There is a popular misconception regarding the workings of the typical house thermostat. Suppose your house is currently at 65 degrees, and you'd like to warm it to 75. Which do you think will heat the house faster, setting the thermostat to 75, or to 90?

They will both take exactly the same amount of time. Many people think that if you'd like to heat your house quickly, you should turn the thermostat all the way up for "maximum heating". This thinking is based on an incorrect mental model of the thermostat. The thermostat does not control quantity of heat; it is a simple on-off switch. It turns the heater on, at "full power", whenever the room temperature falls below the target temperature. When the target is reached, it shuts the heater off.

Along the same lines, people tend to (thinking the house is too hot) set the thermostat to its minimum (50F), presumably thinking that this will cool the house down faster than simply setting it to their real target. (I assume they don't actually want the house at 50, but rather would prefer (say) 65 over the prevailing 75.) This is incorrect, for the reasons above. The house will only cool as fast as the insulation and weather will allow.

As Webster 1913 points out, a thermostat works because of "unequal expansion of different metals, liquids, or gases by heat". Your car's Thermostat is no different. It typically uses wax as the agent to operate the heat motor, or the part of the thermostat which converts heat into work - in this case, kinetic energy.

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Basically, there is a spring holding closed a valve on the thermostat. The flange on the thermostat otherwise closes the opening. When the temperature reaches around 180 degrees fahrenheit, the wax melts; going from a solid to a liquid provides even more motion than simply heating a liquid.

In order to test a car's thermostat, you can slip a feeler gauge or other similar thin, flat piece of metal in between the mounting flange and the valve. Now hang the thermostat in a pot of water with a thermometer to watch the temperature. When the water temperature reaches (or at least closely approaches) the intended opening temperature of the thermostat, it should open and fall off of the feeler gauge. Now gaze in awe as the valve opens quite far; sometimes more than an inch.

Replacing a thermostat is generally as simple as removing the housing from the engine via two or three bolts, pulling out the old, and dropping in the new. You will typically also need a gasket. One generally does not apply RTV or other silicone sealant to the gasket, but this is not true of all cars. Check your service manual.

Incidentally, the kind of thermostat most often found "in the wild" (which is to say, in your home) is two strips of different kinds of metal with different rates of expansion (bimetallic strip). They may be simply straight strips bonded together (as in my aquarium heater) or, more frequently, arranged in a coil, as in most thermostats intended for control of heating and cooling. The strip will be seen to curve or deflect itself slightly until it makes contact. The coil, by contrast, provides rotation, and is usually used to control a mercury switch.

In many applications, the thermistor (coupled with some sort of computer control) is replacing the thermostat, because it is a solid state component which seldom if ever wears out. Any "electronic" or "computer controlled" thermostat works on this principle, and has a microcontroller which monitors the resistance of the thermistor to determine its current temperature. This is even used in automotive applications, for example to switch an electric fan on or off.

It should be obvious that the reason there is a thermostat in your car is to regulate the temperature of the block and the head. When your car is completely cold, having sat overnight and cooled down, the coolant temperature will be the same as the ambient environment. It is good for your engine to operate at normal temperature, and so when you crank your car its best to get the engine to operating temperature as soon as possible. The thermostat therefore remains closed until the coolant circulating through the engine has reached nominal temperature before it opens, allowing the hot coolant to begin circulating through the radiator.

It should be obvious from these points that if the thermostat was to fail, i.e. become jammed, the engine would almost instantly overheat because coolant can't get past the thermostat into the radiator. The thermostat is a good thing to replace (usually they are \$5 at autozone or the like) if your car is overheating badly for no good reason. Better a \$6 fix than a warped head or blown head gasket.

Ther"mo*stat (?), n. [Thermo- + Gr. to make to stand.] Physics

A self-acting apparatus for regulating temperature by the unequal expansion of different metals, liquids, or gases by heat, as in opening or closing the damper of a stove, or the like, as the heat becomes greater or less than is desired.

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