Has anybody come up with an idea of modulating fresh air via a central Fresh air unit serving several AHU's using occupancy sensors located in different zones
The answer is yes. I've seen it in a number of buildings. Basically, if you can afford it, a separate fresh air unit that tempers the air (supply air = space temperature) and controls humidity is the most effective way to make sure that each occupied space gets the ventilation needed and no more.
A couple of military bases I've surveyed had buildings in which CO2 control (like a VAV system) was coupled with standard occupancy sensors to absolutely minimize fresh air intake but keep it at the level that meets codes.
i have to have multiple AHU's that are serving areas with different application like a institutional multipurpose room, library, an auditorium, class rooms and labs. Want to feed them treated fresh air via one central Fresh Air Handling Unit. Whereas the AHU's will recirculate the room air only. By doing so, i want to reduce the equipment cost. AHU coils will have lesser rows since they are not treating the outside air. While a single Fresh Air Handler (if) can serve them will save cost of more than one Fresh air handlers, the valves as well as controls.
Assuming your outdoor pathway is weatherized, size the make-up air duct to serve all AHUs in economizer mode (100% OA) at no more than 1,500 feet per minute duct velocity. Modulate the OA fan using a VFD to achieve about +0.05 in. w.c. static pressure at the most remote AHU at all times (suggest a 0-0.25 in. w.c. device).
Use NO conditioning for the make-up air unit, assuming the make-up air system is insulated and weather-tight.
Each AHU should operate with standard economizer controls (preferably enthalpy comparison), with the variable make-up provided.
You could have all the circulating AHU's draw in outside air as needed and have VSD on the fans to limit the circulating air volume when it is not needed.
Outside air could vary between 0 and 100%.
If the different systems/rooms are spread out over the building the ducting for the fresh air might get expensive.
Required fresh air volume for an auditorium or classrooms can vary a lot because of the varying occupancy.
On the other hand, lots of circulating AHU's are expensive as well.
Would you size the fresh air units to 100% for all the rooms simultaneous or size it to maximum number in the building at say, 50 m3/h per person? I mean, a student is either in the library or in the classroom but not in both at the same time.
