The actuator keeps moving. The building management system (BMS) sequence of operations appears correct, and the temperature sensor is working. Yet the HVAC control valve does not settle into a stable position.
If the control sequence is correct, why won’t the valve stop moving?
The answer often lies in valve hunting, a common yet misunderstood issue in hydronic HVAC systems.
In a typical HVAC system, the BMS provides the broader supervisory layer for scheduling, visibility, alarms, and trend data. The local direct digital control (DDC) controller executes the control loop by interpreting sensor feedback and sending a command signal to the actuator. Valve hunting occurs when a modulating control valve fails to settle at the required operating point to maintain temperature. The issue is usually not the supervisory BMS sequence itself, but the inability of the local control loop to stabilize. This can often look like:
- An actuator that continuously repositions
- A discharge air temperature that repeatedly rises and falls
- A heating or cooling coil that continuously over/undershoots its setpoint
Because the actuator is the visible moving component, it is often blamed for hunting. However, the actuator responds as instructed, and the real cause of the instability lies elsewhere in the control loop.
The result directly impacts building performance, from energy waste and higher operating costs to accelerated equipment wear and tear to occupant discomfort. There are five main areas to investigate to identify the root cause.
1. Control stability depends on the control loop
A correctly sized and commissioned control valve should make smooth, gradual adjustments as building loads change. For example, in a chilled-water air-handling unit, the valve opens as cooling demand increases and then stabilizes once the leaving air reaches the target temperature.
Valve hunting begins when the control loop becomes too sensitive, too delayed, or too unpredictable to maintain the correct valve position. There are several reasons this can happen, including:
- Oversized valves
- Changing pressure conditions
- Aggressive control tuning
- Delayed sensor feedback
- Mechanical problems in the valve and actuator
2. Oversized valves create oversensitive control
One of the most common contributors to hunting is an oversized control valve.
Instinctively, you may think a larger valve is a safe design choice because it can easily handle the required flow and appears to offer extra flexibility. However, a control valve is not selected solely to pass flow; it must also accurately regulate flow under normal operating conditions.
This distinction is important because most HVAC systems operate at part load rather than at peak design conditions. When a valve is oversized, it often operates near its closed position, where even a small actuator movement can cause a large change in water flow.
The result? The coil receives more flow than needed, causing the temperature to overshoot the setpoint. The DDC controller responds by reducing the valve command. As the temperature changes, the controller repeatedly opens the valve, creating a cycle of overcorrection.
This is why proper valve selection matters.
Size a control valve for the coil’s required flow, expected pressure conditions, and operating range – not by pipe size or an oversized safety margin. When the valve is correctly sized, the actuator can make smooth, stable adjustments instead of constantly overcorrecting, which leads to hunting.
3. Hydronic pressure changes reduce valve authority
Stable control depends on the pressure conditions throughout the hydronic system.
For a valve to accurately regulate flow, it needs sufficient control over the pressure drop in its branch of the piping. This is known as valve authority. When authority is low, the valve loses some of its ability to produce consistent water flow, resulting in an inconsistent flow response for any given change in its position.
In practical terms, the DDC controller may command a slight valve movement, but the actual water flow may respond too much, too little, or not as expected.
Variable flow systems introduce additional challenges. As pumps modulate and other control valves open and close throughout the building, the differential pressure across a specific valve can change. A valve position that delivered the correct flow earlier in the day may produce a different flow rate later as system conditions shift.
From the controller’s perspective, the temperature is drifting from the setpoint. From the hydronic system’s perspective, the relationship between valve position and water flow has changed.
This is where proper valve and actuator application becomes critical. In variable flow systems, pressure independent balancing control valves (PIBCVs) help maintain a more predictable flow response even as differential pressure changes. When paired with the right actuator and proper tuning, they can support more stable modulation and reduce a common source of valve hunting.
4. Controller tuning and sensor feedback
Control tuning can make an already sensitive system even more unstable.
In most HVAC applications, the local DDC controller executes the control loop. It interprets sensor feedback, calculates the required response, and sends a command signal to the actuator.
If the proportional response is too aggressive or the integral action responds too quickly, the controller may overcorrect before the HVAC coil can respond.
HVAC coils do not respond instantly. A change in valve position first changes water flow. Heat transfer changes next, and then the air temperature responds. If the controller reacts faster than the process can, it begins chasing the temperature instead of controlling it.
This is where the valve, actuator, DDC controller, and BMS ecosystem need to work together. The sensor provides operating feedback; the DDC controller interprets the deviation from the setpoint; and the actuator positions the valve to adjust flow through the coil. The BMS provides visibility through scheduling, alarms, and trend data.
5. Mechanical limitations
Mechanical performance still matters but should be viewed as part of the full-control ecosystem.
A well-applied actuator provides accurate positioning, repeatable movement, and stable feedback, helping the controller position the valve more precisely.
If the actuator or valve assembly has excessive deadband, backlash, hysteresis, sticking, linkage wear, or degraded feedback, the system may struggle to hold a stable intermediate position. Actuators with built-in intelligence and precise control can support more accurate positioning, feedback, and diagnostics where available, but they work best when paired with proper valve sizing, hydronic design, and control tuning.
Why valve hunting matters
Valve hunting is more than a trend-log issue.
At the coil, unstable water flow leads to unstable temperature control. Occupants may experience hot-and-cold swings, while operators see a control system that constantly adjusts but never stabilizes.
Across the hydronic system, unstable valve behavior can lead to low delta-T conditions, unnecessary pump response, and more difficult commissioning.
The actuator also experiences unnecessary wear. Continuous repositioning increases the cycling of motors, gears, linkages, and valve components. Modulating actuators are designed to move as the load changes, not to continuously compensate for an unstable control loop.
PIBCVs can help reduce one contributor to hunting by maintaining a more predictable flow response as differential pressure changes.
Ask a better diagnostic question
When an HVAC control valve hunts, the diagnostic question is not “Why is the actuator moving?” but “Why can’t the control loop settle?”
Valve hunting is rarely the result of a single component failure. It is more often the combined effect of valve sizing, hydronic pressure conditions, controller tuning, sensor feedback, and mechanical performance.
Stable control requires those elements to work together. When aligned, valves modulate predictably, coils respond consistently, and the HVAC system operates as designed, improving energy efficiency, extending equipment life, and enhancing occupant comfort.
To learn more about our valves and actuators, visit our solutions page.
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