A line-type heat detector head is a continuous sensing element, typically a heat-sensitive cable, that activates an alarm when the temperature at any point along its run crosses a set threshold, whereas a spot-type heat detector head monitors a single fixed location with a finite detection radius [S1][S5]. The two device classes are not interchangeable: each maps to a different coverage geometry, a different cost model, and a different maintenance routine under NFPA 72 definitions [S1][S4].
For industrial projects (cold storage, conveyor galleries, cable trays, tunnels, power plants, engine bays) the wrong choice shows up as either a string of false alarms, or worse, a coverage gap a fire can hide in [S2][S5][S7]. This piece lays out the decision criteria a spec engineer can actually write into a submittal, with a side-by-side on cost, coverage, environment, and lifecycle.
Definitions, Scope, and Governing Code Language
NFPA 72 defines a line-type detector as "a device in which detection is continuous along a path," and defines spot-type by implication as one where the detecting element is concentrated at a particular location [S1][S2]. In practice a line-type heat detector head is a jacketed cable with two conductors separated by a heat-sensitive polymer; once the polymer reaches its rated temperature, the conductors short and the control unit reports an alarm [S2][S5]. A spot-type heat detector head, by contrast, is a ceiling- or wall-mounted point device, often a bimetallic strip, fusible alloy element, pneumatic rate-of-rise diaphragm, or thermistor-based electronic sensor with a defined listed spacing of typically 50 ft (15 m) for smooth ceilings under NFPA 72 [S1][S4]. Heat detectors are also classified as restorable (self-reset after cooling) or nonrestorable (replace the element or the whole head), and that binary matters for lifecycle cost [S4].
Both head types fall under the same life-safety logic, but their coverage assumptions differ sharply: a line-type heat detector head has no inter-device gap along its routed path, while a spot-type heat detector head leaves the space between two points uninstrumented unless you add more heads [S5].
Selection Criteria: Geometry, Environment, Access, Cost
Four criteria drive the line-type vs spot-type call. Geometry: if the hazard is long and narrow (a 300 ft conveyor, a cable tray run, a tunnel, an elevator shaft, an in-rack storage row), line-type cable covers the full run with one device; spot-type would need dozens of heads plus the wiring to match [S2][S5]. Environment: line-type cable can be ordered with jackets rated for chemical, UV, moisture, or corrosive atmospheres, and one documented food-truck engine-bay comparison selected LHD because the run was inaccessible during operation and the environment was harsh [S2][S7]. Spot-type is the better pick for clean, accessible rooms with predictable ceiling geometry, where standard 50 ft spacing rules are met cheaply with mechanical fixed-temperature or rate-of-rise heads at roughly 135°F (57°C) or 194°F (90°C) common setpoints [S1][S4].
Access: a key practical test is whether the device can be reached with an inspector's test pole without shutting the line down; if the answer is no, line-type with a remotely located test switch usually wins, because the cable can be tested from one end [S2]. Cost: linear heat detection is generally more expensive per foot than a single spot head, but on large or awkward zones the total installed cost often flips because you eliminate dozens of point devices, conduit homeruns, and the labor to mount them at height [S1][S2][S5].
For a spec summary, weigh: (1) zone aspect ratio, (2) environmental rating, (3) accessibility during operation, (4) installed cost per protected square foot, (5) restorable vs nonrestorable service model.
Direct Comparison on Four Decision Criteria
On coverage geometry, line-type heat detector head gives continuous sensing along the entire cable length with no inter-device gap, while spot-type heat detector head gives a defined listed radius around one point (commonly 50 ft / 15 m spacing on smooth ceilings per NFPA 72 practice) [S1][S4][S5]. On environment, line-type cable accepts polymer jackets rated for corrosive, UV, moisture, and chemical exposure, making it the default for cable trays, conveyor galleries, and outdoor runs; spot-type mechanical heads are typically indoor, ceiling-mounted, and intolerant of dust, heavy moisture, or corrosive atmospheres unless explicitly listed for them [S2][S5][S7].
On access for test and maintenance, line-type systems can install a remote test switch in a safe, accessible location and the cable itself can be exercised from that one point, which is why conveyor and tunnel projects consistently pick LHD; spot-type heads need physical reach with a test pole or a lift, which in active process areas means shutdown [S2]. On service model, line-type cable is field-spliceable: the damaged section is cut out and a manufacturer-approved splice is fitted, restoring the run, while many spot-type fixed-temperature heads are nonrestorable and require replacement of the entire head once they trip, though rate-of-rise and rate-compensation units are typically self-restoring [S2][S4].
