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Most times, the thermostatic valve is on the bottom of the radiator. If the valve is not near the floor, then it can be found at the top of the radiator's left side. Look for a dial that has numbers on it to determine if it's a thermostatic valve. While some radiators just have an on and off switch, a thermostatic radiator allows you to control the amount of heat it puts out. If your radiator doesn't have a thermostatic valve or is a 1-pipe system, you can skip this step. Twist the valve by hand until it doesn't turn any further and the arrow on the dial points to 0. This means the temperature is set so the radiator won't run. This valve is also located near the floor but on the right side. The valve should have a plastic twist cap on top of it. If you don't have a thermostatic valve, all you need to do is turn this valve. Once both valves are closed, the radiator will be completely turned off so you can cool down. Let the radiator cool down for at least an hour before touching it.

Summary:
Locate the thermostatic valve on the left side of the radiator if you have one. Turn the valve clockwise so the dial reads 0. Find the second valve on the right side of the radiator. Twist the valve clockwise until it doesn't turn anymore.