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DIY ventilation heat exchanger (2010) (ecorenovator.org)
142 points by walterbell 9 months ago | hide | past | favorite | 99 comments



For winter WFH in a rental, there aren't many commercial options for energy efficient ventilation to keep CO2 at reasonable levels for cognition. DIY HRV with coroplast can be customized for small spaces.

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 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.

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.
So dozens of studies by disinterested parties are wrong, yet the studies by a single interested party is right? If you really have good standing for believing this, I would like some more information. I am considering installing such a system.

Thank you.


1. It’s not a single interested party. What I called military studies were by at least 3 completely separate groups. The Navy, the Air Force, and NASA. There have also been studies by the EPA, and “disinterested” academic groups.

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...


Thank you, I will read a bit more l.


Hi. It would be interesting to read these military studies could you provide some citations or links?

Are they published in reputable journals?


If you read the comment thread I linked to, you can find links to most of them.

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).


Reminder that the military held that 4 hours sleep was plenty for a century, and had data to justify it.


As far as I know the army said (and still says) 4 hours is enough short term in the field not routinely.

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.


Quite, and given that we are all humans, we should have a fair idea from personal experience.


Yeah, I’m usually a very scientific man deferring to studies over personal feelings, but I can literally feel the mental fog clear up by the minute if I open a window on the afternoon in my study/work room after forgetting to do so in the morning.

Hell, if I have a day where I am quasi-forced to spend most of it in musty rooms, I get a guaranteed headache.


There are so many things other than CO2 that can affect your feeling of mental fog. In addition the placebo effect is very very real.

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.


Here in The Netherlands for classrooms we have regulations that aim for a classroom to stay below 950ppm CO2. I think almost every high school kid can remember how drowsy a full, poorly ventilated classroom would get.

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.


Stuffiness is mostly a function of humidity.

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?


What's the typical temperature change associated with opening the window?


I had some more time today, just with a cursory google I could find at least 5 studies that show cognitive decline with rising CO2 (and PM2.5, but that’s another discussion).

So yeah, my gut feeling was right. It’s probably good to not consider one negative study disproving a body of positive studies.


There’s not one negative study, there’s many studies over many years.

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...


The human brain is a hell of a drug.

This is why we do randomized, placebo controlled studies.

Saying “you can feel the mental fog clear” is likely a placebo effect.


As said elsewhere, there’s at least 5 studies that prove what I felt.


Studies that go against more rigorous studies done over the past 50 years.


In a rental where you can't cut through the walls introduces challenges. I wonder if anyone has created a DIY window unit that incorporates a "ZEHNDER COMFOAIR 70" which is a small contained HRV unit that has c02 detection and can ramp fan speed to match co2.

https://www.smallplanetsupply.com/small-planet-blog/2020/6/6...


Bionaire Everfresh (circa 2005) $150 window HRV was discontinued, https://www.youtube.com/watch?v=OcVowAsYVI4

$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


you can hookup the panasonic unit to a window manifold for a portable AC and put some insulation board over that. that is the only reasonable solution I have found for the states


I've had good success adding various utility and ventilation feed throughs in rentals with sliding glass door inserts, including large ducts for various HVAC experiments.

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.


Are those Aliexpress ceramic heat exchangers as good as the originals? It would be a shame to install that entire system just to have an ineffective heat exchanger component.


You can 3d print one actually, it works pretty well. The only problem is the alumina has a higher volumetric heat capacity, of about 3.5, whereas PLA has 2.25. Which is pretty high. The thermal conductivity isn't a big deal, the main impediment to heat flow is in the air channel, not the solid material, plastic or ceramic. I commented elsewhere in this thread on how to print your own exchanger relatively easily. You can also get your own fans, so this begs the question of why you would pay $600 for them! Also you need a pair of them, you have to have two of those modules operating, one blowing air in and the other out at any given moment. Then they reverse direction. 70 seconds is a bit long for reversal time, it's better to make it shorter if you can. If you are diying one you can do that. If you try to mod an aliexpress one it's only going to cost more and probably not be as good to be honest, because you can't optimize it. Not hackable!


They vary considerably, the aliexpress ones are about as good, but none of them are very good, they give more like 70% efficiency at the medium flow rate, overall. Not what they claim. They do work though.


