Heat detector

A heat detector is a fire-detection device that operates when its sensing element reaches a specified temperature or responds to a sufficiently rapid temperature rise. It detects thermal conditions rather than smoke particles. Its response depends on heat reaching the device and then changing the temperature or physical behaviour of the sensing element.

Hot gases moving beneath a ceiling commonly provide the exposure that heats the element. Gas temperature, gas velocity, detector position and thermal lag all affect when the device operates. The surrounding gas can therefore be hotter than the detector's nominal operating temperature before a fixed-temperature device responds.

Heat detectors include fixed-temperature, rate-of-rise, rate-compensated and combined arrangements. These mechanisms have different responses to fast and slow heating. Engineering assessment connects the selected mechanism to the expected fire, the normal environment and the time available for the intended protective action.

What a heat detector senses

A heat detector responds to thermal conditions at its sensing element. Fixed-temperature devices operate when the element reaches a specified temperature. Rate-of-rise devices respond to a sufficiently rapid temperature increase. Iqbal and Salley describe both principles in NUREG-1805, a 2004 handbook prepared for United States nuclear fire protection inspections.

The detector forms an initiating input to a Fire alarm system; its operation does not itself mean that occupants have received a warning or that suppression has begun. The SFPE Handbook separates detection, processing and signalling when analysing the system. Those later functions can introduce additional time after the heat-sensitive element operates.

Heat detection requires sufficient energy to reach and heat the element, while a Smoke detector instead responds to combustion particles, which can provide an earlier indication in suitable fire conditions. A Flame detector senses emitted radiation and has different exposure requirements. The appropriate detection principle depends on the expected fire signature and the objective of the system.

Heat detection therefore cannot be described as universally interchangeable with smoke detection. NUREG-1805 associates it with situations such as confined spaces where rapidly developing fires are expected or environments unsuitable for other detection. The same source warns that thermal detection may be too slow for an early life-safety warning. These observations concern the relationship between the fire and the sensing mechanism.

Fixed-temperature and rate-of-rise mechanisms

A fusible fixed-temperature element uses a material that melts at its operating temperature. Melting releases a mechanism or closes contacts to initiate the signal; the fusible element is not restored simply by cooling. NUREG-1805 distinguishes this arrangement from bimetallic devices that can return to their original condition after heating stops.

Bimetallic devices use metals with different thermal expansion behaviour: a strip can bend, or a shaped disc can reverse curvature when heated. This movement operates the electrical contacts. Electronic thermal detectors instead monitor a temperature-dependent electrical resistance and can apply a fixed-temperature or rate-of-rise criterion.

A pneumatic rate-of-rise detector uses expanding air in a chamber or tube: a small vent accommodates gradual changes, while sufficiently rapid heating produces pressure faster than it can escape. That pressure operates a diaphragm and contacts. NUREG-1805 describes example operating rates around 7–8 °C per minute; this is not a universal threshold for every product.

Heat detection can cover a compact point or extend along a line. NUREG-1805 describes a continuous cable in which heat-sensitive insulation separates two conductors until the operating condition is reached. Another arrangement uses temperature-dependent electrical conduction within the cable assembly. Pneumatic line detection instead responds to expansion within a length of tubing. The exposed length and the distribution of heating then affect the signal.

Combination devices can include both fixed-temperature and rate-of-rise functions: the rapid-rise element can respond during fast heating, while the fixed-temperature function remains relevant during slower heating. Rate-compensated devices use differential expansion to reduce the effect of thermal lag. Their internal mechanisms differ from a simple element represented by one uniform temperature.

Fixed-temperature and rate-of-rise detection principles. Based on Iqbal and Salley (2004), pp. 12-3–12-4 and Schifiliti, Meacham, and Custer (2002), p. 4-19.

Transport from the fire to the sensing element

Hot gases rise in a plume and spread beneath a ceiling as a Ceiling jet. A detector in this flow receives heat from the moving gases. Both gas temperature and gas velocity affect the transfer of energy to the element. The International Fire Engineering Guidelines (IFEG, 2005) also identify the element's distance below the ceiling as relevant.

The temperature and velocity are not uniform throughout the flow, and their values change with location relative to the fire and with depth below the ceiling. A detector placed outside the relevant hot flow does not receive the exposure assumed for a detector within it. Geometry is therefore part of the response calculation rather than a separate installation detail.

