Heat detectors respond to the air temperature at the ceiling rather than to smoke. That makes them immune to the steam, dust and cooking fumes that plague smoke detection, and it makes them slower in every fire, which is the trade you are making every time you specify one.
Fixed temperature and rate of rise
A fixed temperature detector operates when the air at the head reaches a set point. A rate of rise detector operates when the temperature climbs faster than a set rate, which catches a developing fire before the absolute temperature is high. Most modern heads combine both, so a slow fire in a cold room is caught by the fixed point and a fast one by the rate of rise.
Choosing the temperature class
Heads come in classes with different operating temperatures, and the class has to suit the normal ambient of the room. A standard class head above a commercial oven or in an unventilated roof space in summer will operate on a hot day with no fire in the building. Choosing a higher class solves that and slows the response, which is why the room's actual temperature range belongs in the design rather than being guessed on site.
Where heat detection belongs
Kitchens, plant rooms, boiler rooms, garages, laundries and dusty workshops. In each case the space would generate constant false alarms on smoke detection, and the fire risk is real enough that no detection is not an option. Heat detection is also used in loft spaces and roof voids where dust and insects would make smoke detection unmaintainable.
Where it does not belong
Anywhere people sleep, anywhere on an escape route, and anywhere the fire you are worried about is a smouldering one. Heat detection in a bedroom is not a substitute for smoke detection: by the time the ceiling is hot enough to operate the head, the room is already beyond the point at which an occupant could have escaped easily.
Questions people ask about heat detectors
What is the difference between fixed temperature and rate of rise?
Fixed temperature operates at a set point; rate of rise operates when the temperature climbs unusually fast. Most modern heads do both, which covers slow fires in cold rooms and fast fires alike.
Can a heat detector replace a smoke detector?
Only in rooms where smoke detection cannot be used reliably, such as kitchens and plant rooms. It responds later, so it is never the right choice on an escape route or where people sleep.
Why did my heat detector operate on a hot day?
Almost certainly the wrong temperature class for the room's normal ambient. It is a design fix, not a fault in the device, and it is common in roof spaces and above ovens.