My contractor has suggested the 17 seer retrofit infinity A/C unit instead of the 21 seer version. Apart from a little less expense is there any reason why the 17 seer model would be preferred, My contractor maintains that the added seer would not be obtained because of my old house etc.I have seen some comments on this board suggesting that in some applications the 17 seer model would be preferable. I live in Washington dc, hot humid summers. in a house that was built in the 1930's with a forced air system. Thank you all for your help. I apologize for the volume of my questions but here is so much to consider once you get involved in a retrofit. thanks again all.