Thermostatic Radiator Trap Basics

Sectional view of a Dunham Company thermostatically operated valve commonly used on radiators.

Thermostatic Radiator Trap Basics
A thermostatic radiator trap is what was once called more appropriately a ''thermostatically operated valve''. Its function is to allow the air in the radiator to pass through itself as the steam pushes the air out of the radiator, but close against the flow of the steam itself. It must also be able to pass the condensed steam (the water) into the return piping, but again close against the steam once the water passes through.
The dominant method to achieve this is to have an element inside the valves housing that expands when its temperature is raised close to the temperature of the steam.
The element usually consists of a flexible member (thermostatic disc) partially filled with a volatile liquid (alcohol water mix). As the steam approaches this element the mixture inside it flashes into vapor which expands the element. This closes the valve by driving down the disc upon the seat. As condensate water builds up in front of it, the flexible member starts to cool down. When its temperature has dropped anywhere from 10 to 20 degrees Fahrenheit the flashed vapor inside the flexible member cools off and condenses which contracts the element. The disc then raises off the seat allowing the water of condensate to drain out.  The approaching steam again heats up the element, the volatile liquid again flashes into vapor, the element expands, the disc again closes the valve.
This happens again and again and again, over and over. After about 5 or 10 years the element fails. The valve can fail open or closed. If it fails closed the radiator won't heat. If it fails open, the steam passes through the radiator into the return piping and wreaks havoc there. Check those radiators traps frequently, (yearly). A failed radiator trap costs you money in extra steam made by the boiler that is not condensing in a radiator but in the basement pipes after it blew right past the radiator.