What is the definition of a modulating valve? A modulating valve is a device that regulates the flow of a fluid, such as water, air, steam, or other liquid and gas mixtures, by varying the size of the opening in the valve. This type of valve is used to control the pressure and temperature of the fluid, as well as the flow to a specific point, such as a control valve or a check valve. Modulating valves are usually actuated by either a plunger, piston, or diaphragm, and they are typically controlled by either an electric motor or a manual handle. The amount of opening in the valve can be adjusted depending on the specific needs of the system. For example, a modulating valve with a large opening can be used to increase the pressure in a system, while a small opening can be used to reduce the pressure.