Heat detector Spacing are tested and given as listed . For example. one detector might have a listed spacing of 30 ft. while another might have a listed spacing of 5o ft. On ceilings up to 10 ft high.the heat detectors can be installed using the listed spacing. That is. you can’t exceed the listed spacing measuring between detectors and you cant exceed one- half the listed spacing measured perpendicular to a wall. Note that when the detectors are evenly spaced. the corners of the room are actually farther than one-half the listed spacing. This distance is actually 0.7 times the spacing.
On ceiling heights greater than 10 ft. the NFPA 72 code has a table with reduction factors.For example. on an 18 ft high ceiling, the factor is 0.71. So. A detector with 30 ft listed spacing would be spaced at a distance no more than 0.71 x 30 ft. equaling 21.3 ft between detectors
These rules apply to smooth. level ceilings. A smooth. Level ceiling is one where there are no continuous obstructions such as beams or joists that are more than 4 in. in depth. The Code contains very specific definitions of non-smooth joisted and beamed ceilings and also has spacing reduction factors that you will have to apply in those situations.
This leads to another rule that says you can exceed the allowable Heat Detector spacing, provided that all points are within 0.7 times the spacing. This is sometimes called the “radius of protection” or the “radius of coverage.”
Any room that can fit in the circle of coverage is effectively covered by that detector as per NFPA72.