What is a Thermostat?

A thermostat is a cooling system component that is responsible for regulating the flow of coolant through the radiator. When the thermostat is closed, coolant is prevented from leaving the engine block and entering the radiator. When the thermostat is open, coolant can freely flow through the entire cooling system. Most automotive cooling system thermostats utilize a wax pellet design.


A common thermostat viewed from both ends.


What is the Purpose of Thermostats?

The main purpose of a thermostat is to maintain optimal engine temperature by regulating the flow of coolant. Since a thermostat starts out closed when an engine is started cold, coolant is initially prevented from circulating. This allows the engine to reach its optimal operating temperature before the cooling system kicks in. If the temperature of the engine ever drops below that level, the thermostat will close and allow it to warm up again.

The History of Thermostat Design

Prior to the 1930s, water cooling systems didn’t have thermostats, and they also weren’t pressurized. Coolant circulated through these systems from the moment the engine was started up, which effectively slowed the process of warming up. Then, in the 1920s, it was discovered that this actually caused excessive engine wear due to the condensation of fuel on cool cylinder walls.

In order to solve the problem of excess cylinder wear, the first cooling system thermostats were developed in the 1930s. These early thermostats used a cylindrical bellows design that was operated by a sealed capsule that contained an organic liquid. The liquid was designed to have a boiling point that was right around the desired engine operating temperature. That allowed the liquid to boil at the right time, which operated the bellows and allowed coolant to flow through the system.

bellows thermostat

Bellows-style thermostats are ill-suited to pressurized cooling systems.

This bellows design was widely used until the adoption of pressurized cooling systems. These pressurized systems allows engines to be operated at higher temperatures, since pressurized coolant has a higher boiling point than unpressurized coolant. Unfortunately, bellows-style thermostats adapted poorly to pressurized environments. When exposed to pressure, these thermostats would sometimes be forced closed, which would prevent the flow of coolant.

How Do Thermostats Work?

Most modern thermostats utilize a wax pellet design. This design includes a few basic components, including:

  • a wax pellet in a sealed chamber
  • a push rod
  • a valve
  • a bypass hole or valve

Like earlier bellows-style thermostats, wax pellet thermostats are installed in-between the engine and the radiator. The end that contains the temperature-sensitive wax pellet is installed facing the engine, and the other end faces the radiator.

When it is cold, the wax inside the sealed chamber is solid, and the valve is closed. Then, as the engine warms up, the wax melts. Since wax expands predictably when it melts, that forces the rod to open the valve. When the wax cools down again, its volume reduces, and the rod allows the valve to close. In that state, a small amount of coolant is allowed to circulate through the bypass hole or valve, which ensures that enough coolant will circulate to heat up the wax pellet at the proper time.

Double Valve Thermostats

Some engines are sensitive to sudden temperature changes, so they use double valve thermostats. These operate similarly to single valve thermostats, but they have two valves that are both operated by a single wax pellet. The purpose of this kind of double valve thermostat is to allow coolant to recirculate through “constant inlet temperature” cooling systems that are designed for engines that require especially tight operating temperature control.

Thermostat Failure

Thermostats are exceptionally simple components that combine a sensing element and a control valve in a single device. They are also relatively simple in design, and the basic wax pellet and valve concept has been around for over half a century. That means they tend to last a long time, but it doesn’t mean that they can’t fail.

When a thermostat fails, it will typically either stick open or closed. When a thermostat sticks open, the engine will never reach operating temperature. That will often result in poor fuel mileage, but it can also cause premature cylinder wear. When a thermostat sticks closed, it will typically lead to overheating issues.

Thermostat gaskets can also develop coolant leaks, and the housings can become pitted and leak. In either case, it’s typically a good idea to replace the thermostat at the same time that the gasket, seal, or housing leak is fixed. This is due to the fact that thermostats are typically inexpensive in comparison to the labor cost associated with replacing them, and there is no additional labor cost to replace a thermostat when the seal, gasket, or housing is already being replaced.

JD Laukkonen

JD Laukkonen turned wrenches in the north end of Seattle for a decade, so he's no stranger to the inner workings of modern automobiles. He has worked as a freelance writer since closing his shop in 2007, and he currently covers automotive technology for Lifewire.com.

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