Get the top HN stories in your inbox every day.
danans
pzduniak
>that they are very expensive niche products
My entire "medium sized European suburban house" runs on a $2.5k 400m3/h unit with HEPA filters made in Lithuania - and that was the more expensive model that I can directly control over MODBUS / 0-10V signal (even turning it into a "dumb" unit). Most of the expenses were running the ducts. YMMV
It's just awesome. Every single room has fresh-smelling air and after fine tuning all my heating systems with algos implemented in Home Assistant - I'm getting ~60-100ppm over outdoor CO2, perfectly clean air, temperature within 1C of the set value, on-demand humidity extraction after showers etc. All it needs to be properly overengineered now is a bunch of dampers and per-room CO2/humidity feedback :)
tomashubelbauer
I wish this was as common as having a fridge in the house. The productivity gains from people not being sleepy and tired from shit air would be insane.
tashoecraft
When having my mini splits installed I pushed for an erv system in the bedroom. The installer had only ever done them in commercial units, and he hemmed and hawed about it, but I had two c02 monitors in my room showing it getting to above 2000 whilst sleeping. I've noticed a big improvement in how groggy I feel in the morning.
BizarroLand
If there were good enough ERVs that could be installed by the DIY'er for about the cost of a cheap refrigerator (~$600 or so) then they would be more common.
I get that manufacturing the transfer plates isn't simple or cheap, but other than that one thing they're basically fans, they shouldn't cost that much.
michaelmior
> after fine tuning all my heating systems with algos implemented in Home Assistant
I'd be very interested in hearing the details of this.
pzduniak
I'll definitely prepare a longer write-up when I have everything figure out, but here's a summary:
I have 4 systems:
- Komfovent HRV for ventilation
- NIBE F-series heat pump for floor and water heating
- Vaillant gas boiler that "supports" the heat pump
- Samsung multi-split AC units
HRV - Komfovent uses the same controllers in all of their units, so you get all the communication goodies you'd want - though it took me a long while to figure out that basic features need to be toggled on :) There are existing YAML presets for their C6/C6M controllers on HA forums. The only caveat is that if you want to feed it a virtual thermostat, you need a stuff a device simulating a 10k NTC inside of the ventilator. Otherwise it's just a single Ethernet cable.
Heat pump - I'm not exactly sure if I'm happy with NIBE, but thanks to the community the integration ended up being quite easy. I wasted a bunch of money on their MODBUS40 just to learn that you need to use a certain MODBUS address in the internal bus to make certain registries writeable (eg. thermostat values) - so I took an ESP32 with Ethernet, a galvanically isolated RS485 dongle, a 12V to 5V converter and used https://github.com/elupus/esphome-nibe. The firmware extracts my templated HA sensor's value and feeds it to the heat pump as a virtual thermostat.
Vaillant uses this weird "eBUS" protocol, there's a bunch of cheap PCBs that you can use to connect to it - I'm using https://github.com/danielkucera/esp-arduino-ebus. That's the last system that I haven't touched :)
Samsung ACs use their MIM-B19N modules installed in the outdoor units. There's some magic around enabling remote control, but once you plug their diagnostics device into their indoor units, you can flash all of them at once. I had to mess around with internal NASA addresses to have all the units appear at once.
For indoor sensors I have 3 types:
- AirGradient units measure CO2, tempeature, humidity, PMx etc. - these are mounted at ~150cm and feed the "current house temperature" template.
- I have like 8 Everything Presence One devices, powered by a custom PCB that converts 12V/24V sent over wired alarm cables to the device. They have built-in temperature, humidity and motion sensors. These are mostly installed for motion sensing and their height makes the temperature measurements quite useless.
- Everything else (and most importantly bathrooms) is done using custom ESP32-C3 devices that use SHT31 sensors to measure humidity and LD2412 for movement sensing. Also using the same adapter PCB for powering.
Thermostats are synchronized across all the devices with HA scripts. The HRV specifically uses its own wired temperature sensor to determine if it should enable heat recovery ("free cooling mode"), since its extracted air temp is always a bit lower than room temp (laziness :-)). "Current temperature" template fed to other heaters is derived from multiple room temperatures (currently using an average), with rooms "ignored" if AC is heating there (or was turned on recently). Ventilation has 2 modes set up - 20% and 80% - with the latter toggled by a bathroom humidity threshold.
There are 3 remaining things I want to set up:
- auto switching to gas heating if it's cheaper / the house is running on batteries - so far I've only imported electricity / gas prices into HA and quickly realized that I'm missing a power monitor on the heat pump circuit
- dampening of air ducts to reduce the temp drop when high humidity extraction boost gets triggered
- using more of the HRV range by auto-adjusting fan speed depending on real CO2 values - there's max 2 ppl at the house most of the time, so even at 20% the HRV is quite wasteful
karussell
How is the noise of these systems? And how often/long need they run to give you clean air and humidity extraction. Is there extra noise when it is windy outside? Is it installed inside the windows somehow or do I need to drill through the wall?
sgerenser
Typically 2 holes, one for air intake and one for air outtake are drilled through the wall. They’re often installed when a home is being built or heavily remodeled. They can be installed after the fact (especially if you have an accessible basement or attic) but it might be a bit invasive running ducts where you need them.
danans
> My entire "medium sized European suburban house" runs on a $2.5k 400m3/h unit with HEPA filters made in Lithuania - and that was the more expensive model
That is just the HRV, not the design of the system, the ductwork, and the installation. All those add up. In new construction those costs can be shared with the regular HVAC system design, but in a retrofit its far more expensive
tinco
Anyone know if there's a good way to control 0-10V dampers? I looked for a solution to control 5 dampers but I didn't find anything, so I started to design my own a couple years ago but never finished the project. I'm having a hard time imagining something so common doesn't have a common solution.
pzduniak
If you're OK with large controllers, the cheap Chinese RS485 stuff seems to work perfectly fine. For "digital inputs" I started out with Polish $150 devices, eventually ended up using the cheapest AliExpress listings for some expansions and so far they've been working exactly the same, with the only difference being the quality of the docs. I'd expect the 0-10V modules to be exactly the same.
yrcyrc
Would you have a link/reference?
pzduniak
https://www.komfovent.com/en/products/domekt-r-400-f-c6m-573
I think this is the one I'm using for ~200-ish sqm.
throw0101b
> One problem with ERV/HRV systems right now is that they are very expensive niche products.
