a (deliquescent) desiccant and a flocculant for clay colloids used as ceramic glazes; i'm not sure to what extent clay with muriate of lime in it can function as a solid desiccant
but maybe i'm missing some basic knowledge here, so plausibly these are stupid ideas; wikipedia is no substitute for experience
Aside: paraffin wax is used in actuators, e.g. automatic green house window openers, on account of its impressive expansion. The MVHR I have also uses it for automatic "summer" bypass of the heat recovery path.
The ideal space-filling "core" of a rete mirabile recuperator is (I argue) a checkerboard, with black and white squares flowing in the opposite direction.† This makes all adjacent channels opposite polarity (unlike a honeycomb) for maximum heat transfer, and also reduces internal drag and manufacturing complexity (unlike triangles). Mass manufacturing can be done via extrusion, or by gluing together 90° corrugated sheets. If the core consumes 80% of the total system volume this reduces 3D printing cost.
Trees typically branch 1-to-3, which is probably optimal because evolution. For an 54x54 grid that means 1458 endpoints, or 6.6 branchings. That's one 2-to-1 branching (common on the first "trunk" branch of a tree) followed by six 3-to-1 branchings, for seven total branchings. This is a typical value for a real biological tree, so we're probably moving in the right direction re:biomimicry.
In real biological trees the fluid flow slows as the branches get smaller, so it's probably good to emulate that.
† arguably a honeycomb shell-and-tube is more efficient geometrically, but it's non-symmetric and (more important) the manufacturing and rigidity/robustness is questionable
don't you need at least two fans and two chambers of heat storage material?
This balances the airflow, so you don't get non-regenerated (or more accurately, less regenerated) airflow "somewhere else" like you do if you use a single unit.
"in thermal regenerators, the hot and cold fluids pass through the same channels in the packing, alternately, both fluids washing the same surface area. In recuperators, the hot and cold fluids pass simultaneously through different but adjacent channels."[0]
While they are indeed simpler, my understanding is that they don't really perform well. There's no humidity recovery, the CFM is too low, the cheap units don't do any active pressure balancing, you can't continuously exhaust from bathrooms/kitchens, and the lack of net flow to distribute air throughout the house means you're mostly exhausting the fresh air you just brought in. Not great.
If you're going cheap, skip the regenerator-type and the HRVs altogether and go straight to a jury-rig ERV.[1] Though again, ideally you can run flex duct (or temporarily on the interior, even just dryer hose) to pull exhaust air from the opposite side of the house, and specifically from bathroom/kitchen areas.
However, first you should test your CO2 levels and/or do a blower-door test. Most buildings aren't tight enough to need an ERV anyway!
i'm surprised there's no humidity recovery; i'd think that, with a regenerator, humidity recovery is unavoidable even if you don't want it. maybe you're talking about cases where humidity or ΔT is so low that you don't get any condensation? https://news.ycombinator.com/item?id=38746150 points out that a little desiccant can solve that problem
Yet I have never seen a system in a house that just moves air between different levels of a house.
There is a whole class of energy recovery ventilators called decentralized, ductless or sometimes push-pull which are quite amenable to DIY. All you need is a good reversible fan and the regenerator and a pipe.
I have shared elsewhere how to 3d print a regenerator. I use a 0.3 mm nozzle and it takes about 24 hours to print a regenerator that can do about 45 cfm at about 80 percent sensible efficiency. Basically just take a cylinder in Cura of the desired diameter, and use lines infill, no top layer, no bottom layer. That's basically it. Make the lines as thin as you can and the line spacing as small as you can while getting the desired airflow. About 2.7 mm on center spacing was about right for me and a typical fan with 6.5 mm stall pressure. The operating point was about 3.5 mm h2o and 45 cfm at maximal fan power.
These devices do capture some water vapor during cold weather because the water vapor condenses on the heat exchanger then re-evaporates.
I have tried sprinkling zeolite and silica gel in there to get good latent heat recover as well and it seems to work pretty well. You don't need much, only about 5 grams of silica gel, more for zeolite 3A molecular sieve.
I have built two window mount units. One for myself, the mega sized one, which there are pics of on my twitter (@open_erv) and also one for my friend Alex, whose landlord wouldn't let him use it, so I sold it for the cost of the parts.
It took forever to sell that thing even at the cost of the parts, so clearly window mount units are not a great business proposition. Also for other reasons, mostly people don't think of them as long term propositions so they aren't willing to amortize the cost of manufacturing a good machine over long periods.
There is one called the purifresh, for windows too.
The ductless models for houses include the lunos e2 for about $1800 CAD and only 25 cfm, and the blauberg Vento (there is another related one called Twinfresh), however I don't believe their noise or efficiency claims, and I have investigated.
There are many cheap chinese knockoffs and some of them aren't too bad Holtop makes one. But mostly they are very poor.
Ultimately I would like to make a hackaday or something that explains how to do all this, with a raspberry pi pico etc. Also an anemometer based on thermistors and the hot wire technique is useful to regulate flow, any extra flow in either direction hurts efficiency.
Both Window HRV and Quiet Corsi-Rosenthal MERV13 Air Filter are business projects which deserve attention from those with more money than time. They are compatible with decentralized manufacturing, field testing and new building regulations for indoor air quality.
12 May 2023, https://news.ycombinator.com/item?id=36238958
The US Centers for Disease Control and Prevention has extensively updated its ventilation guidance on helping prevent indoor transmission of the virus that causes Covid-19. The agency had advised people to ventilate indoor air before, but this is the first time a federal agency has set a target – five air changes per hour – for how much rooms and buildings should be ventilated.
Air quality experts cheered the updated recommendations. “It’s a monumental shift. We haven’t had this. We haven’t had health-based ventilation standards,” said Joseph Allen, director of the Harvard Healthy Buildings Program. Allen says that although it’s easy to see the guidance only in the context of Covid-19, it will help with many other airborne hazards like wildfire smoke, allergens and other infectious diseases, such as the flu.
The move comes the day after the US ended its public health emergency for Covid-19.Need to find a wealthy individual who wants their name permanently attached (CR_) to an improved fan design that will 10X or 100X the TAM for interior air filtration.
Perhaps unequal societies with declining state governance will need an auction site to match named philanthropy with engineering projects that make inter-generational contributions to human civilization.
> Also... useful to regulate flow, any extra flow in either direction hurts efficiency.
I wouldn't get too discouraged just because you had difficulty selling an early unit. Marketing and sales is hard. Reaching the right people and then effectively communicating a product's value is both an art and a science.
[0] https://www.youtube.com/watch?v=LGB83pSLMiI