If the thru the wall unit is rated for 20A, then yes, use the existing 12/2 wire for 240V. Simply take out the 1 pole breaker and replace it with a 2 Pole breaker, making sure the new double breaker is attached to different legs in the panel. If you don't know how to do this, check a wiring book. Then attach the black wire to one breaker of the pair, and the white wire to the other breaker. Place some black electrical tape on the white wire at the panel and the new unit.

If the new unit is 15A, then use a 2 Pole 15A breaker. If the new unit is rated at 25A or more, you will need to rewire as well as bigger breakers, assuming you now have 12/2.

Thanks for the reply bigbird.

Here is a link to the unit:

http://www.airconditioner.com/specpix/ES12L33.jpg

Please check it out and let me know if you think I can still just switch out the breaker. I checked the current wire that they ran and its 12/2

Thanks again!

Yes, according to your link, you can switch out the breaker to a 2 pole 20A and use the existing 12/2 wire.

Actually, this is a better link with more specs...
http://www.friedrich.com/current/php_print_model.php?modnum=ES12L33&line=TTEH

Thanks!

bigbird_1, sounds good, thanks again for your replies.

one more question, in your original reply you said "Then attach the black wire to one breaker of the pair, and the white wire to the other breaker. Place some black electrical tape on the white wire at the panel and the new unit."

i'm confused as to what white wire you are talking about to place electrical tape around. You said to attach the white wire to the other breaker how do i put electrical tape around it then?

"You said to attach the white wire to the other breaker how do i put electrical tape around it then?"

You put black tape around the white insulation where the wire enters the breaker. You don't tape the bare copper, just the white jacket. The same goes for the white wire inside the box where the unit plugs in. An inch or two of black tape is fine to let anyone working on the wiring know that this white wire is now hot.

Ok makes sense now. I just wanted to make sure I only needed 1 12/2 wire for each 2-pole breaker I'm going to put in.

thanks again!

I have a Frigidaire a/c- heater window unit it says 220 or 110 on the cord I have black, white ground, on the power thru wall is 4 wire b,r,w,g do I hook it up black to black, white from cord to red on power side and ground from cord to white and ground on power side,?

Dave, I don't know what your question is, but if you have a 4-conductor cable coming from the breaker box, usually the black goes to one breaker and the red goes to an adjacent breaker, so that the voltage between them is 220. That gives you both 110 and 220 at the wall box (using the appropriate receptacles).