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HVAC Hydronic Control

Understanding the difference between Direct Acting and Reverse Acting valves is crucial for those in hydronic systems, building automation, or HVAC control valves.

Why it matters: Knowing the correct valve action can save hours of troubleshooting and prevent expensive commissioning errors.

  • Identifying valve action in the field is key to aligning with BAS outputs, fail positions, and actuator configuration.

What’s new: Our guide breaks down what valve action means, how to identify it, and why a wrongly moving valve can disrupt an entire control loop.

  • Whether programming a new sequence or replacing an actuator, this guide simplifies what to look for.

The bottom line: This resource is your go-to for quickly understanding and resolving issues related to valve action.
Hydronic Controls

Direct Acting vs. Reverse Acting Valves

If a coil gets warmer when it should be cooling, or a valve closes when the BAS says it is opening, you may be dealing with a Direct Acting / Reverse Acting mismatch.

The good news: valve action is easier to understand when you focus on one question — what happens to water flow when the control signal increases?

Signal goes up.
Does flow go up or down?

Why it matters

  • A mismatched valve can invert the entire control loop.
  • PID tuning will not fix a valve that is moving the wrong direction.
  • Field verification should confirm actual flow, not just stem movement.
Jump to a section
Core Question DA vs. RA Field Check Configurations BAS Output Controller Action NO vs. NC Cheat Sheet
The Big Thing

The core question is simple

Direct Acting and Reverse Acting describe how a control valve responds when the control signal increases. That signal may be 2–10 VDC, 4–20 mA, or pneumatic pressure.

Direct Acting

Signal increases = valve opens

More signal creates more flow. This is common on chilled water valves, heating valves, and many modern modulating control valves.

Reverse Acting

Signal increases = valve closes

More signal creates less flow. This is often used in fail-open or freeze protection applications.

Quick Definition

DA vs. RA at a glance

Do not start with the actuator label, pipe orientation, or stem direction. Start with the flow response.

Action As Signal Increases Valve Position Effect on Flow
Direct Acting Opens More open Flow increases
Reverse Acting Closes More closed Flow decreases
Direct Acting Example
Signal increases then Valve Opens then Flow increases
Reverse Acting Example
Signal increases then Valve Closes then Flow decreases
Signal Behavior

What this looks like with a 2–10 VDC signal

The same signal can create opposite valve movement depending on whether the actuator is configured as Direct Acting or Reverse Acting.

Direct Acting

2–10 VDC opens the valve

  • 2 VDC: mostly closed
  • 6 VDC: about halfway open
  • 10 VDC: fully open
Reverse Acting

2–10 VDC closes the valve

  • 2 VDC: fully open
  • 6 VDC: about halfway open
  • 10 VDC: fully closed

Watch for this: If the BAS output is at 90% cooling demand and the chilled water valve is closing, the loop is probably not a tuning problem. It is likely an action, wiring, or configuration problem.

Field Check

How to identify valve action in the field

The best field test is to command the actuator and confirm the system response. Stem movement alone can be misleading.

Method What To Do What To Watch
Command from BAS Command 0%, 50%, and 100% Most reliable when paired with flow or temperature confirmation
Check pipe temperature Watch supply/return or discharge air temperature Confirms whether flow is actually changing
Watch stem movement Observe movement as signal increases Use caution because stem direction varies by product
Check actuator settings Look for DA/RA, CW/CCW, DIP switch, or software direction Modern actuators may be field-selectable

Pro tip: Command the output from the BAS and verify with temperature or flow evidence. That tells you what the valve is actually doing in the system.

Common Applications

Where each action is typically used

These are common patterns, not universal rules. Always verify the actual installed actuator, valve body, wiring, and BAS setup.

Application Typical Fail Position Signal Increase Typical Action
Chilled Water Coil Closed Opens valve Direct Acting
Heating Coil / Freeze Protection Open Closes valve Reverse Acting
Bypass / Differential Pressure Valve Varies Often closes bypass Often Reverse Acting
3-Way Mixing Valve Depends on piping Varies Design-specific
BAS Output

What the BAS output should mean

For a correctly configured Direct Acting valve, the BAS output should generally match valve position.

0% Output

Valve Closed

Little to no flow expected.

50% Output

Valve Mid-Position

Partial flow expected.

100% Output

Valve Open

Maximum commanded flow expected.

If the valve does the opposite: check actuator action, BAS output configuration, wiring, and any selectable actuator direction settings.

Common Confusion

Controller action is not the same as valve action

A controller can be Reverse Acting while the valve actuator is Direct Acting — and the loop can still be correct.

Example: Cooling Loop

  1. Room temperature rises above setpoint.
  2. The controller increases the cooling output.
  3. The Direct Acting valve opens.
  4. Chilled water flow increases and the space cools.

The goal is not for every component to use the same action. The goal is for the entire control loop to produce the correct system response.

Do Not Mix These Up

Normally Open vs. Normally Closed

Normally Open and Normally Closed describe the fail position. Direct Acting and Reverse Acting describe response to signal. They are related, but they are not the same thing.

Normally Open

Flow with no power

The valve passes flow when power is removed. This is common where freeze protection or fail-open operation is required.

Normally Closed

No flow with no power

The valve stops flow when power is removed. This is common where fail-closed operation is preferred.

Modern Actuators

Many actuators can be configured either way

Modern electronic actuators may allow field-selectable direction through DIP switches, actuator setup tools, or BAS configuration.

Important: Do not assume the label tells the whole story. Someone may have changed the actuator direction in the field without updating documentation.

Quick Reference

Field cheat sheet

When you are standing at the valve, keep it simple.

Direct Acting

Signal increases = flow increases

If increasing the signal opens the valve and increases flow, it is Direct Acting.

Reverse Acting

Signal increases = flow decreases

If increasing the signal closes the valve and decreases flow, it is Reverse Acting.

Need help matching the valve, actuator, and BAS sequence?

Stromquist can help you confirm valve action, actuator setup, output signal, and application fit before it becomes a commissioning headache.

Email the Stromquist Team View Valves

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