Thermal lag is the delay caused by the sensing element taking time to heat. During rising gas temperature, the element can remain cooler than the surrounding gases. Consequently, the gas temperature at operation can exceed the detector's nominal operating temperature. The rating applies to the element's response, not to instantaneous equality between gas and element temperatures.

The Detector activation time contains the effects of exposure and thermal response. A steady-exposure calculation begins when the assumed hot gases reach the element, so it does not automatically include the earlier transport of those gases from the fire. Schifiliti and colleagues explicitly identify this transport omission in the simplest steady ceiling-jet model.

Heat-detector response depends on ceiling-jet exposure and thermal lag. Based on Australian Building Codes Board et al. (2005), p. 2.7-2 and Schifiliti, Meacham, and Custer (2002), p. 4-7.

Thermal response and response time index

A common model treats the sensing element as a single uniform thermal mass. This is called the lumped-mass approximation. It assumes that temperature differences within the element can be neglected. The temperature then increases according to the heat transferred from the gas and the element's thermal response.

The Response time index (RTI) expresses thermal sensitivity in this model. For a given gas exposure, a smaller RTI gives faster heating of the element. The governing equation relates the rate of element heating to gas velocity and the temperature difference. As the element approaches the gas temperature, that driving temperature difference becomes smaller.

$$\frac{d{T}_d}{dt}=\frac{\sqrt{u}}{\mathrm{R}\mathrm{T}\mathrm{I}}(T_g-T_d)$$

Symbol	Description	Unit
$T_d$	Temperature of the detector's sensing element	°C
$T_g$	Gas temperature at the sensing element	°C
$t$	Time measured from the start of the assumed exposure	s
$u$	Gas velocity at the element	m/s
$\mathrm{R}\mathrm{T}\mathrm{I}$	Response time index	m1/2s1/2
$T_a$	Initial element temperature, equal to ambient in this model	°C
$T_r$	Element temperature required for operation	°C
$t_r$	Time to operation under the assumed constant exposure	s

For constant gas temperature and velocity, the equation can be integrated to obtain the time to reach the operating temperature. The expression below uses the natural logarithm, and it applies when the element starts at ambient temperature and the gas temperature exceeds the operating threshold. It represents fixed-temperature response under the stated thermal assumptions.

$$t_r=\frac{\mathrm{R}\mathrm{T}\mathrm{I}}{\sqrt{u}}\ln \left(\frac{T_g-T_a}{T_g-T_r}\right)$$

The ratio inside the logarithm uses temperature differences, so consistent Celsius differences can be used. The response time becomes longer when the operating temperature approaches the imposed gas temperature. If the gas never becomes hot enough, this constant-exposure model does not predict threshold operation. A nominal temperature rating alone is therefore insufficient to calculate a response time.

Element temperature lags behind gas temperature. Based on Iqbal and Salley (2004), p. 12-3 and Schifiliti, Meacham, and Custer (2002), p. 4-7.

Worked example under constant exposure

The SFPE Handbook's second example uses the gas conditions calculated for a 4 m² kerosene pool beneath a flat ceiling. The ceiling is 6 m above the fire and the detector is 6 m horizontally from the plume centre. The ambient temperature is 20 °C. Its calculated exposure is a gas temperature of 153 °C and a velocity of 2.4 m/s.

The example detector has an RTI of 55 m^1/2s^1/2 and an operating temperature of 57 °C. Substituting those values gives an exposure response time of approximately 11.6 s. Rounded upwards to whole seconds, the required time is 12 s. This reproduces the response reported in the handbook example.

$$t_r=\frac{55}{\sqrt{2.4}}\ln \left(\frac{153-20}{153-57}\right)\mathrm{s}\approx 12\mathrm{s}$$

This is the time for the specified element to heat under the specified exposure. It is not a claim that a detector will alarm 12 s after every kerosene fire ignites. The example assumes the calculated gas conditions are already present and remain constant. Fire growth, transport to the detector and subsequent alarm-system delays require separate treatment.

For growing fires, Schifiliti and colleagues divide the changing exposure into short time intervals. The updated element temperature is carried from one interval into the next. Gas temperature and velocity can then change as the fire grows. This approach preserves the element's heating history instead of restarting the calculation at ambient temperature each time.

What tests and prescribed inputs represent

A plunge test exposes the device to a gas flow with known, constant temperature and velocity. The measured operating time can be used to infer a thermal time constant and RTI. The SFPE Handbook explains that this is a simplified representation of heat transfer. Flow around the device and heat loss into its supporting body can limit the approximation.

