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Explanation: Depending on how you phrase your sentence, you may be able to get away with "central heating" alone, wiht no additional noun.
If yu feel you must have a noun, "system" and "installation" are generic enough.
If the reference is to the actual, physical installation, "plant" is the most commonly used term for a big installation (big enough for a hospital). "Unit" may be taken by some readers to imply a smaller installation.