Page 4 of 5

The example unit served some perimeter rooms that had hot-water baseboard heat. The baseboard heat canceled the heat loss through the walls and windows, making the rooms work like interior spaces for winter cooling-load purposes; therefore, their winter cooling load did not decrease to zero. However, the transmission portion of the cooling load was zero, and the solar gain sometimes was quite low (particularly for the rooms that did not face south). Also, at various times, some rooms were empty with their lights off. Even though the 20,000-cfm design supply airflow represented the peak cooling load, a winter cooling load of 8,000 cfm was not unreasonable or unrealistic for this system.

Introducing a minimum outside-air quantity of 4,000 cfm at 0°F with a total airflow of only 8,000 cfm would have resulted in a mixed-air temperature of 35°F, which was too cold to send down the duct and well below the usual 38°F set point for the freezestat. To solve the problem, a mixed-air low-limit override control was incorporated into the system, limiting the outside-air damper position to prevent the mixed-air temperature from going below a desired minimum, such as 50°F. If the override is activated, the building would have been short of outside air. Because the conditions occur only a few hours per year (probably during unoccupied hours at night when toilet and other exhausts could be shut off), the expense of installing a preheat coil in the AHU could not be justified.

WHY IT WORKS

A damper open to a fixed position acts like a fixed orifice. The pressure drop through an orifice (or fixed damper) varies with the square of the flow. As long as the damper does not move, a constant pressure drop across the damper translates to a constant flow (Point D to Point C in Figure 1). Regardless of the total airflow through the mixing box, if the pressure drop across the outside-air damper (Damper 3, Figure 1) is constant, the outside-air quantity will be constant as well.

In the mixed-air-plenum-pressure control scheme, the minimum outside-air control drives only the return-fan speed or other fan-capacity control, such as inlet vanes or a discharge damper. The return-air-damper position is a function of mixed-air temperature, not building pressure or return-fan speed. The return-air damper moves in conjunction with the outside-air damper in the economizer cycle. If the outdoor temperature is within a range in which the economizer cycle is not active, the return-air damper will remain in one position, regardless of system airflow.

If the supply fan slows, the airflow through the mixing box is reduced, and the mixing-box pressure becomes “less negative.” A smaller pressure drop across the outside-air damper means the system has less than the desired minimum outside airflow. Supply air is the total combination of return and outside air. If outside air is too low, return air must be too high. Slowing the return fan in response to the “less negative” pressure in the mixing box reduces the amount of return airflow. Outside airflow then increases to make up the difference. The pressure drop across the outside-air damper (at its minimum position) increases accordingly, so the negative pressure in the mixing box and the minimum outside airflow returns to set point.

The mixed-air-plenum-pressure concept works for systems with return fans. The sole function of a return fan is to overcome pressure in the return duct. For minimum-outside-air and building-pressure control, return fans are problems to be solved, not solutions to a problem. Avoiding return fans saves costs and often makes sense.

The concept also works for systems with low return-duct pressure drop that do not have return fans and still need minimum outside-air control. If no return fan exists, mixed-air-plenum pressure overrides the economizer control and limits the opening of the return-air damper to maintain the desired negative pressure in the mixing box. Without a return fan, building pressure can be controlled via a gravity damper or exhaust fan to relieve excess outside air introduced by the economizer cycle.