Most building codes in US/CA mandate them since about 2015, so I'm not sure how niche they are (at least in new construction).
Depending on the (air) volumes involved, ERVs can be had for under CA$ 2000:
* https://gasexperts.ca/product-category/air-exchangers/lifebr...
* https://bphsales.ca/collections/high-quality-erv-air-exchang...
HRVs for less, but it's probably worth the extra few hundred for better humidity management.
danans
Installation is usually the most expensive part, and can easily send the total price into 5 digits of $, especially in a retrofit, depending on the market.
batushka5
These units are ubiquitus in northernish Europe, as any new/renovated building needs them to reach A/A++ energy effiency. Brands like Komfovent, SystemAir, offering 200 m³/h ducted units for 2000 Euros, with efficiency like:
Outdoors °C -23 -15 -10 -5 0 25 30 35
After unit, °C 12,9 14,5 15,5 16,5 17,5 22,6 23,6 24,6turtlebits
ERVs are not expensive, 1500 USD will get you a decent whole house unit. The installation is the expensive part, which this project doesn't change.
turtlebits
You have it backwards. Counter flow units have lower efficiency than these "regenerative" type ERVs.
The downside of this is that the high efficiency is limited to small spaces (based on the mass of your core), where counter flow units are great for entire homes.
One point often overlooked with counter flow units, is that you can place exhaust ducts in spaces that you want to purposefully remove air, like bathrooms and kitchens, while providing fresh air to places with little air movement, like closets, basements.
Regenerative core ERVs do little for fresh air circulation.
llm_trw
Do you have a source explaining how these work?
Naively allowing the air columns to thermally mix would result in the average of the inside and outside temp. So how does this do better?
IndrekR
The direction of flow is reversed every 30s. The cycle is short enough that due to the thermal mass there is thermal gradient within the heat-exchanger. So it effectively works as counter-flow heat exchanger. Same principle (but continuous flow) is used in rotor ERV: https://en.wikipedia.org/wiki/Heat_recovery_ventilation#Ther...
Heat exchanger there is usually an extruded ceramic grid (ERV) or rolled corrugated aluminum (functions closer to HRV than ERV)
LukeShu
Counter-flow heat exchangers. A parallel-flow heat exchanger would result in the average, as you say; but a counter-flow exchanger means that as the formerly-warm air gets progressively cooler, it is exposed to progressively colder air.
gwd
I've got a counter-flow heat exchanger, but it looks like they're using a different design:
> Each OpenERV TW4 module has a very quiet pair of fans, pointed in opposite directions, and a heat exchanger in a 6 inch pipe, that goes through a wall. The hot, polluted air from inside goes out for 30 seconds, and the heat from it is stored in the heat exchanger.
> Then, the fan reverses direction, moving clean air from outdoors to the indoors. On it's way in, it picks up that heat from the heat exchanger. This type of heat exchanger is called a regenerative heat exchanger, or less commonly, a regenerator. The kind shown in the video is a recuperative type, not regenerative. Recuperative types are what most people think of, consisting of a thin layer of material that separates two gas streams. Regenerative heat exchangers are different. They briefly store the energy while air flows in one direction, then release it when the air flow reverses.
> The OpenERV TW4 modules are made to always work in pairs. One always sucks air while the other blows air, synchronized over WiFi. This should be done, or hot air would be pushed out from the building through the walls during the ingress phase, causing heat loss.
llm_trw
Duh, thank you for reminding me air flows in a duct.
empiricus
the intuition: if the 2 colums flow in the same direction, the final temp is the average. but if the 2 columns flow in the oposite directions it is posible to fully exchange the temperature
kccqzy
What makes outdoor air "fresh" compared to indoor air? You said that the temperature and humidity of indoor air are preserved. So is it just CO2 concentration? Would installing a chemical CO2 scrubber have an effect similar to an ERV system then?
NavinF
CO2 ppm is the main number that research studies look at, but it's also a proxy for the "freshness" of the air. All the air quality metrics are correlated. Eg a lot of cheap CO2 meters measure something else like TVOC and convert it to eCO2 using a lookup table.
CO2 scrubbing would be better than nothing, but it's really expensive and won't improve other metrics like TVOC
runjake
CO2 and chemical off gassing from indoor items. Eg: manufacturing chemicals from furniture, natural gas, cooking fumes, etc.
jonstewart
Planning a renovation of my 1947 rowhome in DC, and I’m really looking forward to adding an ERV.
bluGill
You probably cannot do a renovation the tightens up your house enough to matter. Of course you have not specified what you are doing, it is certainly possible to do that, but it is a major effort that makes the house unlivable for a couple months and costs a lot of money. If you don't do that level of renovation your house will have enough leaks that a ERV will not make any difference in air quality (and even that level doesn't always make the house airtight enough to need an ERV). Making a house airtight is very hard - worth doing because of the energy savings, but not easy.
