Engineer-to-Engineer Guidance for Specifying Dampers, Louvers

By Brittany Swan

Dampers and louvers are integral to HVAC performance, yet often receive minimal attention in the early stages of system design. Overlooking these components can lead to inefficiency, energy loss, and code-compliance issues. Drawing on the field experience of consulting engineers Sean Beilman, PE, and David Sellers, PE, this article provides tips for the effective design, specification, and installation of dampers and louvers.

Static-Pressure Loss

Pressure loss depends on the type, design, and location of a duct system. According to Beilman, control and balancing dampers in medium-pressure systems typically do not cause significant pressure loss, even when ducts are sized unevenly and the dampers are fully open. However, fire dampers, smoke dampers, and combination fire/smoke dampers can introduce substantial pressure drop, if not properly designed or correctly located. Beilman advises using curtain-style fire dampers, the blades of which are outside an air stream, whenever possible to minimize turbulence and noise, especially in smaller ducts. Also, he notes that actuators for smoke dampers, combination fire/smoke dampers, and some fire dampers can be installed either inside or outside ductwork and recommends external placement when possible to reduce pressure loss and simplify maintenance.

Beilman adds that, while outdoor-air intake louvers usually are selected at low face velocity, exhaust-air louvers must be evaluated for pressure loss directly.

“I typically aim for no more than 0.10 in. wc across exhaust louvers,” Beilman said. “Poor duct transitions can amplify pressure drop and reduce weather protection.”

Sellers explains that static-pressure losses also are related to louver and damper blade geometry and cross-sectional area. More aerodynamic shapes and larger free areas mean lower pressure drop, all other things being equal. With long-term operation—say, 20 to 30 years for an office building—those small gains translate to major energy savings.

Sellers also notes that understanding the static-pressure drop and flow-vs.-damper position characteristics of different damper-blade designs is essential, as the fan energy needed to move air through a damper or louver is directly tied to the resulting pressure drop. For dampers that spend most of their life wide open—fire and smoke dampers, for example—blade designs and damper configurations that minimize pressure loss are quite desirable. But for a damper that modulates to provide a control function—air-side economizer dampers, for instance—some measure of pressure drop is needed to achieve control.

Installation Pitfalls

Beilman highlights two critical issues that can undermine damper performance: oversizing and corrosion.

Oversizing dampers can lead to diminished airflow regulation. To match the pressure loss of a smaller damper, an oversized damper must be nearly closed, which reduces its ability to modulate airflow effectively. In many cases, even with a damper fully closed, a system cannot achieve low design airflow.

Beilman also warns against using galvanized dampers in outdoor-air ducts, noting they can corrode from humidity and salinity in under 10 years. He recommends aluminum or stainless-steel construction for durability.

Sellers flags two issues: failing to size modulating control dampers correctly and not providing enough detail in drawings and specifications to ensure proper functionality.

When dampers are oversized, the relationship between pressure drop and blade angle becomes highly non-linear. For example, an oversized damper may deliver 70 percent of rated airflow within the first 15 to 20 degrees of blade rotation and reach 95 to 98 percent by the time it is half-open. Sellers says the result is like trying to fill a teacup with a fire hose—small blade movements cause large surges in airflow. Conversely, large blade movements may yield only minor reductions in flow.

In mixing boxes, oversized dampers can create flow surges during economizer cycles, causing instability in fans, terminal units, and building pressure control. Operators often resort to manual adjustments at startup, as compensating for non-linearity through control strategies rarely is practical. Proper sizing and configuration are essential: Dampers often are smaller than ducts, and their orientation (opposed- vs. parallel-blade action, blade type, placement) strongly affects mixing and pressure drop. Parallel blades promote mixing but could result in less linear operation than opposed blades. Accurate mixed-air temperature measurement also is critical, as plenums are highly dynamic and prone to stratification. Averaging sensors and multiple freezestats are recommended, especially in large plenums. These findings are supported by both field experience and research, including ASHRAE research project 1045 and the work of Keith Peterson.

Design Integrity

Beilman maintains that building owners can formulate non-negotiable specifications for certain products so the products are not subject to value engineering. Specifications can be provided to design teams for consistent application across an owner’s projects. Per Beilman, this approach has been implemented successfully in many cases. By clearly defining acceptable manufacturers, construction materials, and required certifications, owners can ensure that only components with a proven track record of reliability and performance are used.

The Experts

Sean Beilman, PE, is a commissioning authority and mechanical engineer with Melbourne, Fla.-based Thermal Systems Commissioning Inc. He leads the commissioning of commercial and industrial buildings to ensure system performance aligns with design intent and owner requirements. With over 20 years of industry experience, his responsibilities include developing commissioning plans, reviewing design documents, conducting site inspections, leading functional-performance testing, and facilitating project turnover. Active with ASHRAE since 2012, he serves on the committee for ANSI/ASHRAE/IES Standard 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings.

David Sellers, PE, is a senior engineer for Portland, Ore.-based Facility Dynamics Engineering with more than four decades of HVAC experience spanning design, controls, facilities engineering, and commissioning. An engaging instructor for the Building Commissioning Association and various utilities, he authors ASHRAE Journal’s “Engineer’s Notebook” column and the blog A Field Perspective on Engineering, in which he breaks down economizers, data logging, and control sequences into practical, field-tested techniques. The training he conducts emphasizes hands-on diagnostics, retrocommissioning, and controls-commissioning best practices.