A typical HVAC system contains numerous dampers. Many dampers are installed to maintain airflow balance but could be eliminated if different design principles were used. Most dampers are designed to restrict or stop airflow. These include flow-control, balancing, economizer, back-draft, face and bypass, and splitter dampers.
The rule of thumb for splitter dampers is: never use them. Another device will always work better. Their biggest problem is a ripple effect on system balance if the splitter moves. Balancing dampers should always be loose fitting in order to restrict but not stop air flow. A 25-mm (1") clearance around the closed balancing damper and duct is usually acceptable and very easy to install. Other dampers are built to stop airflow and should be factory manufactured so that performance data are available. Opposed blade dampers, the best choice for airflow are effective only through the center one-third of the 90° travel. At low angles, their dampening effect is negligible, and at high angles, flow rates change too rapidly compared to the angular displacement or travel of the damper. Dampers can be a source of noise, particularly if a system is poorly balanced. Computer modeling is recommended for studying these damper effects.
Dampers must be installed in places where airflow needs to be controlled and/or blocked. Dampers located directly behind an outlet tend to be noisy. A better location is in the final branch near the connection to the trunk duct. Wherever a balancing or volume damper is located, it should be accessible. Lay-in ceiling tiles provide good access; in a fixed ceiling, an access door is needed. Dampers should not be installed in hood exhaust systems even if the exhaust duct passes through a firewall. Use the UL approved alternative -- a properly supported, heavy-gauge steel, unobstructed duct.
Dampers have to withstand the maximum static pressure in a system. The maximum static pressure is the maximum that can be experienced in a system, not simply the pressure introduced by the fan during normal operation. Maximum static pressure usually occurs when all dampers in a system are closed except those on one flow path. Simulation computer modeling (T-Duct, 1994) can calculate maximum conditions.