If you are doing that level of renovation is is probably better to just tear down the house and rebuild. The costs will be similar and there are a lot of other things people demand of a new house layout that cannot be retrofitted in the old shell. Often the law will not allow this and so you are forced to renovate just to keep some now illegal feature that is worth keeping, but otherwise a tear down would be better.
jonstewart
You're right in the main, of course, that it takes a major renovation to make an old home tight enough to be worth it, but wrong in your assumptions:
* It's a rowhouse, so there are two party walls on either side.
* Brick.
* The front and back walls are half the length (15') of the party walls.
* The house is small, a footprint of 450 sq ft.
* The renovation will extend the back roof line (pitched roof, front to back).
* The attic (with extended roof) will be renovated with a bedroom and 3/4 bath, plus storage and mechanical.
* Plumbing will be replaced.
* Gas boiler (with radiators) and existing central A/C will be replaced with heat pump.
* We'll be out of the house for months.
* It's in a historic district so it can't be torn down.
* Fortunately most of the windows have been replaced before the historic preservation office started cracking down on replacements. We have light-blocking hex blinds that insulate them nicely at night.
* I will try to sneak in a new front door that's the same design as the old one, and fix up the jambs and sill.
* Historically DC is still more heat dominated than cooling dominated, but climate change is tilting the balance. It may be hard to fight the stack effect in the winter but in the summer I hope to run the house at positive pressure. It's when the A/C's been going for days that the air in the house seems stale.
* Even though there's very little insulation today in the attic (just cellulose strewn between joists), it's not expensive to heat or cool, because it's such a small house.
* I care more about the ERV as a luxury good than as a cost effective appliance. I'm quite sure the cost of it will be negligible when compared to everything else in the reno (we're doing the kitchen, too).
Although the cost will be insane, I expect we'll come close to breaking even on value added to the property, given the neighborhood we're in.
BizarroLand
Anyone can do a renovation that "tightens up your house enough to matter".
Use a qualified professional. Get multiple inputs.
It will cost money, and more money as you approach perfection, but it is doable.
open_erv2
Hey guys, I am the guy behind the OpenERV company, who designed the TW4 and WM12 ERV units.
I'm sorry I don't have a bunch of units ready to ship out, as the site says it's still in beta, I am to be honest kind of taking my time because I have another project, the big quiet fan, which is actually funded a little better, and thus I've been directing most of my time to that. But I do advance this a bit most days. I have a twitter where I tweet my progress : @open_erv, and also I'm on bluesky.
I have shipped a few units to other engineers who have/will test the units so I can share third party confirmation for any skeptics.
To clarify some of the discussion, it is not a counterflow heat exhanger, it is a regenerative type. https://en.wikipedia.org/wiki/Regenerative_heat_exchanger. I prefer this type because they can recover latent heat more effectively than recouperative (such as counterflow) type, and latent heat is 40-50% of the total energy content of the air, seasonal average in Ottawa or Toronto.
I am hoping to get the machine tested by the PassiveHaus institute to show beyond doubt how good the efficiency is with a third party test. I have no doubt, I have tested it myself, though.
These can theoretically handy any temperature differential, but the TW4 and WM12 are currently made of a polymer that I wouldn't trust in an extremely hot climate combined with direct sunlight. For that reason, I am focussed on cold climate scenarios. I am pretty sure it will not frost up even in extremely cold weather like -30. I used it last year in my window and had no problem, and it did get to like -25 at least iirc.
open_erv2
Well this was fun and thanks for the discussion, everyone. People are surprisingly nice and sensible and positive here! I used to have another account but lost the password. Perhaps I'll be a reader here in the future.
Anyway, I've tried to turn the very temporary influx of interest into something positive and lasting by searching for 2 people who can install and document the install of a pair of TW4 energy recovery ventilators, so anyone who wishes to buy thereafter can know what they are in for on that count. There is another guy Alex who will test flow and efficiency, I've already sent him the stuff.
So we get things tested and verified, and I will continue getting a jump on producing units by running the printers and assembling in between when I am doing the more respectably paid work in my life. The kits are on the back burner because even I am still stabilizing the assembly methodology. I even added a new component just recently, a flow straightener that boosts flow by about 10% while allowing noise to be reduced even further.
So stabilize, verify, produce, and then after that, within a couple months, I sell in a more or less ordinary way to anyone who wants them. I'm sorry it's not in time for the cold weather, but we have to remember ERV is about the big picture and long term. Like the rest of a building, it's an investment, and the machines are made for (very) good return and long lifespan.
I will prepare some WM12 units for those who have asked for them. To be clear I only got 12 emails expressing interest, not an absolute flood, but it's encouraging to know some people "get it" at least. I knew there would only ever be a small trickle of relatively wise people from around the world that appreciate good performance and return on investment. I only need to sell a few pairs per month to make it worthwhile, at the eventual $1300 CAD price tag.
I am open to scaling up production with more efficient production methodologies, but I am actually fairly well acquainted with injection molding, machining and other conventional approaches, and they aren't magic. They would help for sure but they wouldn't radically change the price, or the rate of return on investment, and they also take a lot of investment not just for tooling but also re-testing and re-design. I've also changed the design so, so many times after I thought it was done I am highly wary of being locked in.
DamonHD
Good project. I've added links from my MHRV pages which have quite good traction on search.
Small note: the older single-room unit we have with the fan on the outside can ice up and make horrible noises then stall at a few degrees below zero (here in London UK)... B^>
Also: as the creator of a project called OpenTRV, I cannot but help admire your taste in naming! B^> B^>
open_erv2
That's an amazing project! I'm blessed to be in such company. Seriously, I use open source stuff and I prefer to do business with such relatively wise people. If you want a beta unit, email me and I'll put you at the top of the list!
