Why buy an OpenERV?

Why buy any Energy Recovery Ventilator?

You can think of an ERV as a machine that gives you fresh air in cold/very hot weather without causing heating/cooling problems, or think of it as a machine that saves you energy/money/carbon while you get that fresh air, compared with just using a fan/open window/air blowing through cracks.


The main claim to fame of the OpenERV is that it's half the up front price of the next cheapest option, and less than a quarter the price if you make it from kit, for about the same flow and efficiency, and it has even better long term durability/lower long term total cost of ownership, due to a lack of planned obsolescence. As a result, it is far more affordable, but still good re performance.

For some more arcane design details discussion, see here.

On the importance of fresh air:

The main reason to want fresh air is actually immediately obvious to all of us - simply open a window and observe the immediate and clearly improved personal well being. Except for the cold/heat that gets in. Who doesn't feel invigorated, empowered, by the fresh air on a summer day? Let's try to achieve this valuable improvement in well being more, through engineering.

The positive impact of fresh air on well being in general, is all well within our lived experiences, and we know it's worth achieving. There are many less obvious reasons, one of the more recent ones is the relationship between fresh air and COVID-19 transmission (or any aerosol transmissible disease).

Some people don't seem to mind poor indoor air quality on a subjective level, in which case it may make sense to investigate the more medicalized health impacts. These include a remarkable variety and quantity, caused by VOCs, CO2, mold spores and mycotoxins, and excessive humidity and condensation. More recently, fresh air has been recognized as an important factor in reducing indoor COVID-19 transmission. It was found that, on average, 20 CFM per student gave a reduction of 80% in the probability of transmission between students in a classroom.

There are many other more authoritative sources of detailed arguments in favor of fresh air that litter the internet.

I can save you the trouble of trying to find some other way to remove most contaminants, by the way: there aren't any. There is no filtration technology, ozone or ion generator or anything else that can do it. Fresh air is the only way with current technology, to escape most such pollutants, economically or at all. Those fragrance things just add more pollutants to the air, personally I don't use them. (filtration works for Covid-19, though, probably, it's just not as general purpose.)

Secondly, fresh air from outside often contains pleasant odors from nature, produced by sunlight acting on the air or biology - the flowers and other plant life, the smell of the rain, etc.

An efficient, quality ERV is the closest thing to a magic fresh air machine there is.

The OpenERV currently does not include any filtration for pollen or other stuff, engineering that is another project for another time, and it would increase the cost. It is made so one can be interfaced, when it is designed/sourced.

Here is a respectable document that discusses the big picture on why fresh air is valuable, although they leave out the subjective fatigue reducing impact, which, IMO, is arguably the most important.

What's an ERV and why use one instead of just a fan or open window?

Here is a video that explains what an ERV does/is fairly well:



The OpenERV is a single-room - or "decentralized", also called a "ductless"- ERV. It's meant to replace one open window, or perhaps two, for one or two rooms, while eliminating the heating/cooling problems year-round.

Most people just don't understand the issue, unknowingly pay the heating cost, or live with poor quality indoor air, unfortunately. Even many new buildings actually have serious indoor air quality problems. Many brand new homes actually have condensation problems near the windows in winter, which shows inadequate ventilation, and which causes mold, further reducing indoor air quality.

We live in a society where it's pretty much everyone for themselves, this stuff isn't being taken care of by some regulatory agency. The Ontario building code specifies that buildings must have at least 1 fan to bring in fresh air - but the bathroom fan counts! That's as far as they got in making sure things are done properly.

Standard recommendations are about 20 CFM per room, but to be precise you should think in terms of ACH, or air change rate, and that still isn't very precise, because the amount of pollutants being released in a given volume can vary greatly depending on the contents of that volume. I will avoid duplicating content,
and leave you to find other sources which can explain this with more authority.

Using a ductless solution like the OpenERV can also allow you to selectively turn off the ventilation in the rest of the house during the night, when you are not home, etc, saving 2x, or more, energy. You can do this simply by using a smart plug on the OpenERV.

The typical way to get fresh air is to just force it in using a fan, or for older buildings just allow wind to push air in through cracks and windows etc. By default, they don't try to recover any of the heat from the outgoing air.

The heat loss from this escaping air is more than 50% of the total heat cost of a building, according to this document from the Canadian government, in the example case of Toronto (it will be a similar fraction elsewhere, this figure stays mostly consistent despite climate variations).

Heating of ventilation air is responsible for about 20% of all carbon emissions! An ERV can save more than 85% of that, so 17% of all global carbon emissions, theoretically. What other single bit of tech can compare with that? That's something.

An ERV can also help during air conditioning situations, but not as much, because latent heat is a bigger deal in that scenario. I haven't done much calculating or measuring or optimizing of the design for that kind of scenario, but if an ERV is what you want, the OpenERV is a good one. I have experimented with the addition of desiccant to the heat exchanger core, and it does work some.

At the bottom of this page, I share some detailed calculations on payback period and a spreadsheet which you can use to calculate the figures for your own locality with data from the climate, energy prices and carbon impact of different energy sources where you live.

The executive summary is that at $650 CAD per unit (500 usd), an OpenERV has from twice to five times as much return on investment as PV panels, depending on your climate, energy prices and heat energy source. If you make it from kit for $325 CAD, it's twice again that - 4 to 10 times!

It also has a far superior carbon payback period. This is because the embodied energy is far lower for an ERV than it is for PV panels. PV panels have extremely large amounts of embodied carbon.