Listed spacing is derived through defined test arrangements, but it does not directly state the time to respond to every fire in an actual building. Ceiling height, beams, airflow and detector exposure can differ from the test. Schifiliti and colleagues distinguish compliance with spacing rules from quantitative assessment against a specified fire-size or response objective.

In New Zealand, C/VM2, second edition (2025), prescribes heat-detector modelling inputs regardless of the installed make or model. These include RTI 30 m^1/2s^1/2, an activation temperature of 57 °C and a radial distance of 4.2 m. The table also specifies a position at least 25 mm below the ceiling. These values belong to that verification method's analysis.

RTI describes the interaction between the sensing element and the moving gases; it is not a temperature rating or a direct statement of the time from ignition. The same RTI can give different response times when gas temperature or velocity changes. Likewise, two detectors with the same operating temperature can respond at different times because their thermal sensitivities differ. These distinctions explain why both temperature and sensitivity appear in the model.

The prescribed values must not be mistaken for a universal specification of actual detectors. A product can have measured characteristics different from those analysis inputs. The source distinguishes the required simulation from the device selected for installation. An engineering report therefore identifies whether its inputs are measured, assumed or prescribed by the chosen method.

Choosing between thermal response principles

A fixed-temperature device and a rate-of-rise device respond to different features of the same exposure. A rapid temperature increase can activate the rate-of-rise function before the fixed-temperature element reaches its threshold. During slower heating, the rate-of-rise condition may be less favourable. Combination devices use both mechanisms to address these different conditions.

The handbook examines this distinction using several fire growth rates, and its examples show that detector spacing and response depend on the growth history as well as sensitivity. A device that responds earlier in one scenario need not do so in another. The result supports comparing detectors against the intended fire scenarios rather than ranking the mechanisms universally.

Rate-compensated and pneumatic rate-of-rise devices require particular care in modelling, and their internal heat transfer and mechanical response are more complex than one directly exposed element. The SFPE Handbook explains that a single lumped mass may not represent these mechanisms adequately. Applying the fixed-temperature equation unchanged would omit parts of the device's operating principle.

Environmental effects and calculation limits

Normal non-fire temperatures influence the selection of a thermal operating rating. NUREG-1805 discusses higher-temperature devices for locations where ordinary ambient conditions would otherwise cause premature operation. The relevant exposure is the temperature near the detector, including expected non-fire heat sources. A rating chosen without those conditions can produce unwanted alarms.

The report's simple ceiling-jet method assumes an unconfined flat ceiling and a fire away from walls and corners. It excludes a developed hot layer beneath the jet during the period of interest. Obstructions and irregular ceilings can undermine the assumed flow. The method also does not explicitly include direct radiant heating of the sensing element.

The assumption of a constant RTI is itself an approximation. Schifiliti and colleagues discuss the complex gas flow around irregular detector shapes and the possibility of heat conduction into supporting parts. A plunge test may involve conditions in which those heat losses are relatively small, but their importance can differ during the slower flows of an actual fire. A thermal parameter obtained from one test condition therefore needs an appropriate range of application.

Modelled response after another protective device operates also requires care. The handbook notes that its basic calculations do not account for sprinkler cooling when predicting subsequent detector or sprinkler activation. The first operating device can change the environment experienced by other devices. Their later responses cannot automatically be inferred by continuing the original unsuppressed exposure unchanged.

The gas-flow calculation and the element-response calculation have separate limitations. Correctly integrating a thermal equation cannot compensate for an unsuitable gas temperature or velocity prediction. Conversely, suitable gas conditions do not establish the correct thermal model for every detector mechanism. The assessed geometry, fire development and device behaviour must all fit the chosen method.

See also

• Multi-sensor detector
• Thermal inertia
• Fire plume

References

• Australian Building Codes Board, National Research Council of Canada, International Code Council, & Department of Building and Housing New Zealand. (2005). International fire engineering guidelines. Australian Building Codes Board.
• Iqbal, N., & Salley, M. H. (2004). Fire dynamics tools (FDTs): Quantitative fire hazard analysis methods for the U.S. Nuclear Regulatory Commission fire protection inspection program (NUREG-1805). U.S. Nuclear Regulatory Commission.
• Ministry of Business, Innovation and Employment. (2025). C protection from fire: Verification method C/VM2 (2nd ed.).
• Schifiliti, R. P., Meacham, B. J., & Custer, R. L. P. (2002). Design of detection systems. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed., pp. 4-1–4-43). National Fire Protection Association.