How does a thermostat use feedback?

A heater may switch on while a room is cool, stop after it warms, and restart much later without anyone touching a button. The thermostat is responding to the result of its own earlier command. It measures a condition, requests a change and measures again. This returning information makes the process a feedback loop.

In a heating system, a temperature sensor provides a reading. The thermostat compares that reading with a chosen target, and its controller decides whether to request heat. The furnace, heat pump or other heating equipment supplies the energy. The thermostat is therefore a decision maker, not the source of warmth. Its measurement must also represent the room well enough for the decision to be useful.

A simple rule could request heat below 20°C and stop at 20°C. Near the boundary, however, tiny changes or measurement noise could repeatedly switch the command on and off. One solution is to use two different switching temperatures. In our example, heat turns on at or below 19°C and turns off at or above 21°C. Between those thresholds, the controller keeps its previous state.

This behavior is called hysteresis: the recent path helps determine the current response. A room warming through 20°C still has heat requested because it has not reached 21°C. The same room cooling through 20°C has heat off because it has not yet fallen to 19°C. The temperature is identical, but the remembered on-or-off state differs. The small band prevents every tiny crossing of one target from triggering another switch.

The complete loop starts with a cool measurement, such as 16°C. The controller requests heating, the equipment transfers energy and the room tends to warm. At the upper threshold, the request stops. Heat already stored in radiators, air and surfaces can continue moving, so temperature need not stop changing instantly. Later heat loss to cooler surroundings can bring the room back to the lower threshold and begin another cycle.

This is negative feedback because the response opposes the temperature shortfall rather than reinforcing it. Real systems may add timing rules, vary heating power or use more elaborate control. The two-threshold example reveals why regulation needs repeated measurement and some awareness of state: maintaining comfort is a continuing conversation between the room's changing condition and the equipment's response.