Control Damper Authority and Leakage

ScopeAuthority is the fraction of the branch's resistance that belongs to the damper itself. When it is high, closing the damper changes the branch's total resistance a lot, so flow tracks position and control is smooth; when it is low -- a small damper drop in a branch full of coil, filter, and duct resistance -- closing the damper barely changes the total until it is nearly shut, and then the flow falls off a cliff. The installed characteristic becomes effectively on/off, and no amount of controller tuning fixes it, because the problem is hydraulic and not a control-loop setting. The target depends on the blade: the Johnson Controls Damper and Actuator Manual (268.1) recommends the damper take 20 to 25% of the system drop for a parallel-blade damper and only 8 to 10% for an opposed-blade one in temperature and mixed-air control, because opposed blades have the more linear inherent characteristic. The leakage half matters for a different reason entirely: a closed damper that leaks is a closed damper that does not close. On an outside-air damper in a cold climate that is a freeze-stat trip and a burst coil; on a smoke damper it is a life-safety failure; on a VAV box minimum it is a comfort complaint and an energy penalty that runs all year. Leakage is classified as cubic feet per minute per square foot at 1 in w.g., and it scales as the square root of pressure like any other orifice. A damper with 0.15 in w.g. of drop in a 0.60 in w.g. branch has an authority of 0.25 -- on target for a parallel-blade damper, and more than an opposed-blade one needs -- while leaking only 11.3 cfm of 2,000 at half an inch, which is excellent. Fixing the authority means raising the damper's own resistance or reducing the rest of the branch, and the first is almost always the answer because a damper sized to the duct is usually oversized for control. This is the air-side companion to the hydronic control valve authority question. A design screen; the damper manufacturer's published leakage class and pressure-drop data, and AMCA's classifications in full, govern.

Formula or tableauthority = damper_dp_inwg / branch_dp_inwg, target 0.20 to 0.25 parallel blade and 0.08 to 0.10 opposed blade (Johnson Controls Damper and Actuator Manual 268.1, p. 6); leakage_1in_cfm = leakage_class_rate x face_area_sqft; leakage_actual_cfm = leakage_1in x sqrt(closed_dp / 1.0); leakage_pct = leakage_actual / design_cfm x 100.

Source and editionControl damper authority as the damper's wide-open pressure drop over the total variable-branch drop, with the JCI 268.1 target band by blade type (parallel 20 to 25%, opposed 8 to 10% of the system drop), and AMCA-classified leakage in cfm per square foot at 1 in w.g. scaled as the square root of the actual pressure, by name. The leakage class rate is entered rather than bundled. The damper manufacturer's published leakage class and pressure-drop data, and AMCA's classifications in full, govern.

Free accessAuthority is a ratio of the user's own measured pressure drops, and the square-root pressure scaling is public orifice physics; no leakage class table is reproduced.

What governsEstimate. AHJ and licensed professional govern.

API field namesdamper_dp_inwg, branch_dp_inwg, face_area_sqft, leakage_class_cfm_sqft, closed_dp_inwg, design_cfm, authority, leakage_1in_cfm, leakage_actual_cfm, leakage_pct

• Target band 0.20 to 0.25 for parallel blades and 0.08 to 0.10 for opposed blades in temperature and mixed-air control (JCI 268.1)HVAC controls practice
• Leakage scaling classified at 1 in w.g. and scaled as sqrt(actual pressure), like any orificeAMCA leakage classification
• Not a tuning problem low authority is hydraulic; controller tuning cannot fix itHVAC controls practice