If an AC system is working properly, any air pressure changes it creates should be brief and transient. The ideal HVAC system functions as a closed-loop. For every cubic foot of air that the AC conditions and pushes into your building, a cubic foot of air is pushed outside through what is known as “return vents.” This creates a pressure balance that leaves the building essentially neutral with the outside world.
When an AC system first turns on there may be a slight pressure imbalance in different rooms. This is because the unit has introduced more air into the system but that air has not had a chance to disperse throughout all the rooms and push air out of the return vents. However, if the HVAC loop if functioning correctly this pressure imbalance is quickly rectified, returning the building to neutral pressure.
On the other hand, if your AC system is not working as a complete loop because of improper ventilation or if your business uses large fans or blowers, AC can absolutely contribute to pressure changes in a building.
Positive Pressure, Negative Pressure, and Slamming Doors
Positive pressure is when the air pressure in space is greater than the environment outside. Have you ever had to put your shoulder into pushing open a front door, you have experienced positive pressure. Positive air pressure can be caused by a variety of reasons, some of the AC related. They include:
- return vents blocked by either dust or outside debris
- Closed doors that break the airflow loop
- Rising warm air raising air pressure in upper floors
- Leaky ducts in the return system
Essentially, any obstruction or leak in the HVAC system can create a positive pressure system in part or all of your buildings.
Negative pressure is just the opposite of positive; it is when the air pressure inside of a building or room is lower than the pressure outside. This can happen in buildings with large server rooms or restaurants with powerful fume hoods. In essence, more air is being pumped out by the exhaust fans than is brought in via the AC system and other small, non-airtight portions of the building. If you notice doors slamming open suddenly, often negative pressure is the culprit.
Clearly, an AC system in need of a tune-up can change the room air pressure. If you are noticing signs of a possible air pressure imbalance in your home or business, give us a call at 408-295-2182 today.