Thermostat

A thermostat is a regulating device used to maintain a specific temperature. It senses the temperature of a physical system and performs actions to keep it near a desired setpoint. This setpoint is the target temperature chosen by a user or a system.

Thermostats are essential components in many heating and cooling systems. They are found in building HVAC systems, water heaters, and air conditioners. They also control kitchen equipment like ovens and refrigerators. In medical and scientific settings, they are used in incubators. In the United States, these devices are known as thermostatically controlled loads (TCLs). These loads account for roughly 50% of the total electricity demand in the country.

Thermostats operate as a closed loop control device. This means the device constantly seeks to reduce the error between the measured temperature and the desired setpoint.

Most conventional thermostats act as bang-bang controllers. In this method, the system operates at full capacity when the temperature is below the setpoint. It then turns completely off once the setpoint is reached. This method is simple to implement but lacks high precision. It often results in temperature oscillations of about 1-2 °C. To prevent the equipment from cycling on and off too rapidly, engineers include something called hysteresis.

To improve performance, some thermostats use an anticipator. An anticipator helps prevent overshoot, which happens when the temperature exceeds the desired range. Overshoot occurs because a system continues to produce heat or cold for a short time after the device turns off. In mechanical bimetallic sensors, a physical anticipator might use a thin wire. When current passes through this wire, it generates a small amount of heat. This heat is transferred to the bimetallic coil to help the timing. Electronic thermostats use an electronic equivalent to achieve this same goal.

There are several ways to sense temperature. Early technologies used mercury thermometers with electrodes. When the mercury reached a fixed temperature, it closed electrical contacts.

Modern sensors are much more diverse. Bimetallic sensors use the different expansion rates of two metals to create movement. Expanding wax pellets are used in many mechanical applications. Electronic thermostats use a thermistor, which is a semiconductor sensor. These sensors process temperature changes as electrical signals. Other common sensors include electrical thermocouples. These signals can then control equipment through direct mechanical, electrical, or pneumatic means.

Mechanical thermostats can function without any electricity. A common example is the wax pellet thermostat used in internal combustion engines.

This device contains a sealed chamber with a wax pellet inside. As the engine reaches a specific temperature, the wax melts and expands. This expansion pushes a rod that opens a valve. The valve allows coolant to flow to the radiator, which helps regulate the engine temperature. The specific operating temperature is determined by the chemical composition of the wax. This allows the thermostat to dynamically balance coolant flow to keep the engine in its optimum range.

The history of thermostatic control spans several centuries. The earliest recorded examples come from around 1620. A Dutch innovator named Cornelis Drebbel invented a mercury thermostat. He used it to regulate the temperature of a chicken incubator. Modern thermostatic control saw major progress in the 1830s. A Scottish chemist named Andrew Ure designed a bimetallic thermostat for textile mills. These mills required steady temperatures to operate correctly. Later, Warren S. Johnson patented a bimetal room thermostat in 1883. He also sought a patent for the first multi-zone control system in 1885. In 1886, Albert Butz patented the first electric thermostat.

Thermostats connect to many different scientific and industrial fields. While simple bang-bang controllers work for homes, industrial settings require more precision. Factories for semiconductor manufacturing or museums often use PID or MPC controllers. These are more advanced mathematical methods for maintaining exact temperatures.

Even specialized plumbing uses this technology. A thermostatic mixing valve uses a wax pellet to mix hot and cold water. This helps prevent scalding by ensuring the output water is at a safe temperature. From the tiny sensors in a car to the large systems in a factory, thermostats allow us to control the physical world with great accuracy.