Unlike smoke detectors, which have a defined nominal spacing of 9.1m x 9.1m (30ft x 30ft), heat detectors do not have a fixed coverage area in the code. Their spacing is dependent on a critical variable called the Listed Spacing, which is determined by the Manufacturer.
Therefore, there is no single "fixed" answer.
However, the methodology according to NFPA 72 (Section 17.6.3: Location and Spacing) is as follows:
1. Consult the Datasheet: Identify the Listed Spacing provided by the manufacturer for the specific detector model. Let’s assume it is 12m, and we will refer to this as (S).
2. Application Scenarios:
o Flat and Smooth Ceilings (Height up to 3m / 10ft):
The coverage radius (R) is calculated as . In this case: .
o Sloped/Irregular Ceilings or Heights exceeding 3m (10ft):
This involves more complex code details. For example, you must refer to the Reduction Factor Table in NFPA 72. If the reduction factor is 0.91, the adjusted spacing becomes your new basis. The radius would then be: . This adjusted value is what you use for your layout and distribution.