February 23, 2012

Sizing Lunos e² through-wall ventilation units

Exploded View of Lunos  e²

475 is excited to offer the Lunos e² HRV to the US marketplace – a residential ventilation system of extraordinary value. The value is from very high heat recovery efficiency, providing not just monetary savings but exceptional comfort, ease of installation and the elimination of a ducted distribution system.

Installed in pairs, the Lunos e² provides a unique high performance ventilation solution for retrofits and new homes. A Lunos system provides between 10 and 22 cfm, depending on the controller setting – a small amount by any standard. For larger spaces, installing multiple systems can achieve higher ventilation requirements while still pocketing significant savings compared to high efficiency centralized systems. To make sure that homes achieve proper ventilation levels, we offer the following guidelines for sizing your Lunos systems. Our recommendations are based on ASHRAE 62.2 residential ventilation standard for buildings three stories and less, which is required by some jurisdictions and certifications.

Lunos Air Flow Diagram

Lunos Air Flow Diagram

To add to the fun, the ASHRAE standard was recently revised with increasing ventilation levels – with significant implications on flows, equipment sizing and heat losses.  Joe Lstiburek of Building Science Corporation (BSC) has called for these new standards to be ignored. In fact, he has issued his own guidelines that give credit where the ventilation is balanced, distributed and in an airtight enclosure. If these conditions are met, then the new BSC standard suggests ventilation rates like the old 62.2 standard. But there’s more (or less):  Joe says that even these rates are too high and are not needed to generally insure good health – and that they can be lower still. How low is anybody’s guess. The bottom line is that the BSC flow recommendations generally align with Passive House Institute guidelines. 475 sees the BSC standard as a valid alternative to the current ASHRAE 62.2 standard.  See the BSC ventilation standard here.

We offer two ventilation sizing recommendations below, starting first with the BSC guidelines, then followed by the new (2013) ASHRAE 62.2 standard recommendations.


Per BSC total cfm required = 7.5 per occupant + (area x 0.01).   (Occupant number is determined by the number of bedrooms + one.)

For the output of a Lunos pair we use the high setting of 22 cfm/pair – assuming it will be typically running at the lower medium setting.  Let’s illustrate using a few common rough home size/configurations:

  • 450 sf studio = 7.5cfm*1 + (450 x 0.01) = 12 cfm min. = One pair
  • 650 sf one bedroom  = 7.5cfm*2 + (650 x 0.01) = 22 cfm min. = One pair
  • 1,000 sf two bedroom = 7.5cfm*3 + (1,000 x 0.01) = 32.5 cfm min. = Two pair
  • 1,500 sf three bedroom = 7.5cfm*4 + (1,500 x 0.01) = 45 cfm = Three pair
  • 2,000 sf four bedroom = 7.5cfm*5 + (2,000 x 0.01) = 57.5 cfm min. = Three pair

As the numbers suggest, more fans are required as the home moves from a modest size to a larger size.

eGO Room Ventilation

eGO Room Ventilation

The Lunos e² systems are installed in the living areas of the home – including bedrooms, living rooms, offices, etc. Bathrooms and kitchens generally require separate dedicated ventilation systems. For these spaces, our best practice recommendation is to install the Lunos eGO system for optimal comfort and efficiency. Alternatively, traditional exhaust fans can be used with intermittent operation.  The exhaust fans should have timers, motion sensors or switches to limit their use and their impact on the overall ventilation efficiency.


  • 450 sf studio = 7.5cfm*1 + (450 x 0.03) = 21 cfm min. = One pair
  • 650 sf one bedroom  = 7.5cfm*2 + (650 x 0.03) = 34.5 cfm min. = Two pair
  • 1,000 sf two bedroom = 7.5cfm*3 + (1,000 x 0.03) = 52.5 cfm min. = Three pair
  • 1,500 sf three bedroom = 7.5cfm*4 + (1,500 x 0.03) = 75 cfm = Four pair
  • 2,000 sf four bedroom = 7.5cfm*5 + (2,000 x 0.03) = 97.5 cfm min. = Five pair


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10 Responses to Sizing Lunos e² through-wall ventilation units

  1. Ken Neuhauser March 13, 2012 at 2:18 pm #

    How about explaining how this energy recovery ventilation system works (or convincing the reader that it works)? From the exploded view it seems that each unit can perform in exhaust or supply mode at any one time but not both simultaneously. So how is the energy transferred from the exhaust stream to the supply stream? Reversing the flow?
    Without a compelling explanation of how and why this system works, I fear that this device will be seen as another euro gadget distracting designers from the fundamentals of performance.

