What is a Radiator Thermostat?

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  • Written By: Ray Hawk
  • Edited By: E. E. Hubbard
  • Last Modified Date: 06 October 2019
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A radiator thermostat is a type of automatic valve that is designed to open or close, allowing heated air or liquid to pass through a pipe at a predetermined temperature setting. These types of control valves are commonly built into building heating systems, as well as cooling systems on automobiles and other types of engines. The way they function is determined largely by the system they work within.

Heating distribution systems in homes, offices, and apartment buildings have radiator thermostats installed just before the point where the external heating element itself exists. When air or hot water reaches a predetermined temperature from the furnace or hot water tank, the radiator thermostat opens. This allows the mixture to flow into a series of metal coils and veins, which is the radiator itself. It spreads the hot air or water over a large surface area, allowing the the hot air or water to rapidly dissipate its energy into the surrounding room, raising the temperature of the room to the desired level.

The opposite tends to be true with a radiator thermostat when it is designed to keep an engine cool. It opens when the temperature of coolant reaches a high level to allow it to flow to a radiator that spreads the coolant out. Air flow over the radiator removes heat from the liquid, and it is then pumped back into the engine.


Despite these different uses, a radiator thermostat functions in the same basic way regardless of where it is installed. Radiator thermostats are not interchangeable, however. Each unit is specific to a manufacturer and model of heating and cooling system and will not function properly elsewhere.

Simple by design and function, a radiator thermostat is an inexpensive yet crucial element in a heating or cooling system. Since it is the primary switching mechanism that the system depends on to release heat automatically, if it fails, the results can be severe. If a radiator thermostat were to fail in a closed position, it would cut off a channel for heat distribution, and that excess heat and pressure would be forced elsewhere in the system. For this reason, radiator thermostats are designed to fail in the “open” position, allowing the free flow of air or water as if they were not even there.

Radiator thermostats are extremely reliable, but they do degrade over time. They will often fail if they are old and channeling air or water that has reached a temperature beyond their operating parameters. When they fail open, the result in an interior living space is that the room is not heated as expected. In a car engine, it means the coolant flows freely to the engine, but the heater inside of the car, which is also dependent on the radiator thermostat, will simply pump out cold air.


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