Operating Under Pressure: The Importance of Maintaining Building Pressure
When changing from one event to another, building pressure and air flow can be a major concern for venue operations. Not only can they affect energy efficiency, a major concern for many owners, they can also affect air quality in the building, and, by extension, both patron experience and the event itself.
In 2016, SSR was awarded a contract by the NBA to assess all of their arenas after they encountered a few operational issues that impacted the events and the players. When a building’s mechanical, electrical, and plumbing systems stop functioning properly, not only can the building develop odors and mold growth, games and events can be delayed or cancelled due to poor inside conditions, costing teams, promoters, and building owners revenue.
When it comes to building pressure, airflow, and heat loss, there are three main factors: wind pressure, stack effect (or the buoyancy of air — rising warm air causing pressure changes within a building), and the building’s mechanical systems.
Outside air plays a huge part in maintaining and adjusting the inside air pressure. One way to combat the pressure that can be imposed upon the building from outside forces is the use of pressure sensors. The placement of these is crucial.
The readings from a pressure sensor can vary widely depending on the direction and severity of the wind. On a nice calm day, you can get fairly accurate readings from a pressure sensor, but in any other condition, the wind pressure can create a negative pressure in the building despite what your pressure sensor may say. The best way to prepare for this is to place sensors on all sides of the building and average the readings to get a more accurate result.
Another element that comes into play is the stack effect, or the buoyancy of air. Air will escape a building through the gaps in the enclosure, and when it does, it must be replaced by outside air coming through doors or other uncontrolled openings. Air leakage occurs when the building enclosure isn’t completely sealed. As air escapes and is replaced with unconditioned, humid air, the facility’s mechanical systems have to work harder to condition the air and maintain pressure, causing energy and cost inefficiencies.
The only way to control the pressure is to make sure the airflow differential is constant. The building pressure can be maintained if your AHU system puts out more airflow than the building takes in through uncontrolled openings like doors, loading docks, or windows. The general rule of thumb is to put in 5% more air than you take out, which can be scaled as necessary for the size of the building. The goal is to have the building push out air when the doors are opened.
It is important, however, not to forget about the exhaust air driven by the toilets and cooking exhaust. If humidity or temperature is a problem, turning off the outside airflow won’t fix the problem. The cooking and toilet exhaust created inside the building is constant. If there is no air coming from outside, you get negative pressure in the building, which causes unconditioned air infiltration when the doors are opened, which can cause greater humidity and moisture problems.
In March of 2017, a Minnesota Timberwolves game was postponed due to condensation on the court floor. Bookended by events utilizing the ice floor, the game required the court to be placed on top of the ice, which is the standard practice in a multipurpose event space. With the sheet of ice under the court, the court floor was cold at the same time the arena had a temperature and humidity problem. By lowering the temperature on the AHU, the air would have been dehumidified and the moisture controlled, but instead, the outside air flow was turned off, which caused outside air to come into the building. With the dew point of the floor lower than that of the outside air coming into the building, condensation settled on the court, making for unplayable conditions. Similarly, moist outside air can cause fog or frost on the ice sheet during those events.
Not conditioning the outside air is not an option in most venues. If cost is a concern, one solution to still maintain comfortable inside conditions and a positive or neutral pressure could be to reduce the exhaust air flow instead of just reducing the outside air. This can be achieved by only running cooking exhaust during events, replacing cooking hoods with higher efficiency hoods that require lower exhaust airflows, or scheduling exhaust to only operate when needed during events.
In any building, airflow must be controlled to maintain favorable indoor conditions. Pressure constantly fluctuates, both inside the building due to supply air and exhaust air and outside due to weather conditions. It must be closely monitored, but understanding the changes in pressure and the causes of inside air issues can lead to quicker, more cost efficient solutions.