This dude built their own and got bad mold problems https://youtu.be/LiptsaKmq80?feature=shared


Ive been deep down the HVAC rabbit hole. You really need 3 separate systems to do it right:

- ERV - Dehumidifier - HVAC

And basically no regular installer does it right. Lots of efficiency nerds over at Green Home Builders, subscription required.


Should throw in a UV bath as well - to kill any airborne spores/other critters.


It’s been a long time since I looked into it, but IIRC the exposure time doesn’t make sense in a duct. RealSexyCyborg talked a lot about this during the pandemic. Though that analysis was limited to the Covid virus.

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.


It’s probably a win if it just keeps anything from growing in the exchanger.

Condensation and all that.


Wow, even just that bit about mold being able to colonize a dehumidifier without its own MERV/HEPA filter is an important and hard to realize insight.


It seems that the problems he had were in summer. There's many climates where HRVs are only really needed in winter - in which case those problems wouldn't happen.

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.


Absolutely, there are plenty of climates where HRV (aka MVHR) is quite sufficient. In inland areas (most of the US, and a fair chunk of Europe), the temperature difference between summer and winter is enough that a HRV is unlikely to be sufficient. Somewhere like the UK or somewhere near the equator and high altitude, a HRV system probably will be absolutely fine.

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.


In my climate the problem is air being too dry inside during winter.

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.


Fascinating video. There was a comment on his video asking "Wouldn't this introduce the same amount of humidity/mold as opening a window?"

Curious if anybody has any more insight.


I think the issue is that when the heat exchange occurs one way, the HRV is effectively a condenser.

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


There could be some issue with condensation. The moisture from the indoor air could get trapped and leak.


Exactly my concern... Just buy the Aprilaire 8100 and don't risk mold...


How does that remove the condensation though? That's the cause of the mold problem.

Edit: looks like somehow it moves moisture between the exhaust/intake somehow... not just heat.


What does that system cost? There is no pricing information on the website.


Interesting subject tackling two important home issues at the same time : ventilation and heat exchange. It seems like a way to get the best of both worlds. However, I do not know why this forum is relevant or interesting. DIY solutions are easily doable but is it really surprising ? I am sure people have done this with old fridge parts, as someone suggest in the last post. Yes, DIY can be cheaper but reliability is key for such systems. Size can also be key.


Yes, DIY can be cheaper but reliability is key for such systems.

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.


Reacting to your first point: As far as I know (western europe), heatpumps mostly are used to heat water to then heat your house, and renovating generally means reducing breathability of the house itself, which, combined with the temp difference, means condensation/mold problems. You then have to correct that by opening windows intelligently or installing air ventilation, and then a heat exchanger to not lose too much heat. Like you say, it seems to be a no-brainer then to instead combine heat pump _and_ ventilation, so that the ventilation itself works with a heat pump.

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…


It makes a lot of sense to feed your ventilation output to a heatpump input for energy recovery with an air-source heatpump.

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.


> if all the heating were done using ventilation, you might have to live in a very windy house

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...


There's also significant engineering calculations and guidelines/rules about the expected volume of air that is expected to enter/exit vents vs their size and grille restrictions. This is more to prevent loud airflow, but there's also charts for air distribution for room sizes.

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.


> renovating generally means reducing breathability of the house itself, which, combined with the temp difference, means condensation/mold problems

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


There was a period where I considered DIY for such machines impractical. I now think that to diy all the way through is impractical, the design process is too laborious even for those suitably skilled. However once the design is worked out it is actually pretty practical, with a 3d printer. You definitely have to own the printer, they charge way too much to have things printed at cloud services for some reason.

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.


DIY is practice at learning new things and implementing what you've learned. It's a very good thing in that sense. Also, a healthy DIY (or maker, if you prefer) community is good as a check on commercial offerings. If businesses try to overcharge for something or don't offer it on acceptable terms, and you have a great need, having a 2nd way is important.

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.


Commercial units coupled with a heat pump rival ground source heat pumps at a massive 4 - 5 COP. They even heat water and can take in solar heat piping.

https://www.nilan.fi/tuotteet/poistoilmalampopumput/vgu/vgu-...


Love the idea. Curious if there's any maximal theoretical efficiency. It seems to me that if you had a wide pipe with a narrow pipe inside (each going different directions) that you could theoretically have a system that transfers heat with no loss.


You can in theory extend the length of a counter flow heat exchanger to get whatever efficiency you want. In practice, such a long heat exchanger would transfer significant amounts of heat to/from its environment. So then you are limited by how well you can insulate things.