DamonHD
That's very kind, but I have all the (SR)MHRV units that I can reasonably fit!
FWIW the email address on your site page is bouncing for me.
Please do add my email (in my profile here) to a low-volume mailing/updates list if you have one.
And if my limited experience of bringing an open hardware project to market might be of help, let me know!
undefined
schneems
This is neat! I had some issues with ventilation in a foamed house and the only product that’s not a whole home ERV (which, I didn’t have space or ducting) was the Panasonic whispercomfort which actually has some requirements that were hard to meet (minimum duct length) and the overall efficiency isn’t that great. We put in two and have fresh air intake on our HVAC units. Still we’ve taken to running at least one bathroom or laundry fan non-stop.
I’m excited for more competition in this space. Beyond the hardware I’ve found that HVAC installers are way behind the curve on air quality. I hope education and awareness increases in the industry.
benj111
>To clarify some of the discussion, it is not a counterflow heat exchanger, it is a regenerative type.
Can someone expand of this?
intuition tells me that a regenerative design can be no better than 50% efficient, and would be worse at recovering latent heat
ljosa
I think it means that it's like [Lunos](https://www.lunos.de/en/for-heat-recovery). The unit alternates between exhaust and intake every couple of minutes. The air being exhausted heats up a core, which in the next cycle warms the air from the outside. Lunos e2 is advertised to recover 90% of heat and 20–30% of humidity.
benj111
yes i get that, but say you have an indoor temp of 30c and an outdoor temp of 0c. the average of this heat exchanger is going to be 15c. so on average youre only cooling the exhaust down to 15, and heating up the intake to 15c.
a counter flow heat exchanger can get the temperature higher than the average because 30c exhaust is meeting partially warmed intake, and 0c intake is meeting partially cooled exhaust.
Unless theres a phase change???
gregwebs
The ductless design seems great for smaller units or open spaces. Although for smaller units you want to get building owners to install these. Have you seen interest from them or are you expecting a company to take up this design and sell it to them?
I would consider installing this in my open finished attic even though I already have a whole house ERV. The problem with a whole house ERV, particularly in a multi story house is that it doesn’t necessarily produce a lot of fresh air where you are in the house.
open_erv2
I'm planning to just putter along selling units, making about 50% of my living like that, and if a company comes along and wants to buy the design/company that's good. If not, I get some stuff done, earn a living. This isn't a get rich quick scheme, it's an honest living type stuff.
nabakin
Fyi the mobile image swipe mechanic on the linked page is inverted. Swiping left takes you to the image on the left, instead of the image on the right and vice versa.
Abekkus
Is there a link to the big quiet fan project?
open_erv2
There is some stuff in the other projects page on the website, that project went really well too, I spend most of my time on that these days, the ERV is a longer term thing, the fan has a finish line but in a way is more epic/important because it's about halting airborne disease transmission. It works extremely well, about 1900 CFM through merv 13 filters (10x 20 by 25 filtrete) at less than 41 dBa.
wakeforce
I'd love to build this. I have access to a 3d printer, use Python, and have some electronics experience. I live in a northern climate and have been eyeing ERV systems for a while. Basically, I'm the perfect target for this.
However, reading the docs, they seem written more to discourage any kind of DIY attempt by saying things A, B or C are difficult, than actually explaining how to do them correctly. I'd love to contribute to the project, but it feels like it's not set-up to foster community contribution.
If I'm mistaken, I'd love to donate some of my time on this!
turtlebits
Unfortunately the recovery core, which is the interesting part of an ERV, is not included in the 3d stl's.
IMO, this feels like a more marketing project than anything open. ERVs are already very simple (a recovery core + blowers/fans). Commercial units last an extremely long time (some with 10 year warranties) and have comprehensive parts availability.
Also a long term window install is a bit janky and is likely to lose out on efficiency due to glass being a poor insulator.
NavinF
I'm pretty sure stls/regen.stl is the recovery core you're taking about tho I can't open it in a online stl viewer: https://drive.google.com/drive/folders/1py2YwmwBEcvmdw18SKwx...
In the photos it seems to be a bunch of nested single-perimeter cylinders that are joined at a few points to maintain spacing. Easy enough to model, but I agree the documentation is horrible and there's no way to contribute.
Commercial units are not comparable because they're way more expensive despite being so simple
bittercynic
Prusaslicer opens it, and it is the right size and shape to be the core, but there's no internal geometry in that STL, at least not how Prusaslicer renders it.
I'd be interested in seeing a diagram of how the air flows through it, if such a thing is available.
Edited to add: There are instructions in the WM12 manual to use your infill settings to make the old version of the core. Page 15 of the manual states there is a python script in the source files to generate the new core, but it only works for their particular printer. I wasn't able to find the script.
turtlebits
You can get a commercial unit for under $400 USD. Search for "Pioneer 50 ERV", which claims 97% efficiency or look on aliexpress for units under $200.
That said, I find these "regenerative" heat exchangers too limiting as they generally only work for a single room/space.
open_erv2
There is the tw4, which is made to be put in a wall, and there is the WM12, which goes in the window. The main focus is the TW4. There are instructions in the manual for making an ERV core. It is not trivial.
fudged71
I understand that the addition of desiccant material is the core aspect of what makes this an ERV. I don't actually see any clear explanation of how the desiccant material is added to the printed part. While the creator (open_erv2) mentions that sorbent/desiccant can be used to handle moisture ("If you have sorbent, it gets grabbed out of the air before it can condense"), they don't specify how it's incorporated into the design.
Is it added mid-print? After printing? Is it difficult to add?
NavinF
Interesting. I'll paste it below.
