When installing detectors on an open joisted ceiling, what should be the maximum spacing for heat detectors or alarms measured at right angles to solid joists?

The maximum spacing for heat detectors or alarms installed on open joisted ceilings is specifically set to one half of the listed spacing when measured at right angles to solid joists. This guideline is in place because open joisted ceilings can create a unique environment for heat accumulation and airflow, which directly impacts the performance and effectiveness of the heat detectors.

Heat detectors are designed to detect temperature changes and activate in response to rising heat, typically from a fire. When installed on open joisted ceilings, it’s important to ensure they are close enough to detect heat effectively, especially since the airflow dynamics can vary significantly compared to solid ceilings. By reducing the spacing to one half, it allows for better coverage and increases the likelihood of detecting rising temperatures sooner, which can be crucial in fire detection scenarios.

Thus, utilizing one half of the listed spacing ensures compliance with safety standards and improves the reliability of the fire alarm system in the given configuration of an open joisted ceiling.