Heat detector

Offshore F&G layout practice (ISA 5.1-style letter codes) / ISO 7240 / NFPA 72

Drawn as

A detector bubble with "HD" (sometimes a TSH bubble, or a square in building legends).

Meaning

Fixed-temperature or rate-of-rise heat detector in enclosures, turbine hoods and accommodation. Becomes heat-detector.

Variants

Building fire-alarm legends (NFPA 170, BS 5839) use their own forms; check the legend
Linear heat-detection cable is drawn as a dashed line with LHD