This is approximately correct, except for one big problem with counterflow exchangers, which is frost. Even the membrane types still frost up. What you really want is a regenerator, either a thermal wheel or a fixed matrix regenerator, if you really want maximal efficiency. And it should be what they call the total sorption type, so it has some adsorbent in it. Even then the latent efficiency could be improved with a better adsorbent, the industry standard is zeolite 3A. It's good because it lasts. Silica gel becomes slowly inactive as it adsorbs other stuff that clogs the adsorptions sites basically.

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.


Dang. Read a comment in this discussion pointing out that an always-on ERV would work well in bathroom / laundry. I just installed a Panasonic ERV for a whole (~500sq ft) space and a Panasonic whisperquiet vent fan turned down to about the same CFM as the ERV in the bathroom as well. Missed opportunity to simplify, consolidate, and make more efficient the necessary moisture control in the bath by solely having the ERV there.

GOLogic homes[1], IIRC, use a ducted ERV with return from bath, laundry, and kitchen, and supply to bedrooms.

1 https://www.gologic.us/




See also also https://youtu.be/w5sR-0oX1dg?t=184

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.

https://www.youtube.com/watch?v=LiptsaKmq80


heat exchangers are pretty important. does anyone have a summary of the high points in this thread? https://ecorenovator.org/forum/showpost.php?p=54888&postcoun... seems to be kostas's summary of their completed build, documented in more detail at https://diyvmc.wordpress.com/ and with an overview video at https://youtu.be/phIWQD5LAcA

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?


I suppose that heat exchanger ventilation makes most sense at winter time. Not everybody lives in San Diego, a lot of people live in New York, Boston, Chicago, Seattle, etc, where a winter is a thing.


You can understand quantitatively how this goes for different areas using Heating degree days and Cooling degree days, factoring in latent heat in both cases, ideally. There are calculators for this. I made a spreadsheet on my website www.openerv.ca that shows some of how to reckon this, there are links in there to get data.

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.


thanks, this is fantastic information!


Also during hot summer when outside temperature is hotter than inside - you don't want ventilation to bring in hot air.


i think heat exchanger ventilation makes roughly the same amount of sense whenever you're spending costly energy to change the temperature of your space, doesn't it? it works just as well to keep summer heat out as to keep winter heat in, and staying cool in summer can be just as critical as staying warm in winter, depending on where you live

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?


The temperature difference most home heaters work on is much higher than the one most home coolers work on.

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.


hmm, are you saying that the regenerator (or recuperator) has lower efficiency at lower ΔT? or lower efficiency when it's more a ΔH than a ΔT? because if that 5° of cooling and dehumidification costs you the same US$100 a month as the 30° of heating, saving 80% of it will still be US$80 a month


Electricity here is €0,30-0,40/kWh after tax. (Netherlands) (For fixed contact, dynamic contacts have bigger variation). That makes it way more interesting to save on energy. Although getting solar panels is still the fastest investment. Heat pumps typically have 10-15years payback time, especially now they are insanely overpriced due to Ukrainian-Russian war and climate goals increasing demand and lowering supply.


so when electricity costs drop by 15–20× the payback time will be 150–300 years?


Winter has much less sunlight, so powering a heat pump via solar panels + moderate battery becomes much harder. It's no longer mostly free.


  >why is everybody using recuperators when a regenerator would be mechanically much simpler?
They exist, search for "ductless HRV."

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.


I've just installed a pair of these standalone units on opposite sides of a mostly open-plan 40m² ground floor. They can run in intake/exhaust/cycle modes, and be paired as primary/secondary for contraflow.

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
Not sure how much good regeneration is doing[0], but it sounds like it put a band-aid on your problem of inadequate make-up air for the vent! :D

Do these standalone units recover heat only (HRV) or heat and humidity (ERV)?

[0] https://news.ycombinator.com/item?id=38743885


I have no external heat recovery system, so yes it's exactly zero while I'm cooking - but a reasonable way to maintain air quality in an open plan, one of my goals.

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).


They all are capable of doing so under some circumstances with limited efficiency, because as the air exits the house and cools down at some point water condenses out of the air onto the regenerator media. Then it evaps again when flow reverses.

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.


Interesting. With EWI and internal insulation the telescopic duct is nearly maxed out as the total wall thickness is close to 500mm, but 50% of that is empty space. I though I might be able to get a secondary heat mass into that in future.


