Very confused - Natural Ventilation

Jeff,

I did a Preschool / Daycare NC project between Philly and Allentown with a similar situation, and our Increased Ventilation credit was rejected.

I'm going from memory here, but their explanation was something along the lines of 'a project cannot utilize both natural and mechanical ventilation to meet the requirements of this credit. There is no need for the air conditioning system to be operational while windows are open and the space is being naturally conditioned, as this would cause unnecessary energy usage. Therefore ventilation is met either when the unit is running or when the windows are open, but additional ventilation is not included in the design'.

The project was smaller, around 20,000 sf, with several rooms assigned to each age group of the children. We designed a rooftop AHU with VAV control in each space, meeting 62.1 mechanical ventilation. In addition, each room had exterior exposure(s) with operable windows meeting the natural ventilation requirements (4% & 25 ft from opening). So I thought the same thing you did, I'm providing +100% additional ventilation air.
Shot down.

However, I was also involved in a larger condo/mixed-use project, with 85 dwelling units where we were directed by the owner to push for a "mixed mode" ventilation system to meet the prerequisite ventilation. I'm referring to a simple residential HP unit with OA damper, and operable windows; not necessarily what I might consider an "engineered mixed-mode ventilation system". In this project, neither the operable windows or the mechanical ventilation were capable of meeting the 62.1 standard minimums on their own (each method was capable of providing about 80% of the requirement by itself). We noted that the HP fan would run continuously, without the compressor during natural conditioning, to circulate air & provide some mechanical ventilation in addition to the operable windows. Although I was very apprehensive, we were in fact approved by both Philly L&I and LEED.
In this case, we convinced the owner that attempting the increased ventilation credit was pushing our luck, and did not attempt it.

In short, I've seen reviewers go both ways on this concept, so I typically approach it as something very much in the "maybe" column. But worth a shot, if you can justify the system and occupant usage in your narrative.

Just a couple of my experiences. Hope that helps.

Are they under-window console type, or ducted?
I think one of the things that helped our case in the condo project was that we could semi-justify the unit fan operating with the windows open, since the air was being circulated from the exterior exposure (unit return near the windows) to the interior of the dwelling units.
If the reviewer sees console units below operable windows, it might be a tougher sell.

As you said, though, worth a shot.
The answer is always 'No' when you fail to ask.

But, I definitely think the energy savings are worth accounting for in other credits. There will be times that no mechanical cooling will be required because you have occupant control of operable windows. Regardless of the increased ventilation credit, I think there are ways you can still make this feature benefit the project in the form of LEED points, as well as overall owner satisfaction.