I am working on project where the owner is leasing a fairly new retail space that used to be a sandwich shop and is converting it to a pizzeria. The previous tenant only had two residential style furnaces (3-ton & 4-ton) up in a mezzanine level above the bathrooms, which did no provide any outside air. The village is following the 2006 IMC, which should have required them to have some outside air, but apparently that one slipped by the inspectors. The project involves installing a new 22 foot long hood that exhausts 4,724 CFM and a makeup air unit that supplies 3,780 CFM at a perforated grill along the front of the hood. Based on the size of the small seating area we are required to provide 300 CFM of outside air, plus provide another 900 CFM for the difference in the hood and makeup air unit. The engineer calls for installing a makeup air unit with a/c to provide this additional 1200 CFM.
The problem we are having is that this is a two story building with offices above the space, and we only have enough room to run the ducts for the hood and the larger makeup air unit. We are trying to come up with a way to eliminate the duct run for the 1200 CFM unit, but are having a tough time coming up with any ideas. I know the rule of thumb is to provide roughly 80% of the makeup air at the hood and the rest out in the room, because it is more efficient, but does code require that? What if we were to slightly enlarge the supply duct and supply makeup air at the hood equal to that being drawn out? Is this allowed by code, even if it isn't the best practice? Then, to get the 300 CFM of outside air required by code for the seating area, could we attach a heat recovery ventilator onto one of the existing residential units in the mezzanine, which we can duct through a side wall? Any thoughts?
Ask the inspectors. They know the code and have seen many applications and variations on the same problem. They may be able to suggest an idea.