Why building pressure matters
In commercial buildings, pressure matters. If you have untreated outdoor air leaking into the building when the indoor pressure is less than the pressure outside, you need to limit or eliminate the infiltration of the untreated outdoor air. The repercussions of not treating the outdoor air, is an increase in HVAC loads and a resultant increase in your operating costs.
Even so, outdoor air leaking into your building isn’t always negative. For example, if you have a little bit of dry outdoor air leaking into your building envelope, it can prevent condensation.
It’s when you have excessive pressure that you run into problems: increased costs, uncomfortable tenants, and complaints. This situation results from the drafts and stratification that interferes with temperature control, and that causes odor migration. When you have doors that swing outward they could become awkward to open, while doors that swing inward could fail to reclose—a major security issue.
Further problems occur during the cooling season if any outdoor air infiltrates the building as it can raise the dewpoint within the building envelope. This increases the possibility of microbial growth and the structural deterioration of your building. If warm, moist air infiltrates it also can affect occupied spaces as it increases latent loads.
In the summer, the exfiltration of cool, dehumidified indoor air benefits your building as it keeps the envelope dry. The repercussions from excessive positive pressure are threefold: it creates havoc with the temperature by impeding the supply airflow into occupied spaces, it makes opening and closing doors difficult, and, finally, it creates noisy high-velocity airflow around doors and windows.
In the winter, the repercussions from even slightly positive pressure can damage your building as it forces the moisture from the indoor air into your building envelope—if the inside walls are cold, then it could result in structural deterioration.
During cold weather, the ideal net pressure inside your building relative to the outside should range from slightly negative or neutral, which minimizes exfiltration. During the warm weather, it should be slightly positive, which minimizes infiltration. You should avoid excessive building pressure. It doesn’t matter whether it’s positive or negative, avoid it at all costs.
Forces influencing building pressure
It’s really quite obvious when you consider the outdoor elements and the operating system: the wind, the weather, and the mechanical ventilation system. When you try to prevent extreme pressure, it’s tough as the indoor and outdoor pressure directly results from the combined effect of wind, weather and the operation of your mechanical ventilation system.
Wind pressure pulls indoor air from the leeward side of your building, and pushes the outdoor air into the windward side of your building. It’s like a stack effect: wind pressure not only varies with building height and outdoor air density, but also with the protection provided by nearby landscaping such as trees and other buildings.
Think of a pipe filled with a column of water. Then think of what results from the weight of a column of air: It results in a head pressure that increases from the top of the column to the bottom. This pressure is known as either “hydrostatic” or “stack” as the weight of the air column is affected by local temperature, barometric pressure, and the humidity ratio.
Now think of indoor and outdoor air density temperature related differences. They generate variances in pressure, affecting infiltration, exfiltration, and air movement direction within stairwells and shafts.
Warmer indoor air
When your indoor air is warmer than your outdoor air, then it results in a less dense column of air inside the building. This leads to a net negative pressure below the neutral pressure level (NPL), and a corresponding net positive pressure above it. As the building envelope contains, as do all building envelopes, unavoidable cracks and openings, this pressure difference causes the outdoor air to enter the lower floors and the indoor air to leave the upper floors. As well, these leakages encourage upward airflow, which is a normal stack effect, within stairwells and shafts.
Cooler indoor air
When your indoor air is cooler than your outdoor air, then it results in a more dense column of air inside the building. This leads to a net negative pressure at the top of the building, and a corresponding net positive pressure at the bottom. Unless you control building pressure, the outdoor air will infiltrate the upper floors while the indoor air exfiltrates from the lower levels. Additionally, you can have a reverse stack effect when the pressure difference encourages downward airflow within stairwells and shafts.
- Mechanical ventilation system
It’s a matter of balancing the intake airflow and the relief airflow with fans—some force air into the building, while others force it out of the building. The balance between intake and relief airflow determines whether the net building pressure is negative or positive. If you have excess intake airflow, then you pressurize your building by generating a net positive pressure. And if you have excess relief airflow, then you depressurize your building by generating a net negative pressure.
How the intake and relief airflows vary during normal system operation depends on your system type, control strategy and operating schedule. For example, the relief fan might operate on demand intermittently, while intake or outdoor airflow will modify with an airside economizer operation.
Managing building pressure
It all depends on the time of year and the height of your building whether the building pressure is positive or negative. In the meantime, because of the combined forces of wind, weather and the operation of your mechanical ventilation system, the actual building pressure can be positive or negative.
In other words, to put it simply, you are establishing your preferred building pressure under continuously changing conditions.
Positive or negative building pressure
If you want your controls to maintain either a positive or negative building pressure through modifying relief air, then the minimum intake airflow must exceed maximum exhaust airflow. After, you can use the relief airflow to trim the difference between the airflow into and out of the building.
Positive building pressure results from minimum relief airflow, while negative building pressure results from maximum relief airflow. If your system has an airside-economizer operation, your minimum relief airflow might be zero, while your maximum relief airflow might approach maximum intake airflow.
Passive or active building pressure
You can regulate your building pressure to be passive or active. Most commonly, passive building pressure consists of a gravity-operated damper in the occupied space. When you operate your economizer, you prevent over pressurization as the positive pressure that develops inside the space pushes air out of the damper. You can install a relief damper, they are inexpensive to install, although they are susceptible to wind and the stack effect, limiting their use to single-story buildings and small systems.
It depends on the air distribution configuration system whether your active building pressure control requires either a return fan or a relief fan.
If you opt for a return fan that operates whenever the supply fan does, you pull return air from the occupied space, pressurizing the return air plenum at the air handler. The plenum air either passes through the recirculating damper into the mixed air plenum, or it leaves the building through the relief damper. If you want to reduce the burden on the return fan use an energy-efficient enhancement to the return-air-plenum control, or optimized damper control. This keeps one damper nearly wide open; thus reducing the burden on the return fan. Further, based on the positions of the relief and recirculating dampers, it modulates the plenum-pressure setpoint and it modifies the return-fan speed.
Building pressure is critical for a comfortable and healthy indoor environment. To learn more about building pressurization, please contact us.