A switch that feels the room
How Does a Heater Know When to Stop?
A thermostat is a switch that opens when it is warm enough and closes when it is not — everything else follows from that.
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Step 1 of 5
It is a switch, not a dial on the heat.
The heater is either fully on or fully off. What the thermostat changes is how much of the time it spends on, not how hard it burns.
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Step 2 of 5
The old ones bend, the new ones count.
The whole movement is a fraction of a millimetre — enough to break a circuit A mechanical thermostat is a coiled strip of two metals that curls as it warms until it tips a contact apart. A digital one reads a sensor and decides.
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Step 3 of 5
It always overshoots.
By the time the switch opens, the radiators are still full of heat and go on releasing it. The room drifts past your number before it settles.
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Step 4 of 5
So it deliberately waits before switching back.
If it flipped at exactly one temperature it would chatter on and off every few seconds. A built-in gap — switch off at 21, on again at 20 — stops that.
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Step 5 of 5
Where you put it decides everything.
The rest of the house never gets a vote. It only knows its own corner. Near a draught it runs the heating forever; above a radiator or in sunlight it shuts the house down while the rest is cold.
Turning the dial to 30 does not warm a room any faster. The heater was already at full power — the number only changes when it stops.
The short version
A thermostat is a switch that opens above your set temperature and closes below it, with a deliberate gap between the two so it does not chatter.
Try it yourself
Put a thermometer next to your thermostat and another across the room. The two rarely agree, and the thermostat only believes its own.