How a Thermostat Decides When to Heat or Cool

A thermostat is a switch that watches temperature. It senses how warm a space or object is, compares that with a target called the setpoint, and turns heating or cooling on or off to stay near it. Older ones do this with a bending strip of metal. Newer ones use an electronic sensor and a small processor. Both rely on a deliberate gap between the switch-on and switch-off temperatures so they do not flick on and off constantly.

The basic idea: a feedback loop

The Wikipedia article on thermostats describes the device as a closed-loop control. Closed loop means the device measures the result of its own action and uses that to decide what to do next. If the room is colder than the setpoint, the heater runs. Once the room reaches the setpoint, the heater stops. The temperature then drifts down and the cycle repeats.

The bimetallic strip

A classic mechanical thermostat uses a bimetallic strip, which is two different metals bonded into one strip. Metals expand by different amounts when heated, so one side grows more than the other and the strip bends. That bending can open or close an electrical contact. Turning the dial on the wall changes how far the strip must bend before the contact moves, which changes the temperature at which it switches.

According to Wikipedia, the Scottish chemist Andrew Ure designed a bimetallic thermostat in the 1830s for textile mills, and Warren S. Johnson of Wisconsin patented a bimetal room thermostat in 1883. The same article says the Dutch inventor Cornelis Drebbel built a mercury thermostat around 1620 to regulate a chicken incubator. Albert Butz is listed as patenting an electric thermostat in 1886, and Johnson invented a pneumatic one in 1895.

Mercury switches

Some mechanical models replace the contact on the end of the strip with a small glass tube holding mercury. When the strip tilts the tube, the liquid metal rolls to one end and completes the circuit. Wikipedia notes that many countries restrict these because mercury is toxic, and a broken unit counts as chemical waste.

Why there is a gap: hysteresis

If a thermostat switched at exactly one temperature, the heater could click on and off many times a minute as the reading hovered around the setpoint. That would wear out equipment. To prevent it, designers add hysteresis, a gap between the temperature at which the system turns on and the one at which it turns off. Wikipedia explains that a small magnet or the weight of the mercury can create this gap in a mechanical unit, so the temperature must rise several degrees before the contacts reopen.

A result is that a simple on-off thermostat never holds the room perfectly steady. Wikipedia says conventional units oscillate about 1 to 2 °C around the setpoint. The room swings slightly above and below your target, and the swings are normally small enough that people do not notice.

Electronic and smart thermostats

Digital thermostats have no bending strip. They use a thermistor or similar sensor whose electrical resistance changes with temperature, and a small processor reads it. Wikipedia says most run on batteries, though some draw power from the 24-volt AC line in the wall, and a relay or other electronic switch turns the equipment on and off. The processor can apply the hysteresis gap in software and can follow a schedule, such as a lower setpoint overnight.

ENERGY STAR, the US government-backed efficiency programme, describes certified smart thermostats as Wi-Fi enabled devices that can learn your temperature preferences, set schedules that lower energy use while you sleep or are away, and use geofencing, which adjusts the temperature as your phone approaches home. They can also be controlled from a smartphone, enter a low-power standby mode when inactive, and receive software updates.

What about savings?

ENERGY STAR's smart thermostat page states that certified models were independently verified, using field data, to deliver energy savings, but it gives no percentage or dollar figure for the savings. It says almost half of the annual energy bill goes to heating and cooling, which is more than $900 a year for the average American household. That figure is the total cost of heating and cooling, not what a thermostat saves. Real savings depend on the home, the weather, the equipment and how the thermostat is set up, so treat any single number with caution.

Summary

• A thermostat senses temperature, compares it with a setpoint and switches heating or cooling.
• Mechanical models use a bimetallic strip, two bonded metals that bend as they warm.
• A built-in gap, called hysteresis, stops rapid on-off cycling.
• Simple on-off control lets the temperature swing about 1 to 2 °C around the setpoint.
• Electronic and smart models use sensors, processors and schedules, but the same feedback idea underlies them.

This is general information about how the devices work. For installation or wiring of a thermostat, follow the manufacturer's instructions or ask a qualified electrician.

Sources

• Thermostat, Wikipedia
• Smart Thermostats, ENERGY STAR