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If the existing electrical box is not fan-rated or improperly supported, it must be replaced. Option A (Direct Joist Mounting): If a ceiling joist is accessible and close to the desired fan location, remove the old box. Install a new "pancake-style" fan-rated electrical box by screwing it directly into the underside of the ceiling joist. You may need to enlarge or adjust the existing ceiling hole to align the new box with the joist. Option B (Expandable Fan Brace): If no joist is directly accessible and there is no attic access (common in sunrooms), use an expandable fan brace. This brace consists of an adjustable metal bar with a fan-rated electrical box attached. Carefully slip the brace through the existing ceiling opening, expand the bar so its feet firmly brace against the ceiling joists from within the ceiling cavity, and secure it according to the manufacturer's instructions.