Thus, whether you want to save money or the environment, an OpenERV makes sense.


The OpenERV compares very favorably with competing commercial off the shelf single room HRVs and ERVs. A respectable ERV like the lunos ego with 20 CFM and which claims 85% efficiency or more goes for more than $1200 CAD, plus tax and shipping, plus an adapter for the window and even a power cable. Suppose $1450 total.


An Open ERV is much more economical, practical, compact and ethically made.

It also is expected to get better real world efficiency than all of the systems I have seen, because the fan speeds will be automatically adjusted in real time by the microcontroller and temperature sensors to optimize the heat exchange efficiency regardless of wind conditions. Other systems don’t do this.


Beware the common units which have only a single fan that reverses direction periodically. They are measuring the temperature of the air going into and out of the unit, and saying that calculating from those figures the unit is ~90% efficient. But when the unit pushes air into the house, air escapes from elsewhere. Heat cannot be recovered from that air that escaped. It’s gone to the great outdoors. Thus, the de facto amount of energy per liter of fresh air you get is way, way lower, than they claim. They save some energy, but not much. However, some systems like the lunos e2, have a paired unit connected by wireless link that is synchronized to blow air out when the other one is blowing in, and vice versa. That can give good efficiency, and surprise, surprise, they actually cost a lot more. Remember that the flow rate quoted for such units is the amount of fresh air you get with a *pair* of units.

The OpenERV both sucks and blows equal volumes of air at the same time, so energy is always recovered from the outgoing air.


An OpenERV makes particular sense for these situations:


  • The heating of the fresh air would be provided by electricity, which is more expensive than natural gas, and usually implies more carbon emissions. See spreadsheet.


-This may occur because your home is electrically heated, but it can also occur because the heating of a given room is not enough to keep it comfortable when the extra ventilation air is added, so you (would) have to use an electric space heater.


  • You are more after comfort than saving money or carbon. The OpenERV provides a regulated, reliable supply of warm, fresh air that is neither too much nor too little, while an open window tends to be too much or too little, depending on the wind, temperature, etc.


  • You are renting. With an OpenERV You don’t need to get permission to install the unit, and you can take it with you to your next place when you go. All other units on the market are meant to be installed in a hole cut in a wall, not a window. Renters cannot install PV panels, either, so this is a valuable alternative to help you reduce your carbon footprint and save money by reducing your energy bills.


  • You want to do your part to reduce carbon emissions. The system saves quite a bit of carbon no matter how you work it. It saves more carbon per dollar investment than PV panels, by a factor of 16 or so, see spreadsheet for details.

  • It's better in a bathroom than those periodically reversing flow types, because it does not force air out of the bathroom during any part of it's cycle. Great for solving kitty litter smell.

  • You want the best performance for your dollar, in CFM and energy savings.





Aside from direct performance and financial reasons, why else would you buy an OpenERV, instead of another ERV?


  • You want to support open source. Open source has proven to be a real force for good in the world of software, and this is part of a pioneering effort to extend that into the hardware world, where even more benefit may accrue. Let’s do this!

  • It’s repairable. All conventional modern manufacturers do the planned obsolescence thing. There is no planned obsolescence in this unit. Moving parts never last forever. The fans in any unit will stop working or start making excessive noise eventually, usually after a few years, because they are on 24/7. Will you be able to replace them? With an OpenERV, the answer is yes. Even in twenty - or fifty - years from now, you will still be able to buy and print replacement parts at low cost, easily and quickly. An OpenERV is made to last decades, not end up in the landfill as soon as the manufacturer thinks they can get away with it! I encourage you to read up on planned obsolescence and it's impact on people, our cost of living, and the environment. Unfortunately, it is more real and prominent than most people think.

  • Transparency. With OpenERV, you know what you are getting better than any other option. You can have a look at the design files, see test data, see actual pics of the dismantled unit, videos of it's operation, ask other members of the community that have bought one (when there is an actual community).

  • Customization. You may have some unusual requirements. You can hack and modify the unit to your liking, with the source code in hand, and everything very hacker friendly. Want to integrate it into your smart home and monitor efficiency 24/7? If you have the programming skills, you can. Add desiccant? Add an integral co2 or motion detector?

  • You’re supporting a fellow worker bee entrepreneur in making an honest living with ethical business practices, not an oversized psychopathic company. The downsides of giving our business to the Amazon and Walmarts of the world truly are adding up on us. It’s like pollution. Sometimes it seems unrealistic to boycott those guys, but sometimes it becomes clear just how unethical they really are and how it’s catching up to us. It’s very much a real concern.

  • Satisfaction guarantee. You can return the unit within 60 days for any reason for a full refund. Please have a good look at the pictures etc. to be sure it’s what you want, to avoid unnecessary returns, though, that’s better for everyone. Some people like the 3d printed look, some don’t.

  • Manufacturing warranty. If the unit proves to be no good in any way within 5 years from purchase, you can return it for a replacement or repair. This allows you to be confident that despite the unusual manufacturing process, it will work well.



Here is the spreadsheet you can use to calculate return on investment and the environmental benefit figures for yourself, given your realities. To see how the spreadsheet works, you just have to look at the formulas, I tried to keep them short so you can figure out how things work easily. Notice the very favorable figures when compared with PV panels, the gold standard in green tech. Input cells are in blue, the most important output cells are red:

OpenERV financial and carbon impact reckoning