Valve authority is a key determinant of system stability in hydronic HVAC. Explore how hydraulic design, pump control and valve dynamics interact — and how poor authority leads to instability and comfort issues.
Control valves are typically sized based on flow requirements or percentage opening, but a critical dimension is how much of the total pressure drop the valve actually controls. If the drop across the valve is too small compared to the entire circuit, the valve may become overly sensitive — even slight movements cause large changes in flow. This is the essence of valve authority in a hydronic loop.
In a system where branches close, loads vary and valves modulate, the effective resistance of the circuit changes. When authority is insufficient, the valve’s position becomes unstable, leading to oscillation, inconsistent flow rates and unpredictable temperatures.
The distribution of pressure loss between pipework, fittings, valves and heat-emission units defines the environment in which a valve operates. When one path has significantly lower resistance than others, flow will preferentially travel that route unless balancing is in place. The role of balance valves is to ensure branches are hydraulically similar, thereby stabilising flow distribution and improving valve performance.
A well-designed hydraulic network also accounts for how pump performance shifts with circuit resistance. A pump operating near its design point may behave very differently when loads drop or multiple valves close — the entire network’s behaviour changes.
Rather than viewing the valve in isolation, it must be seen in relation to the pump and the controls. A pump runs along its pump curve and a change in system resistance alters its head-flow point. This, in turn, modifies the pressure drop available across a valve and can reduce its authority unexpectedly.
If a control strategy keeps differential pressure too high, or uses an inappropriate mode for the system, valves may operate in a range where small changes trigger large variations in flow. In contrast, matching the pump control mode to the network hydraulics enables valves to stay within their stable authority region and behave predictably.
Indicators of low valve authority include:
Preventative steps include:
When authority is managed correctly, valves operate smoothly, flow stabilises and overall system efficiency improves.
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