Note on printing the regenerator/heat exchanger:
The latest and greatest heat exchanger is produced directly with python script generated gcode specific to the printer I use and cannot be practically produced diy, unfortunately. However the old model can be, and the STL is included for that, in the source repository. To do this, simply use Cura, load th STL in, put it in the center of the build plate, and set it to do “lines” infill with about 2.5 mm on center (between centers of the lines) spacing and 0.45 mm width, no top layer and no bottom layer (set them to zero). Check the preview and it should show you a structure which is much like grid infill, parallel channels which are square in cross section, with the outer wall. Tape can be applied over the nubs on the side to fit in an oversized pipe, or they can be sanded if the pipe is too small. You could also use grid infill, but the roads tend to have problems where they intersect. When the nozzle goes over one road, it wipes the plastic off, and not enough is deposited on the lee side. I don’t know how to solve this in Cura without using lines infill. If you could make it so the nozzle went in alternate directions each layer that would probably solve it well enough.
Aurornis
I expected a community open source project from the title, but reading the docs led me to the same conclusion: The website is about convincing you to buy one while discouraging you from attempting to build one.
It looks like a fun project. I don’t want to discount what has been designed and built. It is confusing to start reading about the project and discover that it’s more of a business than a community project while simultaneously being unavailable for purchase. The person who built it commented on HN that they’re focused on a 3rd different fan project right now, which brings the future of this project into question.
It would be great if a community effort could fork this project and work on making it easier to DIY so the community could push it forward.
EDIT: After exploring the files I’m not sure I’d even call this open source. I either can’t find some key files or they’re deliberately excluded. True open source projects would also include the CAD source, not only .STLs so others could adapt and modify the source. I think the open angle on this project is more marketing than substance.
open_erv2
The step files are also there, which is the best common denominator for CAD files. Again, it's open source for the purpose of maintenance and repair, not cloning, and frankly earlier on I did make it more community oriented and nobody ever contributed even a little bit, so I just gave up on that idea.
The most likely scenario for longer term is that people may submit minor patches or suggestions, which I roll into the hardware or firmware. In reality, hardware is not like software. You can't make changes easily. Some wizards may take it upon themselves to spruce up the firmware with fancy features and release something, which anyone is free to do. There would then be multiple compatible versions of the firmware, one which I curate for reliability with minimal features, and others which others can provide. Same as for 3d printer firmware.
The firmware is Micropython, which is extremely easy to understand and modify.zajio1am
It is not open source (per OSI definition), as it is under CC BY-NC-SA.
outlog
just saw this video https://www.youtube.com/watch?v=U-hVUczzlL4 and you get a very smart solution for a fair price, that does coordination etc.. and it's even esp32 based should the need arise. see https://www.bpcventilation.com/bsk-zephyr-single-room-heat-r...
I also have this DIY bookmarked: https://www.youtube.com/watch?v=wJB3dyHDa-8
ilyagr
I'm confused how the heat retention could be around 80-90% without expending a ton of energy.
Naively, if on average the same amount of air goes in and out, I'd expect the temperature of the heat exchanger (on average in space and time, eventually) to be the average of the outside and inside temperatures. If the outside is hotter, the air coming in would be cooler than the outside air (which is a win), but it couldn't be cooler than the average of the temperatures. So, it would still not be anywhere as cool as the inside air, which doesn't sound like 90% heat retention.
Is the heat exchanger attached to a heater or a cooler? The linked video, https://www.youtube.com/watch?v=CDCu0IbEn8Q , would suggest not, as it talks about saving the energy needed for cooling or heating. Is there another clever trick?
singron
What you describe is kind of like a theoretical heat exchanger that only averages temperature at a single point.
You can improve this by exchanging heat across a continuous length along opposing flows. Imagine two parallel pipes thermally bonded where fluids flow in opposite directions. Each point still averages the temperatures, but the average temperature varies across the length and approaches the interior temperature on the interior side and the exterior temperature on the exterior side.
ilyagr
Yeah, I think this makes sense. If you connect many of my heat exchangers in series, the temperature gradient increases; only the middle one will work at the average of the inside and outside temperature (the example of 3 in a row makes sense to me). At the limit, it becomes what you described.
Thanks!
vanderZwan
The mechanism described is called a countercurrent exchange. One fun detail is that it's quite commonly found in biological systems in nature too!
snewman
There isn't a uniform temperature across the entire exchanger. There's a smooth gradient extending from one end to the other. If the outside is hotter, then the inbound air gradually cools as it gives up heat to the outbound air which is gradually warming.
GistNoesis
I find the idea of reversing the air flow direction every 30s simpler to understand than two counter-flowing pipe side by side.
Imagine a pipe filled with 3 metallic grid sections (such that the air temperature in the section will equalize with the metal temperature) separated by plastic grids (such that the heat isn't conducted through the metal), and you push air alternatively from one hot side at 20°C to a cold side at 0°C for 30s and in the other direction for 30s.
For symmetry reason, the pipe will passively (we don't count the energy required to move the air) have a gradient of temperature from the hot side to the cold side. The first section will be ~15°C, the second ~ 10°C, the third ~5°C. (Each section temperature is the temporal average of the temperature of the air flowing from previous sections : so because air switch direction, it means it's the average of left and right sections.)
From the point of view of the house, you only lose energy from the first section of the pipe which will be more like 15°C rather than 0°C.
LukeShu
Counter-flow heat exchangers can be very efficient, without a heater or cooler attached. That said, I don't think I've seen a commercial ERV claim to be more than 80% efficient, so I'm skeptical of the 90% measurement.
