Consider replacing your heating system at the same time you replace your A/C unit, especially if your furnace is more than 15 years old, because connecting an older furnace to an energy-efficient A/C may prevent it from performing at its rated efficiency.
Proper A/C sizing and installation is the key element in deciding efficiency. Too large of a unit will not remove humidity from your home. A unit that's too small will not be able to provide a comfortable temperature on extremely hot days.
What did you do when faced with the decision to repair or replace an A/C? Tell us about it in the comments below.
Editor's note: This is an updated version of an article originally posted Oct. 10, 2012.