I don't like the telescopic ducts because they can leak into the wall cavity. There is nearly no point in adding more mass. It does increase efficiency but the typical regenerators are alumina which already have a lot of thermal mass. The problem is their surface area is too low. I made a simulator in python to evaluate various designs, and the efficiency of them is pretty poor. The main reason the fan reversal time is so long is the fans suck, they take a long time to accelerate, which affects average flow rate, the shorter the period the more time it takes to accelerate.

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.


i think the solution to the viscous-drag problem is a fractal regenerator similar to a mammalian circulatory system, in which a branching tree of "artery" air passages feeds a set of "capillaries" distributed over a surface of high hausdorff-besicovitch dimension which separates the "arteries" from a second branching tree of "vein" air passages; this squares the circle of low air resistance (because the capillaries are short) with high heat flux (because the total surface area of the capillaries is large). if you have two counterflowing sets of capillaries instead of one, you have a rete mirabile recuperator

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


Fluid dynamics is indeed a complex subject, I found this on my travels, it seems to be one of the more approachable and comprehensive guides: https://www.mcgillairflow.com/pdf/productlit/ductSysDesign_g...

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.


Interesting. This is in concordance with my own thoughts.

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


the idea with the adsorbent is that you can get humidity exchange even when the ΔT isn't low enough to get actual condensation?


Yes, that's correct.


thanks!

don't you need at least two fans and two chambers of heat storage material?


That's how the wireless kind works. One unit blows in while a second identical unit (theoretically located across the house) blows out, and they switch roughly every minute.

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] 
So these regenerative-type HRVs alternate the flow direction of the air, using the packing material as a thermal "battery" to warm the incoming fresh air from the outgoing exhaust air.

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!

[0] https://www.thermopedia.com/content/1087/

[1] https://www.youtube.com/watch?v=CnLwJxCbxfM


this is great information, thank you!

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


commercial systems often use rotary regenerators


indeed!


I would absolutely love to simply have a system that moved air around the house. If you have a house that is 3 stories (one of them an underground basement), you could probably get away with a heat exchanger on the ground floor and simply running the circulation either up or down depending on the season.

Yet I have never seen a system in a house that just moves air between different levels of a house.


I'm the OpenERV guy. It's been a long road and it's not very open source any more I'll admit, however I did publish a bit Wikifactory article with all the cad etc. for a quite efficient window mount unit. I now have the know how to boost the flow through that thing by 2.5x or so so it could actually be a viable product/machine. However it's not a good business proposition.

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.


Thanks for posting with tech details and biz context!

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.


Yes, I agree :). I don't know if you saw my proposal to Kanro, an anti-pandemic charity, to develop a fan for a superior CR box. I think it will be really good if they chose to fund it, it bothers me that they take several months to approve a proposal though. I see no reason for such waste, this is an emergency...


Didn't see your proposal, will look that up.

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.


Very cool project. Thank you for sharing your work.

  > Also... useful to regulate flow, any extra flow in either direction hurts efficiency.
I think this factor is what ultimately tips the balance in favor of a centralized "smart" auto-calibrating unit[0] (using simple and reliable pressure sensors) over distributed ductless. However centralized units do have other advantages.[1][2]

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

[1] https://news.ycombinator.com/item?id=38743885

[2] https://www.youtube.com/watch?v=TChYcSSNXrI


I can actually get small thermistor based hot wire anemometers for about a dollar, so I can just use those with a PID control loop to control fan speed. Then the flow is quite well regulated. However high winds will overwhelm the fan, a problem you don't have so much with centralized units, however that's because the fans are so powerful in centralized units. This implies quite high electrical energy consumption and also noise production. The noise production is not as bad because the fan is in your basement or whatever, but it's still serious and not as good for the most part as a well designed, flow compensated decentralized unit, even for the flow vs noise ratio. I added an automatic valve which only costs about $30, which closes in a storm. So as long as most of the time the wind isn't crazy everything works pretty good with the decentralized approach.


BTW don't know if I said this, but there are some reversible fans on digikey that should work. I was going to open them up and paint them with dielectric grease for water resistance. I ordered the 136 mm diameter one, they are pretty expensive and we will see how quiet they are.


BTW that regenerator did the 45 cfm at 3.25 mm H2O of air pressure.


Shouldn't it also recapture moisture as well?


Wtf are the Ads that replace the images attached to the forum posts?


Why is HN obsessed with air quality?




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