How does room thermostat work? Does it really control the room temperature?

Answer by Arûn Râinâ:

Feeling too hot? You'll be wanting to cool down, then. Feeling too cold? You'll need to warm up. Our bodies are amazing, self-regulating mechanisms that can constantly adjust to keep their temperature within a whisker of 37°C (98.6°F). But the rest of the world isn't quite so helpful. If we want our homes to keep their temperature more or less constant, we have to keep switching our heaters on and off—or, alternatively, rely on clever gadgets called thermostats to do the job for us. What are they and how do they work? Let's take a look inside!
You might have a temperature control on a wall in your home to control the heating system but, although it's probably marked in degrees, it's not a thermometer. It's called a thermostat, a modern word based on two ancient Greek ones: thermo (meaning heat) and statos (which means standing and is related to words like stasis, status quo, and static—meaning to stay the same). We can tell just from its name that a thermostat is something that "keeps heat the same": when our home is too cold, the thermostat switches on the heating so things quickly warm up; once the temperature reaches the level we've set, the thermostat switches the heating off so we don't boil.
Let's just be clear about the difference: a thermometer is something that measures the temperature; a thermostat is something that tries to maintain the temperature (keep it roughly the same).
How thermostats work
So how does a thermostat work? Most things get bigger when they heat up and smaller when they cool down (water is a notable exception: it expands when it heats up and when it freezes too). Mechanical thermostats use this idea (which is called thermal expansion) to switch an electric circuit on and and off. The two most common types use bimetallic strips and gas-filled bellows.
Bimetallic strips
A traditional thermostat has two pieces of different metals bolted together to form what's called a bimetallic strip (or bimetal strip). The strip works as a bridge in an electrical circuit connected to your heating system. Normally the "bridge is down", the strip carries electricity through the circuit, and the heating is on. When the strip gets hot, one of the metals expands more than the other so the whole strip bends very slightly. Eventually, it bends so much that it breaks open the circuit. The "bridge is up", the electricity instantly switches off, the heating cuts out, and the room starts to cool.
But then what happens? As the room cools, the strip cools too and bends back to its original shape. Sooner or later, it snaps back into the circuit and makes the electricity flow again, so the heating switches back on. By adjusting the temperature dial, you change the temperature at which the circuit switches on and off. Because it takes some time for the metal strip to expand and contract, the heating isn't constantly switching on and off every few seconds, which would be pointless (and quite irritating); depending on how well-insulated your home is, and how cold it is outside, it might take an hour or more for the thermostat to switch back on once it's switched off
Gas-filled bellows
  1. The trouble with bimetallic strips is that they take a long time to heat up or cool down, so they 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. 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.
  1. Wax thermostat :   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 and 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.

How does room thermostat work? Does it really control the room temperature?


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