Answer: Crown of Thorns (Euphorbia milli) is an excellent houseplant for a sunny windowsill. It has a natural period of dormancy, and leaves may drop at this time, but new leaf buds will appear in a month or two. Provide your plant with average household warmth (minimum 55F in winter), and as much light as possible, but shade from direct summer sun. Water moderately from spring to autumn, and sparingly in winter, allowing the soil surface to dry between waterings.
The central air shouldn't affect your plant, unless it is in a direct draft from one of the airducts. I expect that the leaf drop has more to do with its natural dormancy than with the new air conditioning. If you don't want to look at your bare plant, store it in another room until it develops new foliage, then bring it back into view.
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