We have a packaged HVAC on the roof and the AC needs to be replaced. It's 23 years old and dying/dead. We were considering buying just an AC and keeping the package unit running for heat.
We feel that the AC always dies first anyway so why buy another package unit when the gas furnace part of it is always still in good operation when the AC goes. On top of that, split-system AC units are always rated for a higher efficiency and we live in the Vegas desert so that is very important to us.
Are we being crazy? This means we'd have to split the ducting somehow. Also, when it's time to replace the furnace, where will it go? Can it go on the roof or will we have to change the gas piping, and the ducting again, to place the unit somewhere inside?
Is this a heat pump with supplemental gas heat?
Sounds like the shuttle valve is stuck or something in the cycle reversing has failed, heatpumps do both AC and heating.
Why double the misery by having two systems to fail and not having the current one repaired/replaced?
As to new systems, you can get very efficient heat pumps with supplemental gas heat, we just installed such a system in our business. During low level heat needs, it runs in heat pump mode, when the temperature outside drops below a certain settable level, the gas portion runs in tandem. The gas section is the new high-efficiency type that extracts so much heat that they use a PVC stack and water drain for the condensate. You'd kill two birds with one stone, having AC and more efficient gas heat.
You'd need an air-handler somewhere downstairs, taking up the same amount of space (and nearly the same cost) as a furnace. I can only assume that the increase in efficiency is due to having the unit being within the building envelope, instead of on the roof.
Do a full, new split system downstairs. I might endeavor to have the old unit remain as a back-up, with its ducts removed and a simple grate on the ceiling. Although, at 23yo it's got to be on its last leg, too.
IME, rooftop units are for when you want more than ~5 tons of AC, and don't want dual-split systems.
Edited excerpt from pitzersonehour.com: (short answer: packaged are cheaper to install and cost more to run, split systems; the opposite)
Split air conditioning system
When you’d want this: It’s a more cost efficient choice if your home already has a [reliable] furnace, and you are purchasing a new air conditioning unit or replacing an older unit.
Packaged air conditioning system
A company I work for will up-charge you if it's a horizontal attic install or in a crawl space ('cause that ain't fun and does require more work).