Control of Building Pressure
Control of building pressure can have a significant effect on energy use, drafts through exterior doors, and comfort. In a hot and humid climate, it is valuable to keep the building at a slightly positive pressure. This ensures that dry air, from inside the building, enters the walls rather than allowing humid air from outside to enter the building through the wall and likely cause mould growth. In a cold climate, the building should be kept close to outside pressure, or slightly negative, to prevent the warm, moist air from inside the building from entering the wall where it could and cause condensation or ice.
When an economizer is running with 100% outside air, the same amount of air must also leave the building. On small systems, no return or exhaust fan is provided, on the assumption that the washroom exhaust plus leakage will be adequate to balance the amount of air coming in.
In milder climates, intermediate size plants can be accommodated with “barometric dampers.” Barometric dampers blow open when there is a slightly greater pressure, than outside at that location in the building ‘At that location’ is included as a proviso, since the wind can make a huge difference to the pressure at different points around a building. If the wind is blowing towards the damper, it will tend to keep it shut. On the other hand if the damper is on the leeward side of the building, the wind will tend to open it.
On the larger economizer systems, typically the ones shown in the figures in this text, complete with a return fan, the return/relief fan and relief damper can be used to control building pressure. The least efficient method is to separately control the relief damper and effectively throttle the relief fan flow. Better, is to add a speed control for the return fan so that it maintains a set minimum outlet pressure. This will ensure adequate return air for the main supply fan and allow the relief damper to control the building pressure.
The Final Step
Chapter 13 is the final chapter. In it we cover two groups of topics that did not fit into the flow of the previous chapters. The first group deals with heating and heat storage. The second group deals with air distribution in rooms and separate outdoor air systems.
Finally, there are some suggestions for you for future courses and other sources of information.