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noun, する verb
A warm cluster is when you get a bunch of warmth together, and then use it to warm your home. This is indoor heating, kind of like the same way how an air conditioner is a "cool cluster."
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
Why did you turn off the heat? All the pipes are completely frozen!