Whats the deal with HVAC?

There are seemingly infinite varieties of HVAC system designs, and lots of different pieces can be centralized or not. In a big building, you might see per-floor or per-apartment air handlers (fan units) with a central source of heating and heat dissipation (distributed through cool or hot water loops). Central systems can be more efficient, quieter, better-maintained, more comfortable, better-hidden, and so on.

Also, I would caution you against assuming that a standalone under-window unit is drawing air from a better source than a central HVAC system. The heat it removes from your apartment gets dissipated right there on the outside of the building, so in air conditioning mode it's either (a) only recirculating indoor air, or (b) working harder because it just heated the air it's drawing into your room. And since outdoor air quality in NYC is typically not great, a properly filtered central HVAC system's outside air intake may be preferable on that score.

It certainly depends on the details of how things are built and operated, and if central AC is not for you, there are of course plenty of buildings that don't have it...

I have HVAC in a 100 year old building. I don't share any air with any neighbors.
The venting is all inter not intra apartment. I imagine most are this way and if not you can cut off vents circulating outside your perimeter.

Now I do have a small breech in my HVAC closet where I am receiving outside air. It seemed at first ok as I thought, hey why not some "fresh" air mixed in. Except I have a neighboring building whose fireplace chimney stack when the winds are right brings smoke into my apartment!
I am waiting for later this year when my building's local law (10 is it?) has the scaffolding up to seal it off.

I can tell you this apartment with a mostly closed HVAC system is 10 times cleaner than my last apartment. The accumulation of dust in my old apartment was ridiculous when I compare to where I am now.
And my old apartment had thru wall units, was over 30 stories and just off the east river where visible polution was nil.
The consequence of my much cleaner apartment is changing my hvac filters rather frequently. Small price to pay for an infinitely cleaner apartment.

Sorry got that reversed.....venting is intra (within) instead of inter (between).

soyiuz, you're thinking of central HVAC as in an office building, where there're air handlers for each floor or for groups of floors.

There's no central HVAC like that in NYC apartment buildings. (The only exception I can think of was a building on Madison in the 80s from 1937.)

When you see "central AC" mentioned, it means either:

1. The apartment has its own compressor(s), air-handlers, and ductwork, or

2. The building supplies chilled water to all apartments, with each apartment having its own air-handler either under the window, up under the ceiling, or in a closet, with ductwork.

No shared air in either case. Where you can get cross-apartment leakage is if your exhausts for interior bathrooms and kitchens overcome the exhausts in other apartments or the hallway, and draw in their air.

The early centrally-AC'd building is 25 E. 83rd: http://nyre.cul.columbia.edu/projects/view/16770#images

There're two HVAC equipment rooms on each floor. I don't know whether that 1930's setup is still in use there.

Ah, thank you much for the explanation!

Thank You I got some informative information about hvac from this discussion