Detection is a sensing problem before it is a wiring problem. Smoke, heat, flame and gas all announce a fire differently, at different speeds, and each sensor is fooled by a different everyday event. Choosing wrongly does not usually produce a system that misses fires; it produces a system that cries wolf until somebody disables it, which amounts to the same thing.
Smoke detection
Optical detectors sense light scattered by smoke particles and respond well to the smouldering fires that kill people at night. Ionisation types respond faster to fast flaming fires with small particles. Optical is the general purpose choice in most buildings. Both are fooled by steam, dust and aerosols, which is why siting near bathrooms, loading bays and kitchens is a design decision rather than a convenience.
Heat detection
Heat detectors respond to a fixed temperature, a rate of temperature rise, or both. They are slower than smoke detection by design, so they are used where smoke detection would be unusable rather than where it would be better: kitchens, plant rooms, dusty workshops and garages. Using them to solve a false alarm problem elsewhere is trading warning time for quiet.
Aspirating, beam and flame detection
Aspirating systems draw air through a pipe network to a central sensor and can detect at very low smoke concentrations, which suits high value spaces and places where access to a ceiling is difficult. Optical beam detectors cover large open volumes such as atria and warehouses with one path. Flame detectors watch for the radiation signature of flame and suit large open areas with a fast fire risk and no smoke path to a ceiling.
Mixing types in one building
Real buildings need several. The skill is in matching the detector to the space, and the tell of a good design is a schedule that says which type goes where and why. A design that specifies one detector type throughout a building with a kitchen, a plant room and an office is either very simple or has not been thought about, and the false alarm log will tell you which within a year.
Questions people ask about fire detection systems
What is the difference between optical and ionisation detectors?
Optical responds better to smouldering fires with larger smoke particles, ionisation to fast flaming fires with smaller ones. Optical is the general purpose choice for most buildings today.
Which detector should go in a kitchen?
Normally a heat detector, because cooking will set off a smoke detector often enough that somebody removes it. That is a deliberate trade of sensitivity for a system that stays in service.
Are aspirating systems worth the cost?
Where very early detection matters or ceilings are effectively unreachable, yes. In an ordinary office they are expensive for what they add. It is a question about the space rather than about the technology.