Balancing Dampers for HVAC Duct Branches
If a branch duct misses its design CFM, balancing dampers bring it back in line. The right type depends on ceiling access and how often the system gets rebalanced.
The table below compares manual, cable, remote, and automatic models. Call 847-674-0000 with your branch CFM schedule for a selection.
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Manual, Cable, Remote, or Automatic Balancing Dampers
The right balancing damper type depends on ceiling access and how often airflow gets reset. Automatic types fit risers and shared exhaust, where static pressure keeps shifting.
Manual, cable, and remote types come in round, single-blade, and multiblade rectangular versions.
| Type | How it is adjusted | Access needed | Power |
|---|---|---|---|
| Manual | Locking hand quadrant, the same setup as manual volume control dampers | Reach the duct itself | None |
| Cable | Push-pull or twisting cable run to an access point | Ceiling port, wall plate, or diffuser | None |
| Remote | Low-voltage actuator run from a handheld controller | RJ-11 port in a wall, ceiling, or diffuser | Controller battery, no building wiring |
| Automatic | Spring-held blade adjusts itself to hold a set CFM | None after startup | None |
Where Should Balancing Dampers Be Installed?
Balancing dampers should be installed in each branch duct, at the takeoff from the main. At the takeoff, throttling noise fades before the air reaches the room. The same damper throttled at a diffuser neck makes noise people can hear.
- Keep the quadrant, cable port, or RJ-11 port reachable after the ceiling closes.
- Leave the branch furthest below design CFM wide open, then throttle the rest.
- Balance to minimize throttling first, then trim fan speed. ASHRAE 90.1 sets that order for fan systems over 1 hp.
Balancing dampers are set once and left in place. Tight shut-off calls for a bubble-tight isolation damper. Airflow that changes with occupancy or temperature calls for a zone damper or another control damper.
Balancing Damper Questions
The difference between a balancing damper and a control damper is when the blade moves. A balancing damper is set once during air balancing and stays there. A control damper moves in service, driven by an actuator and a control signal. It opens, closes, or modulates as occupancy, temperature, or pressure changes. Most HVAC systems need both.
Automatic balancing dampers work by holding a set CFM with a spring-held blade and no power. Rising duct pressure pushes the blade toward closed, and falling pressure lets it open. Most ship set to the CFM the branch needs. One major line is rated from 0.2 to 2 in. w.g. at 25 to 425 CFM per damper.
A fire damper can act as a balancing damper when it is built for both jobs. A multiblade fire damper with a manual quadrant can be set to design airflow. Automatic balancing dampers also come paired with UL 555 fire dampers rated for 1.5 or 3 hours. One damper then covers both roles in the duct.
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