Heat Detector Types Explained: Fixed and Rate-of-Rise

A heat detector is a fire detector that responds to temperature rather than smoke. It operates either when the air around it reaches a set temperature, or when the temperature rises faster than a set rate, or both. They are slower than smoke detectors for most fires, but far less likely to be set off by steam, dust or cooking fumes, which makes them the right tool for a particular set of spaces.

This article explains how heat detectors work, the main types and classes, where they belong and where they do not, and the practical issues of placement and maintenance in buildings in Singapore.

How a heat detector works

All heat detectors contain a thermal sensing element exposed to the surrounding air. In modern electronic detectors this is usually a thermistor, a resistor whose value changes with temperature, monitored by a small circuit. Older mechanical designs used a bimetallic strip that bends with heat to close a contact, or a fusible element that melts at a set temperature, much like a sprinkler head.

The detector sits at ceiling level, because hot gases from a fire rise in a plume and spread outward under the ceiling. As this hot layer reaches the detector, the element warms and, once the response criteria are met, the detector signals the fire alarm panel. In an addressable system the panel may receive the actual temperature reading and make the decision itself. In a standalone domestic heat alarm, the alarm sounds from the unit.

The response depends on more than the air temperature. The detector’s own thermal mass means it lags behind the air around it, and the speed of the hot gas flow affects how quickly heat is transferred. A detector at the edge of its coverage area or behind a beam will respond later than one directly above the fire.

Fixed temperature and rate-of-rise

Fixed temperature

A fixed temperature detector operates when its element reaches a defined threshold. It is simple and very stable in environments where the temperature rises and falls during normal use, such as kitchens and boiler rooms, because ordinary fluctuations stay below the threshold. Its weakness is that a slowly developing fire has to heat the space considerably before the detector operates.

Rate-of-rise

A rate-of-rise detector operates when the temperature climbs faster than a set rate, regardless of the absolute value. It catches fast-developing fires earlier than a fixed temperature detector. It can, however, be triggered by rapid but harmless temperature changes, such as a burst of hot air from an oven door or sunlight on a roof, so it is not suitable everywhere.

Combined detectors

Most heat detectors used in buildings combine both principles, operating on a rapid rise or on reaching the fixed threshold, whichever comes first. Very slow fires are caught by the fixed element; fast ones by the rate element.

Detector classes and temperature ratings

Point heat detectors are tested and classified under recognised standards such as the European EN 54-5, which is one of the standards listed in the SCDF Fire Code’s table of codes and standards for fire detection. Classes are defined by the typical and maximum application temperature for the space and the static response temperature of the detector. The most common class, A1, is intended for spaces at normal room temperature and responds in the mid-50s to mid-60s degrees Celsius. Higher classes are intended for spaces that run hotter in normal use, with correspondingly higher response temperatures.

A class suffix may indicate whether the detector has rate-of-rise behaviour or is a fixed temperature only device. The practical point is to choose a class whose response temperature is comfortably above the normal maximum temperature of the space, but no higher than necessary, so that the detector is neither prone to false alarms nor unduly slow.

Where heat detectors belong

Heat detectors are used where smoke detectors would raise frequent unwanted alarms, or would be damaged or contaminated:

Kitchens and pantries with cooking
Laundries and drying rooms with steam
Car parks and loading bays with exhaust fumes
Boiler rooms, generator rooms and some plant rooms
Dusty workshops and certain industrial spaces
Bathrooms in hotel rooms or serviced apartments where steam is a known nuisance source

They are not a substitute for smoke detection where early warning matters for life safety. In bedrooms, hotel rooms, corridors and escape routes, smoke detection gives the time occupants need. A heat detector in a bedroom may not respond until a smouldering fire has already filled the room with smoke. This is why Singapore’s home fire alarm device requirement relies on smoke detection, as described in our article on the home fire alarm, and why heat alarms in homes are best used as a supplement in kitchens. For a comparison across all detection types, see our guide to the fire detector.

Placement, spacing and the building fabric

Heat detectors cover a smaller area than smoke detectors and must be placed closer together, at spacing set by the applicable standard. In Singapore, clause 6.3 of the SCDF Fire Code 2023 requires automatic detectors in a building fire alarm system to comply with SS 645, the local code of practice for electrical fire alarm installations, for their type, location and spacing.

The building’s structure directly influences coverage. Deep beams act as barriers that hold hot gas in one bay, so detectors may be needed in each bay. Sloping roofs collect heat at the apex. High ceilings allow the plume to cool and spread before reaching the detector, which reduces the effectiveness of point heat detection, and above certain heights other methods are preferred. Detectors should also be kept away from air conditioning diffusers and supply air jets, which can cool the element or blow the hot layer away.

When a renovation lowers ceilings, adds bulkheads or changes room layouts, the detection layout should be reviewed. Changes to an approved system are carried out by a competent fire alarm contractor, with submissions made through a Qualified Person where required.

Linear heat detection and maintenance

Linear heat detection uses a sensing cable rather than a point device. Some types use two conductors separated by heat-sensitive insulation that melts at a set temperature, causing a short circuit at the point of heating. Others measure temperature along an optical fibre. They protect cable trays, tunnels, conveyors, escalator trusses and car park ceilings where point detectors would be impractical, and some types can report the location along the cable.

Heat detectors are robust, but they still need periodic functional testing with a heat source, and inspection for paint, damage or obstruction. Detectors that have been painted over during redecoration should be replaced. Electronic detectors have a finite service life and should be renewed according to the manufacturer’s guidance and the maintenance regime.

Frequently asked questions

Is a heat detector or a smoke detector better?

Each suits different spaces. A smoke detector gives earlier warning for most fires and is preferred for life safety in bedrooms, corridors and escape routes. A heat detector is better where smoke, steam or fumes occur in normal use, such as kitchens, laundries and car parks, because it avoids repeated false alarms.

Can I install a heat detector in a kitchen in Singapore?

Yes. A heat alarm is a sensible addition in a home kitchen because smoke alarms there would be triggered by cooking. It should supplement, not replace, the smoke detection required in the home’s circulation area under the home fire alarm device rules for new homes and those undergoing fire safety works.

What is a rate-of-rise heat detector?

It is a detector that operates when the temperature climbs faster than a set rate, regardless of the actual temperature. It detects fast-developing fires sooner than a fixed temperature detector. Most building detectors of this kind combine rate-of-rise with a fixed temperature threshold so that both fast and slow fires are caught.

Why do heat detectors need to be spaced closer than smoke detectors?

Hot gas from a fire cools as it spreads under the ceiling, so a detector further from the fire may not reach its response temperature in time. Smoke travels further while remaining detectable. The standard therefore allows a smaller coverage area for each heat detector than for each smoke detector.

Do heat detectors work on very high ceilings?

Their effectiveness falls as ceiling height increases, because the fire plume cools and mixes with air before reaching the ceiling. For high spaces such as atriums and warehouses, designers often choose beam smoke detectors, aspirating systems or flame detectors instead.

Changing ceilings, layouts or uses in a building and need the engineering checked? Talk to our consultancy or get in touch.