Nursing Home Attic Conditioning
I'm designing the hvac system for a two story nursing home with mezzanines in the attic for the hvac equipment. I've got it all figured out (I think) except for the attic. The insulation is above the roof deck. We're using AHUs for the common areas, ptacs for the resident rooms, and an energy recovery DOAS for ventilation. My boss said we should just put return/exhaust diffusers in the common areas connecting to the attic and use it as a plenum to condition it. There are a few issues with this, and I can't seem to wrap my head around it. We can't use the DOAS to exhaust this air from the return diffusers because all of the exhaust has to be ducted (restrooms, soiled linen, shower areas, etc). We could use the attic as a plenum for the return air for the AHUs but the DOAS needs 500mbh of heat with only 14mbh of cooling so I was planning to use a gas fired DOAS but gas fired equipment is not allowed in plenums so I'd have to have a rather large condensing unit. She showed me a similar project they did a few years ago, but it was a school, and the DOAS was exhausting more than half the air from the classrooms using the attic/mezzanine as a plenum. I'm kind of stumped. The energy model says I need 90 tons of cooling if I use a separate ahu to cool the attic or if I don't condition it at all I need to add about 60 tons to the rooms below (I know, I've got to somehow condition it to avoid possible condensation). My boss insists that we shouldn't need much if any additional cooling because it worked on another project (she didn't do that design or work on the project, but she reviewed the old design). I'm a little stumped. Can you give me any insight? TIA