MVHR inlet/extract positions New advice on distance between them (and why it matters)

18 September 2023 by Jason Quinn

There’s some new advice available about locating the air inlets and exhausts in MVHR systems. If you’re a Passive House designer or otherwise involved in high-performance building, it’s important to keep up with the technical changes, as best you can.

ASHRAE’s latest publication* has pulled together research and studies (including CFD work) in order to offer advice on the location of MVHR fresh air inlets relative to exhaust. This is especially important as we start to do more multi-family Passive House projects.

The general rule of thumb was to keep them 3m apart in any direction but the proposed change to the standard would allow them to be as close as 1.5m if the inlet is above or to the side of a downward facing exhaust, or below a horizontal facing exhaust.

Having inlets and exhaust closer than this is not ideal, as in some wind conditions exhaust air may be recycled into the building. That said, it’s still orders of magnitude better than exhaust-only ventilation in terms of fresh air supply.

Also note that some suppliers have special integrated inlet/exhausts that are designed to prevent mixing. So long as they are designed well, the above is not applicable.

I like ASHRAE standards. They are a collection of rules-of-thumb to try and produce good buildings and systems. This is done with considerable emphasis on cost-effective design.

*Public Review Draft for the Proposed Addendum f to Standard 62.2-2022, Ventilation and Acceptable Indoor Air Quality in Residential Buildings

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