(I've seen ERVs with heaters attached; but for the purpose of avoiding frost buildup when it's below freezing outside.)
cyberax
Commercial HRVs often use rotating disks, not counterflow heat exchangers. Disks are freeze-proof, but they need to be powered.
cheeseface
In my experience 80% plus is quite common in the models sold for colder environments. E.g. Mitsubishi electric Lossnay advertises 86%.
cyberax
> I'm confused how the heat retention could be around 80-90% without expending a ton of energy.
Imagine two air streams counter-flowing. They "swap places" within your heat exchanger, so you can (theoretically) get 100% heat recovery.
This principle is used by animals to minimize the heat waste, by counter-flowing warm and cold blood: https://en.wikipedia.org/wiki/Rete_mirabile
danans
Co-current flow (both flows moving in the same direction) work the way you described.
Countercurrent-flow heat exchangers (where the two channels of the fluid/gas) move in opposite directions on both sides of the heat-transfer mechanism maintain a heat flow gradient over the entire length of the heat exchanger. This can result in an almost complete transfer of heat from one current to another.
High efficiency HRV/ERVs use counter-current flow heat exchangers.
cies
I'm not 100% sure about this. But with ERV (opposed to HRVs), iirc, also the moist of the air is transferred to the incoming air. The moist contains a lot of the energy.
ailawyer
[flagged]
teekert
I so need this, and I so need it to function with Home Assistant. I would love to ventilate based on values of my Aranet 4 (a bluetooth CO2 sensor). Also, would be nice if it coordinates with multiple units, ie what this brand does: [0]
EDIT: It does, if you click on "learn more", you'll learn more: "The OpenERV TW4 modules are made to always work in pairs. One always sucks air while the other blows air, synchronized over WiFi. This should be done, or hot air would be pushed out from the building through the walls during the ingress phase, causing heat loss." ...Perfect!
Currently I have two holes in my wall for ventilation, when it is windy it's too much (feel the wind blowing inside), when some people visit and there is no wind, boom, >3000 ppm CO2 in 20 minutes.
I just really hope it is very quiet, although it says ~37 dBa (which is quite a lot imho), I replaced my bathroom ventilator recently, it produces 25 db! [1]). The previous one [2] produced 52 dB (cheapest around), that was pretty annoying, you'd hear it in the bedrooms above the room it was used in. Maybe 37 dB it isn't so bad, especially since you can wind it down and mostly need it when it's busy/noisy (many people) anyway.
Btw, don't buy a CO2 sensor, pretty soon you're a ventilation nerd, or as my wife would call it, a ventilation curmudgeon.
[0] https://blaubergventilatoren.de/en/series/vento-expert-a50-1...
[1] https://www.filterfabriek.nl/ventilatoren/badkamerventilator...
[2] https://www.hornbach.nl/p/rotheigner-toilet-badkamerventilat...
Y_Y
> One always sucks air while the other blows air, synchronized over WiFi. This should be done, or hot air would be pushed out from the building through the walls during the ingress phase, causing heat loss." ...Perfect!
Absolutely.
> A room is not heated by increasing its internal energy but by decreasing its entropy due to the fact that during heating, the volume and pressure remain constant and air is expelled.
https://pubs.aip.org/aapt/ajp/article-abstract/79/1/74/10418...
The point about balancing airflow is crucial, but I think underappreciated by non-professionals. Thermodynamics is highly non-intuitive in places, and the enclosed climate-controlled spaces we love to inhabit are certainly included in that.
Don't get me started on the idea that you can cool a closed room by running a fan or opening a fridge.
teekert
Don't get me started on the idea that you can cool a closed room by running a fan or opening a fridge.
Oh man I had this discussion with my wife yesterday, we have a small electric heater in a room where a pipe burst and I still need to fix that (no heating means instant fungus problems). It keeps its fan rotating always, that way it determines the input temp for its thermostat more accurately. But wife insists it is sometimes blowing cold air and thus very very bad... I explain what a thermostat is (bimetals and all) and that she experiences "coldness" because a layer of warm air is blown from her skin, it's not blowing cold air... she doesn't follow... I even measure the energy usage and the thing only uses 20 W or so when just blowing, not heating. Even when just blowing it's moving cold moist air from the walls so overall good. It's difficult dealing with her like this.
I'll pay someone to tell me how to deal with someone like this and maintain a positive atmosphere. The thing is, I also do it for things that really are probably not worth discussing... I should pick my battles better, is there ever a good time for some mansplaining? Or should I say... Nerdsplaining?
sneak
The problem as it stands now is that she is experiencing something (“it is blowing cold air”) and your claims (“no it’s not”) run counter to that direct experience.
Place and leave a thermometer in front of it and give her the information that you are using to make your own claims.
You went through a process to learn that moving air feels colder than still air at the same temperature. It seems that perhaps she has not. Surfacing the ground truth of the air temperature may help the situation.
vanderZwan
> Don't get me started on the idea that you can cool a closed room by running a fan or opening a fridge.
On that note, I'm curious if hybrid heating/cooling solutions will ever take off. Other than this OpenERV product I mean, which I guess technically counts!
Low Tech Magazine mentioned some experiments in their article on compressed air energy storage (CAES). Instead of trying to make that form of energy storage an adiabatic process, the idea is to use the heat produced/required in the compression/decompression steps in the household to improve the energy efficiency (e.g. use heat produced during compression to heat water; do the decompression in a space that should be cold anyway like a basement used for food storage).
