Control Valve: Definition & Technical Guide
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A control valve is a plumbing or mechanical valve that regulates fluid flow by varying the size of the flow passage in response to a signal from a controller, thermostat, or manual adjustment. Unlike shut-off valves that operate in fully open or fully closed positions, control valves are specifically designed to modulate flow at intermediate positions, enabling precise regulation of temperature, pressure, or flow rate within a piping system.
Modulating Flow, Linear vs. Equal-Percentage Characteristics
Control valves serve as the final control element in a fluid regulation loop. A sensor detects a process variable such as temperature or pressure, a controller compares the measured value to the desired setpoint, and the controller sends a signal to the control valve to adjust its position. The valve opens wider to increase flow or closes partially to reduce it, maintaining the system at the target condition.
In residential plumbing, the most common control valve application is the thermostatic mixing valve, which blends hot and cold water to deliver a safe, consistent output temperature. The valve’s internal thermostat senses the mixed water temperature and adjusts the hot-to-cold ratio continuously. If the cold supply is interrupted or the hot water temperature spikes, the valve responds within seconds to prevent scalding.
The internal design of a control valve determines its flow characteristic — the relationship between valve position and flow rate. Linear valves produce proportional changes in flow for each increment of movement. Equal-percentage valves produce exponential changes, providing fine control at low flow rates. The correct characteristic depends on the system being controlled.
TMV, Pressure-Reducing, T&P Relief, Balancing & Zone Valve Types
Thermostatic mixing valves (TMV) blend hot and cold water to a preset temperature. These are required by code on bathtub and shower supplies in residential installations to prevent scalding. ASSE 1016 and ASSE 1070 govern their performance.
Pressure-reducing valves (PRV) automatically reduce incoming water pressure to a safe level. Homes receiving municipal water above 80 PSI require a PRV to protect fixtures, appliances, and piping from excessive pressure.
Temperature and pressure (T&P) relief valves on water heaters are a specialized control valve that opens to relieve dangerous pressure or temperature conditions, protecting the tank from rupture.
Balancing valves are used in multi-zone hydronic heating systems to equalize flow between circuits of different lengths and resistance characteristics.
Zone valves are motorized control valves that open and close individual heating or cooling zones based on thermostat signals, allowing independent temperature control in different areas of a building.
Anti-Scald Valves, PRV Installation & Water Heater Services
Bonded Plumbworks installs and services control valves across multiple plumbing applications. Thermostatic mixing valves are installed during bathroom plumbing projects to meet state anti-scald requirements. Pressure-reducing valves are part of water line services, installed where municipal pressure exceeds safe operating limits. T&P relief valves are inspected and replaced during water heater services.
A malfunctioning control valve can cause scalding at fixtures, pressure spikes that damage appliances, or inefficient heating system operation. Bonded Plumbworks’ plumbing inspection service includes testing all control valves for proper operation and code compliance.
ASSE 1016, ASSE 1017 & ASME/ANSI Relief Valve Standards
ASSE 1016 governs thermostatic mixing valves for plumbing fixtures. ASSE 1017 covers temperature conditioning valves. The State building codes require anti-scald protection on all bathtub and shower supplies. ASME/ANSI relief valve standards govern T&P valve performance on water heaters.
Watts LF1170, Honeywell Zone Valves & Caleffi Balancing Options
Watts offers the Series LF1170 thermostatic mixing valve and the LF25AUB pressure-reducing valve. Honeywell manufactures motorized zone valves for hydronic systems. Leonard Valve produces the TM Series commercial thermostatic mixing valves. Caleffi provides balancing valves and pressure management products for residential systems.