don’t react quickly to temperature changes. An alternative design of thermostat senses temperature
changes more quickly using a pair of metal discs with a gas-filled bellows in between.
The discs have a large surface area so they react quickly to heat and they’re corrugated
(they have ridges in them) to make them springy and flexible.
When the room warms up, the gas in the bellows expands and forces the discs apart.
The inner disc pushes against a microswitch in the middle of the thermostat turning the electric
circuit (and the heating) off. As the room cools, the gas in the bellows contracts and the metal discs are forced back together.
The inner disc moves away from the microswitch, switching on the electric circuit and turning the heating on again.
You can also find corrugated bellows thermostats in other applications (for example, older cars), and, instead of gas,
they’re sometimes filled with a volatile (low-boiling) liquid such as a diluted alcohol; the exact chemical inside depends on the range of temperatures over which they need to operate.
Summing up what we’ve discovered already, you can see that all mechanical thermostats (all non-electronic ones) use substances that change size or shape with increasing temperature. So bitmetallic thermostats rely on the expansion of metals as they get hotter, while gas bellows work using the expansion of gases. Some thermostats go further and use the
change in state of a substance from liquid to gas. Wax thermostats are probably the most common example—and you’ll find them in home radiator valves, car engines and mixer showers. They use a little plug of wax inside a sealed chamber. As the temperature changes, the wax melts (changes state from solid to liquid), expands greatly, and pushes a rod out of the chamber that switches something on or off (operating the engine cooling system in a car or regulating the mixture of hot a nd cold water in a shower to ensure your body doesn’t get boiled like a lobster). Wax thermostats tend to be more reliable and longer lasting in the extreme conditions inside a vehicle engine.
Temperature valves fitted to central heating radiators typically use
wax thermostats. When the radiators heat up to the level you’ve set, the wax valves expand and
reduce the flow of water through the radiator until the temperature falls back down again. Coupled with room
thermostats, valves like these can stop your home from overheating—and that’s a good way
both to save energy
and money and do your bit in the fight against global
warming.
Most of us spend only a half to a third of our time at home: the rest of the day, we’re either at work or traveling there. Typically, we’ll have a programmer or thermotstat at home that switches the heating on or off either according to the time of day or the inside temperature. But that’s a crude system at best. By default, to avoid a cold home, a lot of us just turn the heating up high, wasting huge amounts of energy and money. That’s the problem the latest generation of smart thermostats are designed to solve. They learn how you manua lly alter the temperature at different times of day, or differently during the week and at the weekend, comparing that with objective temperature and humidity measurements to establish a reliable program they can follow automatically in future. Typically, they also allow you to program them remotely using a simple smartphone app so you can turn the heating up on the train on your way home, for example.