Sub-Types Inside Each Family
Inside the spot family, the common industrial sub-types are fixed-temperature (trips when the element reaches a preset temperature, often 135°F or 194°F, slow due to thermal lag, ideal where sudden intermittent heat would falsely alarm a rate-of-rise head), rate-of-rise (trips when the local temperature climbs more than 15°F per minute, uses a metal diaphragm over an air chamber, self-restoring), and rate-compensation (responds at a preset air temperature regardless of rise rate, using a tubular metal case that extends when heated) [S4]. Most rate-of-rise heads also include a fixed-temperature fusible-alloy backup that is nonrestorable [S4].
Inside the line-type family, the two practical sub-types are analogue (or digital) heat-sensing cable and fiber-optic linear heat detection. Analogue/digital LHD cable uses the polymer-between-two-conductors short-circuit principle described above and is by far the most common in industrial builds [S2][S5]. A dedicated LHD control unit can report the distance in meters to the alarm point, which turns troubleshooting from a walk-down into a controller readout, a meaningful reduction in mean time to repair on long runs [S2]. For very long runs, fiber-optic LHD measures distributed temperature along a fiber and gives a continuous temperature profile rather than a single threshold, but it sits at a higher price point and is typically reserved for tunnels, mines, and power cable monitoring [S5].
Use Cases and Limitations
Line-type is the documented right answer for: cable trays, conveyor belts (one 300 ft conveyor case study moved from dozens of weatherproof spot heads to a single LHD run with traveler-cable support and a remote test switch), cold storage, in-rack storage of hazardous materials, aircraft hangars, tunnels, bridges, elevator shafts, solar farms, fuel storage, and food-truck engine bays where access during operation is impossible [S2][S5][S7]. Its limitations: higher unit cost, slower response than a well-sited spot head directly above a fast-developing fire, the need for a control unit or addressable interface module, and a one-shot nonrestorable action in the polymer short-cable design unless a restorable digital cable is specified [S2][S5].
Spot-type is the documented right answer for: boiler rooms, kitchens, ordinary office and plant spaces, and any zone where NFPA 72 spacing rules are met cheaply with ceiling-mounted points, where the environment is clean, and where the head is reachable for annual testing [S1][S2][S4]. Its limitations: leaves gaps between devices, cannot cover very long or very narrow runs without dozens of heads, generally unsuitable for outdoor, corrosive, or high-vibration locations unless explicitly listed, and many fixed-temperature spot heads are nonrestorable and must be replaced after one trip [S1][S4]. Heat detectors of any kind are also the slowest fire-detector class, and codes generally prefer smoke detection for life safety where the environment allows it [S4].
Standards, Sourcing, and Trackable Signals
All heat-detector submittals should be reviewed against NFPA 72 for spacing, placement, and testing rules, and against the listing documents (UL 521 in the US, the relevant CE/EN 54-5 class in Europe) for the specific fixed-temperature or rate-of-rise rating on the head [S1][S4]. For an LHD cable, request the polymer activation temperature, the jacket material rating, the maximum single-run length supported by the control unit, and whether the cable is restorable or one-shot, all of which are listed values and not marketing claims [S2][S5]. Verify on the spot head whether the fixed-temperature element is restorable or nonrestorable, since that single attribute drives the maintenance budget for the next 10 to 20 years [S4].
Trackable signals to watch on the next refresh: (1) restorable digital LHD cable gaining UL/EN listings for longer single-run lengths, which would shift cost crossover points further in favor of line-type on mid-size zones; (2) NFPA 72 clarifications on remote test-switch equivalency for LHD, which already drove the conveyor case study but is still project-by-project with the AHJ [S2][S5]; (3) tighter EN 54-5 class definitions on spot-type heat detector heads (classes A1, A2, B, C) as manufacturers push more electronic, addressable spot heads with self-diagnostics into the same form factor.
For component-level specifications, see heat detector, dry type transformer, and heat treatment furnace.
Background reading: ISO 13850 vs IEC 60947-5-5: Function-Level vs Device-Level E-Stop Rules.