I have a 10 year old 10SEER Trane outside and inside unit. The coil is leaking freon in a few spots. The first technician gave me a quote to replace the outside and inside air handler with a 13SEER for around $4800. The second technician I called came out and suggested only replacing the outside unit with a 13SEER for $2000. He told me that the new outside unit would work with the inside unit.

Out of the 8 quotes I received, only two recommended only replacing the outside unit. Is there a reason I should replace both?

I have a 10 year old 10SEER Trane outside and inside unit. The coil is leaking freon in a few spots. The first technician gave me a quote to replace the outside and inside air handler with a 13SEER for around $4800. The second technician I called came out and suggested only replacing the outside unit with a 13SEER for $2000. He told me that the new outside unit would work with the inside unit.

Out of the 8 quotes I received, only two recommended only replacing the outside unit. Is there a reason I should replace both?

Replace both.

While it can be made to work they would have to add a expansion valve and then it will still cost more in energy usage. All systems sold today must be 13 SEER or higher and using the old coil it will not make the 13 SEER level. Anyone who recommends not replacing the inside coil is doing you a disservice and should not be considered for your job. Demand a quality job and get what you are paying for. A old coil that has been used for years is probably dirty as in restricted,the condensate pan is probably rusting out, and will not really perform correctly with the new unit. Important note: The line set (copper lines that connect the inside to outside) should also be replaced. The 13 SEER units require a larger diameter lineset on the suction size in most applications. Typically they went from 5/8 to 3/4 OD for 2 to 3 ton units and larger fom 3 to 5 tons.

See below for my opinion on this from class.

I totally agree with the above answer. You may look into a brand which offers a warranty that will be worthy of price. Now as of March of this year, you can get a full 10 year parts warranty on Payne and Carrier units. All you have to do is register. 10 year warranty will be better investment than saving few hundred now going with a lower priced unit. Installation of your system will be the life of it. Make sure they get it inspected, and do the good to best options. Also as of 2010, just over year and a half the only type units sold will be R410-A. You may want to do it now. Good luck.