  2. 475ken March 13, 2012 at 2:51 pm #

    Hi Ken,
    Yes, agreed – definitely need a blog post on how it all works – it is in the queue. In the meantime I’d encourage all interested to click on the link to the Lunos page of our site here:
    where explanatory brochures can be found.
    But in brief, they are installed in pairs – one exhausting while the other supplies air. After 70 seconds they both reverse direction. As air exhausts it transfers heat to a ceramic core, discharging the heat to incoming supply air. Once a full explanation is up in a new post, I’ll be sure to let you know. Thanks for the comment.
    PS – I exchanged emails with John Straube regarding the devices and he sounded impressed, although put-off by the cost.

  3. Kevin Gallen May 15, 2012 at 2:00 pm #

    Ken, I am inclined to agree. I have reviewed documentation on your website and have a few concerns: if designed to operate in pairs and one unit bring in air first, that air being brought in initially has the potential to be freezing. If all units are designed to remove air first and then supply air, that addresses the heat exchange/freezing concern, but it creates pressure imbalances in both phases of operation. In addition, if you use the fan to exhaust, then supply, much of the particulate (dust/debris) that is removed from the space will tend to be blown back into the space. I am a big proponent of energy recovery, but I am not yet sold on this technology.

  4. 475ken May 15, 2012 at 2:13 pm #

    Hi Kevin,
    Thanks for the comment, but you’re posing real misinformation as questions. There are no pressure imbalances – the system runs in balance. The dust/debris is not blown back into the space. Please come by the office sometime – we’d be happy to run the machines for you.
    Hope all is well.
    – Ken

    • Kevin Gallen May 15, 2012 at 2:51 pm #

      I am basing my comments on the manufacturer’s documentation posted on both your site and on Zehnder’s site. There is nothing in the documentation that gives me confidence when it is very cold outside, that uncomfortably cold air will be brought in by one of the units during every cycle, and there is no documentation included on the types and locations of filters (since they are bi-directional fans, one would assume there would be a filter on either side of the fan). Is there any more detailed documentation that can be posted, that contain enough information to satisfy a consulting engineer? I would love to stop by and see them in action.

      • 475ken May 15, 2012 at 3:26 pm #

        There is Lunos documentation on the Zehnder website? I’m happy to send you the efficiency curve to show how much heat is recovered at various outdoor temperatures. Of course when it is very, very cold out the air coming in is substantially off the room temperature (this is true for the most efficient systems, unless there is a preheat of some sort) – BUT at 18cfm the occupants are not getting blasted with cold air at all, on the contrary, the supply air continues to mix with the room air in a comfortable fashion. The units have been installed widely in northern Germany to great success and they have been recently installed in a house in Maine – where the accumulated dust at the interior of this airtight building was expeditiously removed from the space via these units – and we look forward to seeing them in action during next winter. Not sure why a filter would be required on both sides of the fan…..with one filter the air passes through it going in either direction. When can you come by?

        • Kevin Gallen May 15, 2012 at 3:59 pm #

          Thanks, Ken. This is good information. I am only in NYC about once or twice a week now that I am working full-time on my office near Trenton. I will check my schedule and let you know.

  5. John March 3, 2016 at 9:31 am #

    If one half of a Lunos e2 pair is installed in a bedroom and the bedroom door is kept closed most of the time, is there a recommended size of under-cut for the bottom of the door? By my calculations, if the door is undercut 1/4″, then the air speed under a 30″ door would be 4.8 miles per hour for the maximum e2 flow rate of 22 CFM. Is that acceptable, or would it negatively impact the e2 or its ability to push 22 CFM? If not, what would you recommend? Thanks.

    • John March 4, 2016 at 9:30 am #

      Hello John – 1/4″ would be little tight by our thinking. We suggest a 1/2″ minimum, but our recommended is 3/4″ to 1″, so that area rugs and the like cannot inhibit proper flow.


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