[0] https://solar.lowtechmagazine.com/2018/05/ditch-the-batterie...
pzduniak
Check out Komfovent units if you want a ready solution. My setup is Komfovent HRV (over MODBUS TCP), NIBE heatpump (over MODBUS UDP + esphome-nibe), Vaillant gas boiler (over eBUS-WiFi) + a bunch of AirGradients scattered around the house. Nothing has access to the internet, everything is glued together with HA. Works surprisingly well :)
salomonk_mur
37dba is practically nothing. Like a very soft whisper from a couple meters away. Remember the scale is logarithmic, 37 is almost 2 orders of magnitude below 52.
open_erv2
Thank you, this is correct. Using my class 2 sound meter, if I stand in my house in nowhereland cornwall with power shut off to the whole house, it's 38 dBa. 37 dBa is audible in that environment but nearly inaudible in a normal environment where your computer cooling fan is making 45 dBa at 1 meter, etc. 42 dBa is pretty quiet too, my furnace makes 43 dBa at 1 meter from the duct when it turns on. And that 42 dBa is a full 60 cfm, full blast. That's more than twice the airflow of competing units like the blauberg vento. You don't turn it up that high when you are sleeping.
teekert
Ok, so that 25 dB fan is a lie then because it can very easily be heard over other noise.
gcormier
Why not just buy an ERV? It's available, comparable in costs, and in 5-10 years, there will still be parts, unlike this project where the author hasn't actually shared the most crucial component, the exchanger.
boomskats
I was reading up on counter-flow heat exchangers a few weeks ago after I'd just installed a MVHR system and realised that the actual heat exchanger components themselves were, counter-intuitively, a fraction of the price of the whole unit.
I was surprised when I saw they're mostly made of thin plastic and don't depend on thermal capacity at all (unlike, say, HX espresso machines). The way they work is quite simple:
c
w o
a │ │ l
r └─────────────────────────────────────────────────────┘ d
m
air at 50 deg ────────────────────────► air now 5 deg o
i u
d ───┬──┬──┬──── heat exchanging surface─────┬──┬──┬───── t
o ── ▼ ▼ ▼ ─────────────────────────────── ▼ ▼ ▼ ──── d
o o
r air now 45 deg ◄─────────────────────────air at 0 deg o
r
a ┌─────────────────────────────────────────────────────┐
i │ │ a
r i
r
The lightweight plastic walls are advantageous here - while plastic isn't particularly conductive, the walls are so thin that heat transfers readily. It's how these heat exchangers can achieve 80-90% efficiency without needing any expensive materials or thermal mass. The warm exhaust air leaves only slightly warmer than the incoming cold air, having transferred most of its heat to the incoming stream.
Clever design.
ent
Thank you for the illustration! I was sitting here, wondering that the whole system sounds paradoxical but seeing it drawn down with the arrows really helped grasp how this works!
turtlebits
Not sure what you mean by the heat exchanger core being a fraction of the price, but I've seen replacement cores cost around 1/3 of the total unit.
They should be made of high thermal conductivity material like resin or ceramic.
elric
I installed a central ERV in my home, a Zehnder ComfoAir Q. Installing it was quite involved, but not hard. Definitely within reach of anyone with basic DIY skills.
The hardest part is finding a good spot for the ventilation unit, which is about the size of a large, old CRT TV. You have to run ducting from there to multiple rooms, but their ducting system is easy to install.
Mine draws about 20W/hour at its typical setting, and it greatly improves comfort. Keeps some of the humidity out when it's humid out, keeps some moisture in when it's dry out. Fresh air year round. Keeps mosquitos out. Keeps some dust/particulates out.
Worth the effort, even in my small house.
A decentralized unit would be a lot easier to install, but I imagine it's less efficient, less suitable for larger dwellings, and probably louder. YMMV.
mreiner
Cool project! Around that topic I can also recommend a channel of this engineer (switch to auto-translate): https://www.youtube.com/watch?v=Cv0s6TgwbJg
Paraphrased:
- push-pull ventilation is easy to install and comparatively cheap
- it's prone to hygiene issues like blowing dirt out of the filters back into the air and providing a moist environment for microorganisms in some operational conditions
- it's prone to windy conditions
- the numbers stated by commercial vendors seem to have no basis in reality, there seems to be no vendor providing data based on the relevant testing standard for these systems. OPenERV states they want to get it tested by Passivehause institute but also say no lab data measured yet.
Might be just my counter-factual gut-feeling, maybe a mechanical window opener based on EspHome for short pulsed passive ventilation intervals is actually more efficient, easier to implement and need less maintenance? Not aware of any comparisons though and last time I checked I could only find some finicky 3d printed actors that might not survive a guest opening the window.
snickerer
I use similar decentralized (single-room) ventilation units here in Northern Germany: https://www.bayernluft.de/de/frame.cgi?page=start.
I am super happy with them. We now always have air that feels fresh and warm in winter, and the humidity has dropped significantly.
There are two types of such ERV devices:
1. Those with only one air channel that switches directions periodically. They use a heat storage element in the airflow. OpenERV belongs to this group.
2. Those with two separate air channels for intake and exhaust at the same time. The air does not mix but passes through a heat exchanger. Bayernlüfter works like this.
The only thing I don't like about Bayernlüfter is that it is not open source. It is controlled by a Raspberry Pi (or a similar clone), and I don't have access to it.
jsiepkes
In the Netherlands these systems are fairly common in new houses. Mostly because the law mandates a certain level of energy efficiency of new houses. There are other ways of obtaining this required efficiency level, but an ERV unit is pretty cost effective.
I've personally been looking at installing such a system [1]. However since houses in the Netherlands are almost all made out of concrete installing such a system in an existing house is pretty hard.
[1] https://www.duco.eu/uk-ie/products/mechanical-ventilation/ve...
jacobgorm
Same in Denmark, we pretty much had to install one when building our house, to make up for energy loss from the large window area we wanted. We didn't have it properly calibrated at first, but once that was (professionally) done it has worked perfectly and kept a pleasant indoor-climate ever since.
