There is a massive difference in residential HVAC techs and commercial HVAC work. I had three bids from different contractors in my house and I specifically called out the whistling return vent, showed them how it sucks the filter in. The filter grill was clearly under sized for the CFM the AHU was pulling. Not one of them thought that was anything to be concerned about. None of them knew the first thing about manual D and calculating CFM for ductwork.
I can only recommend you do the homework and tell them exactly what you want installed. ie; be your own specification engineer. Anyone that's actually good at this sort of work goes commercial exclusively, that's where the money is.
Thanks! When we put in solar twenty years ago they kept patching me through to engineers they normally protect from customers, because I'd learned more than any customer-facing staff.
I can only recommend you do the homework and tell them exactly what you want installed. ie; be your own specification engineer. Anyone that's actually good at this sort of work goes commercial exclusively, that's where the money is.