Much has been written about controlling the minimum-outside-air quantity in variable-air-volume (VAV) air-handling systems. Instead of debating the merits of various control methods, this article will discuss how to apply the mixed-air-plenum pressure method and why it works.

BACKGROUND

Although most buildings require outside air to make up exhausts and meet ventilation needs, other buildings require a constant minimum quantity of outside air. In constant-volume systems, the minimum required outside-air quantity is a constant percentage of the constant supply-air quantity. Therefore, a fixed outside-air damper position provides the fixed minimum-outside-air quantity.

In VAV systems, the supply-air quantity varies with load, but the minimum required outside-air quantity does not change. Exhausts, such as toilet exhausts, might be constant regardless of load. Design occupancy and the resulting outside-air quantity required for ventilation also might be constant. If the minimum required outside-air quantity remains constant, but the supply-air quantity decreases, the minimum required outside-air quantity increases as a percentage of the supply-air quantity. When a system operates with minimum outside air (and an economizer is not active), a fixed minimum-outside-air damper position will result in smaller amounts of outside air as the VAV supply-air quantity decreases.

The system needs a way to control the outside-air quantity to maintain the desired minimum. Several methods have been employed, such as:

Direct measurement. Outside-air quantity can be measured directly by pitot or electronic device. The outside-air damper modulates as required to deliver the minimum outside-air quantity. The technique measures outside air directly, independent of and unaffected by other control loops.

The method has been criticized because it can be difficult to measure the low velocities involved accurately, but some of the difficulties have been overcome by advancements in hot-wire anemometers and related technologies. The method also does not address other control considerations, such as controlling the return fan. A return fan that runs too fast can depressurize a building. A return fan that runs too slow can result in outside air being sucked in through the relief/spill damper (Damper 1 in Figure 1).

Fan tracking. Fan tracking measures supply and return airflows. The return-fan capacity is controlled to maintain a constant difference between supply and return airflow.

The method is appealing because it measures the quantities of interest more or less directly. As previously mentioned, the direct-measurement method does not always measure low air velocities accurately; however, fan tracking reduces this risk by measuring airflow at duct velocity.

Nevertheless, the method has been criticized because inaccurate outside-air-quantity measurements are the result of the cumulative inaccuracy of two independent measurements (supply and return airflow). Additionally, the method applies only to systems that have return fans.

Building-pressure control. Gravity dampers or an exhaust fan can be used to control building pressure. An engineer can subtract the amount of exhausts (such as toilet exhausts) from the minimum-outside-air quantity and estimate the extent to which the remaining outside air could pressurize the building. During commissioning, pressurization set points can be determined by taking building-pressure field measurements with known outside-air quantities.

One drawback is that the method does not truly control minimum outside air. The method uses a surrogate, such as the supply-fan capacity signal, as an approximation of total supply airflow. The controls use the surrogate signal to open the outside-air damper further as the supply fan slows, maintaining a nearly constant minimum outside airflow rather than a constant percentage of supply airflow.

Mixed-air-plenum-pressure control. Return-fan capacity can be controlled by pressure in the mixed-air plenum (Point C in Figure 1). The rest of this article will discuss how to apply the mixed-air-plenum pressure-control method.