Fire Sprinkler Glass Bulb Colors & Temperature Ratings Explained

Fire sprinkler glass bulbs play a critical role in determining when a sprinkler activates during a fire. Each bulb is filled with a heat-sensitive liquid that expands as temperature rises. When the liquid reaches its designed activation point, the bulb bursts and the sprinkler discharges water to control the fire.

Glass bulb colors are standardized to represent different temperature ratings. Red bulbs (68°C/155°F) are the most common and are used in ordinary environments such as offices, corridors, and residential buildings. Yellow (79°C) and Green (93°C) bulbs are used in moderately warm or mechanical areas where ambient temperatures are higher. Blue (141°C), Purple (182°C), and Black (204–260°C) bulbs are designed for high-temperature industrial spaces such as boiler rooms or manufacturing zones.

Choosing the correct temperature rating is important for reliable system performance. A bulb rated too low may activate unintentionally in hot environments, while one rated too high may delay activation during a fire. Following NFPA standards and matching bulb color to ambient temperature ensures safe and effective fire protection.