I had a through the wall air conditioner and removed it to replace. I thought it was as easy as going an purchasing a new unit i.e a window unit (not knowing there was a difference or even that a through the wall unit was a option) I puchased a window unit and ripped out the existing unit. (literally ripped out all of it including the sleeve/casing) How do I measure for a new unit? I found a Frigidaire Frigidaire FFRH1222R2 12,000 BTU 230V Compact Slide-Out Chassis Air Conditioner, but am unsure if it will fit. Any help you can give is greatly appreciated.