According to Merriam-Webster Unabridged Dictionary,
- Air-conditioning, when used as a noun, is hyphenated. The house has air-conditioning.
- Air conditioner, a noun, is open. The air conditioner is on the fritz.
- Air-conditioned: Both as an adjective and a verb is hyphenated. The air-conditoned room. Air-condition a building.
The above article is an update.
Back when I wrote this article (June 14, 2013), the below was true. I’m leaving this for posterity and also as a demonstration that English is a living language—it constantly changes. This is especially true of compound words, which usually start as open, and then over time as they become more widely used are hyphenated, and then sometimes they eventually become one word.
- Air conditioning, when used as a noun, is open. The house has air conditioning. I’ll agree with this one 🙂
- Airconditioner, a noun, is closed. The airconditioner is on the fritz. Personally, I think this looks like crap 🙁 My personal choice would air conditioner.
- Air-conditioned: Both as an adjective and a verb is hyphenated. The air-conditoned room. Air-condition a building. Also agree with this one.
Now, check out this photo. I just took these screenshots at @ 8 p.m. June 14, 2013. The “older” version of the dictionary is supposed to be the free version, but yet they’ve got their crap more together on the air conditioning thing than the unabridged version. Merriam-Webster sucks swamp water sometimes, swear to God.