A smoke alarm warns you earlier. A heat alarm warns you later and does not go off when you make toast. That trade is the entire decision, and it is why a house needs both rather than one kind everywhere: the rooms where smoke detection would be constantly triggered are exactly the rooms where fires most often start.
How each one senses
A smoke alarm responds to particles in the air, which appear early in almost every fire including the smouldering ones that happen at night. A heat alarm responds to air temperature, either crossing a fixed threshold or rising unusually fast. Smoke arrives before heat, so a smoke alarm is always the earlier warning where it can be used without constant nuisance.
Why heat alarms are used at all
Because in a kitchen, a garage or a dusty workshop a smoke alarm will sound so often that somebody will take it down, and an alarm in a drawer detects nothing. A heat alarm accepts a later warning in exchange for staying in service. That is a deliberate design choice, not a cheaper option, and it only makes sense in rooms where the alternative is no working alarm at all.
Which room gets which
Circulation spaces, landings and the principal living room take smoke alarms. Kitchens take heat alarms. Garages and integral workshops take heat alarms. Bathrooms and shower rooms normally take nothing, because there is little to burn and steam guarantees false alarms. Loft spaces are a judgement call driven by what is stored and whether anything electrical runs through them.
They must be in the same system
A heat alarm in the kitchen is only useful if it sounds the alarms upstairs, because a kitchen fire at night is precisely the case where nobody is in the room. Interlinking the heat alarm with the smoke alarms is the point of fitting it, and a standalone heat alarm in a kitchen with unlinked alarms elsewhere is close to no protection at all.
Questions people ask about heat alarm vs smoke alarm
Is a heat alarm as good as a smoke alarm?
No, and it is not meant to be. It warns later. It is used where a smoke alarm would sound so often that it would be disabled, which makes it better than the realistic alternative in those rooms.
Can I put a smoke alarm in the kitchen if I move it away from the hob?
Distance helps but rarely enough in a normal kitchen. The standard practice for dwellings is a heat alarm in the kitchen, interlinked with the smoke alarms elsewhere.
Do I need a heat alarm if I have a smoke alarm in the hall?
A hall alarm will detect a kitchen fire only once smoke reaches the hall. In a house where the kitchen door is closed at night, that can be a long time. The interlinked heat alarm is what closes that gap.