They are required on a HVAC unit that exceeds 2,000 CFM and located on the return side when the capacity of the HVAC unit exceeds 15,000 CFM and serves (2) or more floors..
That is the book version, but there is too much information you would have to know about the HVAC system..
Usually the architect spells them out on the plans that have to be approved by the Fire Marshall
There comes a time when people who volunteer their service need to step back and see if it is better for them to retire. A red flag is when they become "cranky" and lose all sense of reality by making decisions that really don't make any sense..