Each closed-head sprinkler is held closed by either a heat-sensitive glass bulb (see below) or a two-part metal link held together with fusible alloy such as Wood's metal and other alloys with similar compositions.The glass bulb or link applies pressure to a pip cap which acts as a plug which prevents water from flowing until the ambient temperature around the sprinkler reaches the design activation temperature of the individual sprinkler head. Because each sprinkler activates independently when the predetermined heat level is reached, the number of sprinklers that operate is limited to only those near the fire, thereby maximizing the available water pressure over the point of fire origin.

The bulb breaks as a result of the thermal expansion of the liquid inside the bulb.[4] The time it takes before a bulb breaks is dependent on the temperature. Below the design temperature, it does not break, and above the design temperature, it takes less time for higher temperatures. The response time is expressed as a response time index (RTI), which typically has values between 35 and 250 m½s½, where a low value indicates a fast response.[5] Under standard testing procedures (135 °C air at a velocity of 2.5 m/s), a 68 °C sprinkler bulb will break within 7 to 33 seconds, depending on the RTI.[6] The RTI can also be specified in imperial units, where 1 ft½s½ is equivalent to 0.55 m½s½.