Should I replace a 9 SEER condenser from 1989 with a used 12 SEER from 2005?

I currently have a 2 ton Trane XE900 installed which is over 20 years old. I believe it's 9 SEER. There's no problems and everything is working fine. I can replace it with a 12 SEER Rheem RAMC condenser for $400. I would only be swapping out the condenser--not the whole system. The Rheem is used and was installed in 2005 and used for a couple of years. It was swapped out when the owner added more living space and they installed a larger unit. Both systems are R22.

My main question is, would I see a big drop in electricity by just swapping out the condenser? I know that some of the additional SEER benefit comes from efficiencies in the coil and blower and I would not be making those changes. This is in central Texas where we have months of very hot temperatures. I'm wondering how long it would take to recoup the $400. And I'm wondering if it makes sense at all to make this switch. I'd appreciate any advice.

 

It's not a great idea...

1. The used unit most likely has no warranty and it may have unforeseen problems
2. R22 is being phased out. Future repairs could be very expensive.
3. Coils don't last forever - eventually the one will spring a leak
4. A 12 SEER unit requires a larger coil than a 9 SEER unit to operate at rated capacity and efficiency. Mismatching reduces capacity and efficiency; it might be okay to put a (newer) larger coil on an older straight cool condenser (with the a expansion valve), but not the other way around.

 

Thanks for the info. I'll stick with what I have until it craps out and then look into replacements.