In any type of building, positive pressures are created by installing a fresh air duct to the HVAC system with the proper HVAC access door and panel, or by proper installation of a Heat Recovery Ventilator (HRV). however, many HVAC professionals don’t really feel comfortable looking beyond the equipment and ducts that they install. It is also known that there can be a reduction in the amount of return air of fresh air. Take, for instance, if 10% of fresh air is brought into the system, the fan can only pull 90% of the return air through the return ducting.
HVAC professionals having the mindset of not really caring about the system after installation can impede their ability to best serve their client’s needs. Having no or less understanding of how the building interacts with the HVAC system. Positive pressure controls ventilation into a building, as well as keeps air from being pulled in from locations that are undesirable. Building pressures are important because they help you see what’s really happening in a building just like static pressure does in an HVAC system. To help you understand the overlooked HVAC system influencer, it’s time that we look at some building pressure essentials.
Duct System’s Building Side
A forced-air HVAC system is a closed-loop and the building also acts as the connection point between the return airflow and supply. Not everybody knows this fact but the building itself is actually a duct system and an HVAC system extension. Although it works like a traditional duct system, it is still different because it is constructed of different materials, including drywall, wood, and insulation.
Consequences that are Unintended
There’s always a high possibility that you’ve encountered building pressure without knowing it. These common problems could indicate that you’ve come across them.
- Problems with dust and air quality;
- Uncontrollable humidity like dryness in winter and humidity in summer;
- Uncomfortable rooms;
- Large temperature swings between floors;
- Installed systems that are properly sized but can’t maintain comfortable conditions; and
- Cold floors.
Knowledge about Infiltration and Exfiltration
When it comes to infiltration, which is the uncontrolled airflow that comes into a building from the outdoors or unconditioned spaces such as a crawlspace or attic, it is important to see it as a return duct leakage– the air is pulled in through the unintentional leaks in the return ducts. This often results in the air coming into a building from areas that you don’t want it coming from.
On the other hand, the opposite of infiltration is exfiltration. This refers to the uncontrolled indoor airflow from the conditioned space that leaves a building. You can then compare it to supply duct leakage where the air is pushed out through unintentional leaks in the supply ducts.
- Building Airflow is Driven by Building Pressure
In a building, airflow moves through an opening only when there’s a pressure difference on either side. For example, you have an open window with no wind or temperature difference between the interior and exterior. No air would flow through the window under these conditions. If the wind blows into the window, onto the opposite side of the house, or if there is a temperature drop, this creates a pressure difference across the window. Because higher pressure goes to lower pressure, the air then moves through the window. Building pressures change and airflow is also influenced by air temperature differences. Keep in mind that there are many interactions that influence building pressure and airflow.
- Natural Building Pressure Influencers
Typically, there are two categories that influence natural building pressure: stack effect and wind-driven.
Stack effect is a naturally-driven airflow that occurs due to differences in temperature and air density between the indoors and outdoors. The airflow is tied to warmer air rise and cooler airdrop to displace it. On the other hand, wind-driven pressure occurs when the wind is blowing. The natural force creates a pressure difference across any opening driving airflow.
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