Functioning principle:

This machine uses what is called a Thermal Wheel or Enthalpy wheel, at it's heart.

Here is a the best video I could find that explains how an ERV works, they even show a mini thermal wheel type unit, which is fundamentally similar to how the OpenERV works.

Fundamentally, the outgoing air surrenders 88% of it's heat and 45% of it's water vapor to the rotating heat exchanger media, and then the airflow through that section of the heat exchanger is reversed, with fresh air picking up the heat and water vapor on the way in.

The OpenERV is an Energy Recovery Ventilator, which means it recovers at least some of the latent heat (water vapor). Under simulation for Toronto weather, it recovered 45% average latent heat, over a year. This is even better than an HRV, which does not recover latent heat. This occurs because water vapor condenses onto the media, then re-evaporates. There is no desiccant required.

It is also gets quite high sensible efficiency, testing has shown it gets about 88%.

For a discussion on the benefits of fresh air it provides and the monetary savings, see the Why OpenERV? page.


This ERV is open source, with all CAD files and the firmware for the raspberry pi pico licensed under the CC BY-NC-SA 4.0, which allows non-commercial use without a license. After the development costs are recovered the ERV will be released under an MIT license which allows for commercial use.

You can make it yourself, and I recommend purchasing a kit to obtain the non-printable parts efficiently. See the Buy Now page to buy a kit.

Development stage:

*The development of the window mount unit has been largely suspended*. The 5-4 unit is the best, it works and gets ok flow rate, and good efficiency, but it's just a bit noisy, and secondly the gap between the manufacturing cost and resale price is not very large. Clearly that doesn't make for a promising business model. Secondly, people don't seem to be able to understand what it does/why they would want one. That's not good for business. Therefore, I have decided to focus on the wall embed model, which has superior properties on many of these counts.

See the Community page for details.

A test was done on the V4.0 beta model, and the data indicates an excellent 88% efficiency, +/- 4% at a full 21 CFM. You can see a video showing/discussion this test, with commentary, on the YouTube channel.