What must be maintained for a heat detector set for a 25-foot spacing when placed in different ceiling conditions?

For a heat detector set for a 25-foot spacing, maintaining a maximum of 64% of the listed spacing is critical in ensuring effective detection under varying ceiling conditions. The rationale behind this is that the performance of heat detectors can be adversely affected by the specific characteristics of the ceiling, including height, materials, and insulation. These factors can influence how heat rises and spreads within an environment, which can impact the detector's ability to sense temperature changes indicative of a fire.

By adhering to the maximum spacing guideline of 64% of the specified distance, you account for these variations and maintain the detector's efficiency in responding to heat. This ensures that detectors are placed adequately close together to detect heat buildup even in less-than-ideal circumstances, thereby enhancing overall safety and compliance with fire code regulations.

This approach contrasts with simply keeping the same spacing as listed or applying minimum criteria, both of which may not provide adequate coverage in altered ceiling conditions.