I own a very old home that was converted into a triplex during the housing shortage in the 40's. At some point a central heat/air conditioning unit was installed which services the entire home, and where both cold air return ducts are located in only one of the units (the larger side that I inhabit). I have been told this is against code due to the circulation of shared air throughout the apartments, and have been trying to find a lower cost solution than having to install separate central units to each apartment. I have also been told that the system in place is too large to service just one of the apartments, so at this point, I would have to install 3 new units at $15K+ to bring it up to code. The other two apartments have window units to supplement and provide climate control, but I would prefer if I could eliminate the shared air somehow while keeping the costs reasonable and without having to remove the system that is in place. I have very little knowledge with how these systems work, so any suggestions would be greatly appreciated... thanks!

Couple of good AC people to answer here, hope they see this post. Additional return air ducts may be an option.

Thx Stratmando- still think there would be common air circulating though

I think you might be forced into separate units, the danger might be from a fire located in the apartment with the return air and blowing the smoke and flames into the other apts.
The only way to get around it would be to bring in 100% outside air, but this would be very inefficient and may not handle the heating and cooling loads. Good luck, Mike

Could have smoke detectors in each unit, so if smoke is detected, AC shuts down?

Thx!- wonder if that is possible...

Also- can you or maybe acetc give me info on central units and what the max/min square footage can be in proportion to the unit's specs/capacity (btu's, tonage, etc.. ) Want to make sure I couldn't use the system to service my apt only as it is fairly large- consists of about half the total sf of the home, and the upstairs unit only gets one of the servicing ducts anyway...

Older homes normally did not have insulation in the walls and were built fairly loose. The old thumb says to figure about 400 square feet per ton of cooling. If you use it just for your area of the house you need to be sure the duct system will carry the air that the unit will need to move or you might have problems with iceing the coil. If you know the size of the a/c and the size of the duct runs to your apt. I can tell you if it will work and what to do if it won't. Mike

With out an alarm system to hook to(They use 12 volts and contacts).
You could use regular smoke detectors, interconnect them. The interconnect wire has 120 or 9 volts. I forget which is yellow, and which is orange, When smoke is detected and alarm sounding, that would energize relay, opening contacts. The contacts would interrupt the red wire to thermostat, shutting down AC. You could do actual connection at air handler.