The old-and-trusted brand here is https://www.genvex.com/en (ours was supplied by Ecovent though https://ecovent.dk/?lang=en )
moooo99
Similar story here in Germany. New energy standards require a ventilation concept. Some people choose to rely on daily ventilation to save some money, but most people nowadays opt for an ERV. At least here in Germany, for some reason there are quite a lot of people who are super against the idea of having an ERV. Personally, I wouldn't want to miss is for having fresh air alone, not having to deal with pollen is an added bonus
open_erv2
My basic understanding is that the thermal energy also costs a lot more over there than it does in north america, like 4x as much.
ra
I'm confused if this is true open source hardware and software or not?
toboche
I don't think it's FOSS at the moment. I actually would build two or three of these units on my own and could provide some feedback along the way. My definition of open source means that I should be able to do so.
Unfortunately, the gDrive files are not providing enough information for me to build one of these in a DIY manner. I didn't find enough information on the hardware side, no BOM, no hardware documentation. I think that, if the author would like to actually boost DIY adoption, it'd be worth having a step by step assembly guide. At the same time, when reading the page, I had a feeling like it's more supposed to be a way of advertising a future commercial product, not really focusing on the FOSS/DIY side.
The software is provided, but from my experience with such projects, it's maybe half of the minimum information needed to build a full fledged device.
I like the project and would love to build it in the near future though.
jeroenhd
Under "source code", there's a link to a GDrive with a ton of design files and documentation, as well as source code.
These are licensed CC BY-NC-SA 4.0, so depending on your personal definitions, they may or may not be "open source" (IMO they're open source but not FOSS but I've seen others equate open source with FOSS).
open_erv2
As a community we need to do some thinking on how open source may sensibly be applied to hardware. Unfortunately Prusa, who used to be a real champion, has departed from the assumed True Path, and they have discussed their reasons, which are largely valid. That said their design at a more fundamental level has also departed from a maintainable, simple and elegant design.
The purpose of the source code is to enable maintenance, not cloning, I say that on the website. That is this context, there are many others. It improves the economics because the machine lasts longer and there is no planned obsolescence. People are welcome to make their own units from the source if they have the skill, but although it would be fun, I don't really have time to make it easy. Some day there may be a kit which is very economical but it will still take a whole day of work to assemble, probably.
thanzex
Me too, when reading "open source" I was expecting some design docs or the like. Aside from the general confusion of the website, I haven't been able to find some of the most important information. For example, there's no diagram or immediate explanation of the general working principle and airflow path. The heat exchanger itself is published only as-is for those designs, while the author writes that he uses a custom python script tuned for the design size and his 3d printer to generate it.
When i saw this I immediately thought of studying it and reuse some of its designs for my custom use case, which does not appear to be currently possible.
At first glance it appears to be "open source" in the sense that you can buy it, but if and when something breaks you can print/reorder it easily.
Correct me if i'm wrong
M2Ys4U
No, it's not. Files are available under the CC BY-NC-SA licence which, because it does not allow commercial usage, is not open source.
MichaelRazum
Looks interesting. Especially since it seems to be much cheaper that the closed solutions...
Have looked in this kind of systems, for my parents. The use case was basically, not about energy efficiency, but rather noise protection - to be able to sleep with a closed windows. I think so far I always had two issues (in that usecase).
- First the device by itself - produces a bit noise like 42db might be too much for some people if you want to sleep. Especially some of the devices are using one ventilator, which switches directions and won't produce homogeneous noise.
- Second 60 CFM is fine, but if you want to have the feeling of an open window - it should be much more and most devices can't deliver that. Also the heat exchange thing is kind of cool in the winter for sure. In the summer, you often have the case that in the evening you house is much warmer than the air outside - so you would like to turn the heat exchange off in the winter.
PS: Actually, maybe looking for a complete different use case. But I think what would be very cool, would be some idea to make at least one room 100% quite (with fresh air ) in a cheap way. Guess this would be a huge life changer for a lot of people, who suffer from noise pollution.
leoedin
I think that summer/winter distinction is really important. In the UK where most houses don't have air conditioning, you really don't want heat recovery for 1/3 of the year. On really hot days you might want to use heat recovery during the peak few hours, but otherwise you are trying to cool the house down with colder outside air.
I would love some sort of intelligent house ventilation system which could do all that. Heat recovery when it makes sense, normal ventilation when it doesn't. All automated based on dT and relative humidities.
ZeroGravitas
I think with this model you can do this as long as you install them in a synced pair (which I think is the baseline assumption).
Normal (heat recovery mode) you have them reverse flow every 60 seconds or so to swap heat.
In cooling mode you just run then continually. One is bringing in fresh air and the other is removing stale air.
The heat store in the intake will soon cool to the outside temp and the heat store in the output is irrelevant (apart from maybe slowing the air flow and creating noise).
If you manually control the system you could combine with a few open windows to create cross breezes even on still evenings.
Get the top HN stories in your inbox every day.
The way I like to describe HRV/ERVs to people who don't know about them is:
"Imagine you could open a window to get fresh air into your house and stale air out, but when you did so, most of the heat/humidity would stay in during the winter, or stay out during the summer, leaving you just the fresh air".
In terms of the effect on environment inside a house, I usually say:
"Imagine it's always a fresh-air spring day inside your house"
This is a great project. One problem with ERV/HRV systems right now is that they are very expensive niche products. While this system doesn't achieve the extremely high heat recovery efficiencies of counter-flow units, the perfect is the enemy of the good, and this seems like it could be orders of magnitude cheaper.