In summer, you need cold angry gas machine.
Sometimes, occasionally, mercifully, something is actually simple in Chinese. This is sort of one of those cases, but sort of not.
The word for air conditioner is 冷气机 [leng3 qi4 ji1]. 冷 means cold, and 气 means air, angry or gas. 机 means machine. So air conditioner is "cold air machine". Makes sense.
Here's the air conditioner from my brother's old apartment in Beijing:
For short, you can call air conditioner 冷气. We don't usually add the 机.
The only mildly confusing thing is that the full word for machine is 机器 [ji1 qi4]. Of course, both of these words mean "machine". So, "machine" in Chinese is really "machine machine".
If you use the common short form of air conditioner, it sounds like "cold machine", but it's really "cold air"!! Rookie mistake to think it'd be as simple as "cold machine". Although, if you pronounce it correctly, no one will know you're using the wrong word.
(Yes, the word for "heater" is 暖气机, or just 暖气 for short. The word 暖, pronounced nuan3, means "warm".)