This small valve keeps a radiator at the temperature you set, on its own. Use the buttons to look inside it and follow hot water from the boiler to the radiator and back.
A thermostatic radiator valve is a small self-acting valve fitted where hot water enters a radiator. It senses the room temperature and adjusts the flow of hot water so the radiator only gives out as much heat as the room needs.
The clever part is that it needs no electricity and no separate room thermostat. A wax-filled sensor on the valve body does the sensing, expanding and contracting as the room warms and cools.
From the outside it is just a knob on a pipe. Cutting it open shows the wax sensor, the bellows, the stem, the plug and the spring that turn a tiny temperature change into a movement that opens or closes a water passage.
Two ideas matter most: heat makes the wax expand, and the valve converts that expansion into a push. The rest of the parts simply translate that push into a change in the water flow.
When the room drops below the temperature you set, the wax in the sensor bulb contracts. The spring is now stronger than the wax, so it pushes the plug away from its seat and the passage opens.
With the passage open, hot water from the boiler flows through the valve and into the radiator. The radiator fills with hot water and starts to give off heat to the room.
As the radiator heats up, so does the air around it. The sensor bulb feels that rise in room temperature, not the temperature of the water in the pipe.
Warmer air makes the wax expand. Because the wax has nowhere else to go, it pushes against the bellows, which presses the stem and the plug downwards towards the seat.
The more the wax expands, the further the plug closes. The hot water is gradually throttled back until the room is just holding the set temperature. If the room cools again, the wax contracts and the cycle repeats.
A wax-filled sensor bulb on the valve body feels the air around it. The wax expands and contracts with the room, so the valve reacts to the real temperature without any electricity.
Wax expands and contracts strongly for a small temperature change, so it makes a simple, reliable, self-powered actuator. There is nothing to wire up and nothing electrical to fail.
The dial sets the temperature you want. Turning it changes the pre-load on the spring, so the valve opens and closes around a different target temperature.
Hot water from the boiler enters the inlet, passes the valve and flows into the radiator. Cooler water leaves the radiator and returns to the boiler to be reheated.
No. It is self-acting: the wax itself provides the push. It only has an effect while the boiler is supplying hot water, so when the heating is off the valve simply sits open.
That is the valve doing its job. Once the room reaches the set temperature it throttles the flow, so the radiator only receives enough hot water to hold that temperature.