The point is that the first floor AC unit dose not have to deal with the roof load.
Kind of irrelevant. A load calculation will determine the size including the the "roof load". One system costs less than two systems, and if you have the proper airflow and duct layout, one properly sized system can handle the load just as well if there were two systems.
Some of the decision may depend on how the ducting is run. If you have one 5 ton to heat/cool then you would somehow have to split the ducting in half to let the two separate 2.5 ton work properly.
I'm guessing that there is NO ductwork in the new addition, which would add greatly to your cost, particularly if its being run in an unconditioned space (sheet metal, flex, collars, tape, mastic... adds up). Added if it's a heat pump, you will have to run electric for the aux heat which adds even more to the cost. Sometimes I scratch my head thinking what the builder thought as far as the space given to install an HVAC system.
The above then asks the question if a mulit zoned dutless mini split system would work?
I have a unit for upstairs and one for downstairs. Great being able to turn the upstairs up to 77 or 78 and the bottom on 70. We have a return air for the downstairs unit at the top of the stairs and so actually the downstairs unit does cool upstairs to some extent so it seems the upstairs hardly ever runs. But no question I would go with 2 units.
The house I bought has three systems. NOT the way I would of planned it, but it's what the contractor was "stuck" to work with. The one nice thing about three systems though is that if one goes down in the heat of summer, you can "live" in within another working system in the house while having the problem fixed.
Since you can control better, you don't have to over cool or over heat one area to get the other one comfortable. Go with the two units and you will operate cheaper and be more comfortable.
Overall, a properly sized system with the proper ductwork, zoned properly will work better IMO than two systems and most likely cost less to run than two seperate systems. Short cycling hurts todays equipment more than helps.
Problem is, no one ever seems to plan for adding on to their house.
I would do a heat load calculation for the 1 st floor & 1 for the 2nd floor
Doesn't that come with AC directs price? LMAO
considering the seer rating being the same on all units:
How old is the existing system? I'd guess probably a 10 SEER?
Is the addition finished or still in the process of building?
I would suggest that your daughter and son in law call three HVAC contractors to see what they propose. Take my word for it, don't always go with the lowest price. Problem is, the information given on the actual newly built space given here is limited and trying to determine the best course of action to take is best by eyeballing the job in person by a person who should know what they are talking about.
