Door Holder & Damper Interface
How the fire alarm system compartmentalizes a building on alarm — releasing magnetic door holders so doors close, and closing fire and smoke dampers so smoke cannot travel through walls, floors, and ducts. Each device has a distinct trigger: alarm or power loss for holders, heat for fire dampers, a detection signal for smoke dampers.
Devices, triggers, and actions
| Device | Trigger | Action |
|---|---|---|
| Magnetic door holder | Fire alarm activation OR loss of power (fail-safe) | Magnet releases; the self-closer shuts the door to compartmentalize smoke |
| Fire damper | Heat — a fusible/thermal link melts at its rated temperature | Spring-driven blades snap closed in the rated wall or floor penetration |
| Smoke damper | A smoke-detection signal from a duct or area detector | An actuator drives the damper closed to stop smoke migrating through the duct |
| Combination fire/smoke damper | Both — a smoke-detection signal and a heat-actuated link | Closes on whichever occurs first (actuator plus thermal element) |
Active vs passive protection
The distinction that matters in the field is heat versus signal. A fire damper is passive: it needs no wiring and no detector, just a fusible link that melts in the heat of a fire and lets a spring slam the blades shut — the same idea as a sprinkler head. A smoke damper is active: it is wired to a detector or the fire alarm panel and driven closed by an actuator the moment smoke is sensed, long before there is enough heat to melt anything. Smoke moves faster and farther than heat, so smoke dampers catch it early; combination dampers carry both mechanisms.
Magnetic door holders work on the same fail-safe logic as everything in life safety: the normal state holds the door open with power to the magnet, and removing power — whether from an alarm command or a genuine power failure — drops the door to its safe, closed position. There is no way for a wiring fault to leave the door propped open.
Common questions
What makes a magnetic door holder release?
A magnetic door holder releases on either a fire alarm signal or a loss of power — it is fail-safe, so de-energizing the magnet is what drops the door. The self-closer then shuts the door to compartmentalize smoke and fire. Because loss of power drops the door to the safe (closed) state, NFPA 72 does not require a secondary power source for the hold-open function.
What is the difference between a fire damper and a smoke damper?
A fire damper is passive and heat-activated: a fusible link melts at its rated temperature — 165°F is standard — and spring-loaded blades snap shut in a rated wall or floor. A smoke damper is active and signal-activated: an actuator drives it closed when a duct or area smoke detector reports smoke. A combination fire/smoke damper does both, closing on whichever occurs first.
What temperature does a fire damper fusible link melt at?
165°F is the standard rating for typical HVAC applications, with 212°F used in higher-temperature locations. It is not a single fixed code value — the link temperature is selected for the location's temperature class, roughly 50°F above the maximum normal system temperature.
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