2) The trap was probably not needed, unless it the unit is a counter flow
system or it is tied into the sewer stack. I doubt that either of these
situations are the case. If it is a counter flow unit the blower can suck
the condensate from the drain and keep it in the unit. If this were the
case you would have had water around the system. If it were tied into the
stack ( you can't do this in many areas) you would probably have had a sewer
smell in the house without a trap. Traps have a problems with getting
clogged, I often remove them for this reason. Hopefully a tee was installed
before the trap so it can be flushed periodically to prevent blockage.
Price to install the trap seems a little high if that's all that was done to
the drain systems.

This particular trap is clear so you can see if it is clogged, and it has
two red lids that can be flipped open to clean it. The technician seemed to
think that having a trap would cause the pan to drain out better than
before, and claimed that there is is even now a code that NEW systems have
to have it. Maybe it was a ripoff. I don't think my system is the kind that
you mentioned, and no it's not tied into the sewer line.

3) must be a large apt/condo unit is a four ton

Yes, 2100 square feet townhouse EXCLUDING the finished basement which has
supply vents in it (although I usually close those since basement is cool
enough in summer)

J.