How Your Thermostat Keeps Every Room Just Right

What does a thermostat actually do?

A thermostat is a regulating device that senses the temperature of a system and acts to keep it near a desired setpoint ¹. You find them in building heating, central heating, air conditioners, HVAC systems, water heaters, ovens, refrigerators and incubators ¹. It is listed as a home appliance, servomechanism and HVAC control system, linked with Warren Seymour Johnson ².

It works as a closed loop control device ¹. That means it compares the measured temperature with the level you want and tries to reduce the gap. It often acts as the main control unit by switching heating or cooling on or off, or by regulating the flow of a heat transfer fluid ¹.

How does it sense heat and switch?

Mechanical thermostats commonly use bimetallic strips, which turn a temperature change into small movement to control heating or cooling ¹. Electronic thermostats instead use a thermistor or other semiconductor sensor, which treats temperature change as electronic signals ¹. Other sensor types include expanding wax pellets, electrical thermocouples and older mercury contacts ¹.

Many common models are bang-bang controllers ¹. That is plain words for full on or full off. To avoid switching too fast around the setpoint, they include some hysteresis ¹. As a result, the temperature swings a little, usually 1 to 2 degrees C ¹. Many designs also include an anticipator, which stops heating or cooling slightly early to allow for leftover heat and avoid overshoot past the setpoint ¹.

Where did the idea come from, and how to use it well?

One of the first recorded feedback devices was a mercury thermostat for a chicken incubator, built by Cornelis Drebbel about 1620 in England ¹. Modern control developed in the 1830s, when Scottish chemist Andrew Ure designed a bimetallic thermostat for textile mills ¹. Warren S. Johnson patented a bi-metal room thermostat in 1883 and sought a patent for multi-zone control two years later, while Albert Butz patented the electric thermostat in 1886 ¹.

For daily use, it helps to think of the thermostat as a guide, not a tap. Choose one steady setpoint and give the system time to respond, since small swings and short delays are normal. Keep the area around it clear so it can sense room air, and avoid changing the setting up and down often, which can cause extra cycling and wear.

• Pick a comfortable setpoint and leave it for a while
• Place lamps and heat sources away from the thermostat
• Ask a heating professional for help with wiring or placement