The heat is on. Summer is beating down on us and it’s bound to stick around for a while. Could a five minute building pressure test provide the answer your customers have been longing for?

Most buildings unintentionally operate under some level of positive or negative pressure. These pressures are usually caused by a defective duct system. Complaints stemming from building pressure problems can include indoor humidity issues when the fan runs and systems that work in the cool of the morning will not cool the building in the afternoon.

Ideally, most buildings should have a positive pressure of about .02-in. w.c. To assure positive building pressures, normally we have to introduce fresh or outside air into the system. This pressurizes the building because if a 1200 CFM fan returns 200 CFM through an outside air duct, it can only pull 1000 CFM from the building. Meanwhile, the supply side of the fan is delivering the full 1200 CFM. This leaves 1000 CFM return and 1200 CFM supply causing a 200 CFM positive CFM in the building. With more supply air than return air in the building, the building is under a positive pressure.



This pressure condition can be designed, but to be sure it actually happens requires air diagnostic testing. It’s hard to say how great the positive pressure reading in the building will be though. It depends on how tight or leaky the envelope of the building is, and what other pressure generating forces exist including the wind, appliances, and exhaust fans and the stack effect of the building.

Because pressures are invisible and variable, we must measure them to understand their effect on a building. In addition to the overall building pressure, pressure measurements can be taken between rooms in the building to further identify the effect of the HVAC system on the performance of a building.