Curing "Sick Building Syndrome"
To meet the requirements of ASHRAE 62 the outside air entering a building should be measured and controlled. There are several methods to control outside air and the improper use of outside air is a contributing factor to Sick Building Syndrome. For well over 15 years, Sick Building Syndrome has been the topic of discussion at the ASHRAE meetings and in a number of publications. In 52% of the cases studied, the cause of sick buildings is blamed on inadequate ventilation.
The most common method used to control the minimum outside air is to allow a fixed amount of outside air into the mixed air plenum. This is accomplished by setting the outside air damper operator to maintain a minimum position. The adjustment on the damper operator is usually set by the balancing contractor. This has several downfalls in maintaining a minimum amount of outside air.
First the negative pressure in the plenum varies based on how dirty the filters get and how much wind is blowing outside the building. Second, the repeatability of the damper operator to return the damper to the same position every time is inconsistent. Rod and ball joint linkage will allow the damper to vary position by 5% and even direct coupled actuators allow a 2 1/2% movement. This can cause a change in the amount of outside air introduced into the building. Changes in mixed air plenum pressure on VAV systems will vary the amount of fresh air introduced in direct proportion to the change in total supply volume.
Another way to control the minimum amount of outside air is to use a separate minimum outside air damper to control the incoming air. This has an advantage in that the velocity through the small outside air damper is higher than if you allow a fixed amount of outside air into the mixed air plenum as described earlier. Using a separate minimum outside air damper makes it easier to measure and set the controls. But, you are still limited by dirty filters, wind pressure on the outside of the building and the variance for VAV systems.
A third approach is to use air measuring stations on the return air and on the supply air. The difference between the two measuring stations is the amount of outside air entering the building. In this case, suppose you have a 100,000 cfm supply fan, an 85,000 cfm return air requirement and 15,000 cfm fresh air requirement. The accuracy of the air measuring station on the supply fan is 3% or 3,000 cfm. The accuracy of the return air measuring station is 3% or 2,550 cfm, so the potential error in the 15,000 cfm outside air entering the building is 5,550 cfm.
The only way to measure and control the outside air with any accuracy is to control and measure the outdoor air rate at the intake damper. To do this an air measuring system must be integrated with the outside air damper so they are tested and function as a system.