Sizing Lunos e² through-wall ventilation units

Sizing Lunos e2 through-wall ventilation units

Installed in pairs, the Lunos e2 provides a unique high-performance ventilation solution for retrofits and new homes. A Lunos system provides between 9 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 achieving 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 (2010 version) residential ventilation standard for buildings three stories and less, which is required by some jurisdictions and certifications.

Lunos Air Flow Diagram
Caption: The Lunos e2 system provides continuous ventilation with one unit providing fresh air in and the other unit exhausting stale air out and then reversing in 70 second intervals. For the full video, click here.

To add to the fun, the ASHRAE standard was revised in 2013 with increasing ventilation levels - with significant implications on flows, equipment sizing and heat losses - and is still operative today. 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 62.2-2010 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 62.2-2013 standard. See the BSC ventilation standard here.

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

SIZING PER ASHRAE 62.2-2013 (through 2016)

As expected, below, we show how the 62.2-2013 guidelines can significantly increase the number of units required - while the benefit is questionable:

  • 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


The ultimate number of fans may also be affected by the layout of the home. As there should be a Lunos e2 in each bedroom at minimum, to ensure indoor air quality in each bedroom. Then depending on the layout the living areas may act as a pass-through space for circulation of the air between bedroom fans. Consequently, there may be opportunities to be more or less efficient in the number of units required depending on the specifics of the layout.

The Lunos e2 systems are installed in the living areas of the home - including bedrooms, living rooms, offices. Bathrooms require separate, dedicated ventilation systems. For these spaces, our best practice recommendation is to install the Lunos eGO unit for optimal comfort and efficiency.

eGO Room Ventilation
eGO Room Ventilation

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.


If you still have questions, request a quote through the form link above and we can help you optimize your e2 and eGO layouts.

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