THE BUILDING SIDE OF THE DUCT SYSTEM

Almost six years ago, I wrote about a concept called “The Building Side of the Duct System.” The article looked at how a forced-air HVAC system is a closed loop and the building acts as the connection point between supply and return airflow.

The building itself is a duct system and HVAC system extension. It works like a traditional duct system but is constructed of different materials, including wood, drywall, and insulation. Individual rooms operate like ducts, while interior doors operate like volume dampers.

It’s a game changer once you understand your customers “live inside” the duct system and that the pressures in it directly influence how they live and feel.

UNINTENDED CONSEQUENCES

There’s a good possibility you’ve encountered building pressure issues and didn’t know it. Here are some common problems that could indicate you’ve come across them:

• Dust and air quality problems;
• Uncontrollable humidity — dry in winter, humid in summer;
• Uncomfortable rooms — hot and cold areas;
• Large temperature swings between floors — hot second floor and cold first floor;
• Properly sized and installed systems that can’t maintain comfortable conditions; and
• Cold floors.

In addition to these issues, it’s important to point out that carbon monoxide (CO) poisoning risks also increase in homes with building pressure problems. Flue gases, auto exhaust, and other harmful fumes can travel into the living space unless pressures are controlled.

INFILTRATION AND EXFILTRATION

Infiltration is uncontrolled airflow coming into a building from the outdoors or unconditioned spaces, like a crawlspace or attic. Think of it like return duct leakage — air is pulled in through unintentional leaks in the return ducts. You often end up with air coming into a building from areas that you don’t want it coming from and shouldn’t be breathing.

Exfiltration is the opposite of infiltration. It is uncontrolled indoor airflow from the conditioned space that leaves a building. Compare it to supply duct leakage — air is pushed out through unintentional leaks in the supply ducts. Expensive conditioned air that your customers want to keep inside their home is pushed outside and lost.

Unfortunately, building leakage locations are plentiful. You find them in attics, basements, crawlspaces, and inside the building. Any opening that connects the building’s inside to either the outdoors or an unconditioned space should be considered a potential pathway.

We’ve traditionally been taught to look at windows and door as the main offenders, but the worst culprits are usually hidden. You must dig to find them. They range from duct chases and soffits to plumbing penetrations and can lights.

BUILDING PRESSURE DRIVES BUILDING AIRFLOW

Airflow moves through an opening only when there’s a pressure difference on either side of it. Let’s say you have an open window and there is no wind or temperature difference between the inside and outside of the building. Under these conditions, no air would flow through the window.

However, if the wind started to blow into the window, onto the opposite side of the house, or if the outdoor temperature dropped, this creates a pressure difference across the window. Since higher pressure goes to lower pressure, air moves through the window.

Airflow is measured in cfm (cubic feet per minute). To picture cfm, think of a 12 x 12 x 12 cardboard shipping box. This represents approximately 1 cfm of air. For every 1 cfm of airflow into an opening, 1 cfm must also flow out.

Let’s say you have a kitchen exhaust fan that’s sucking 250 cfm of air from a kitchen. That means there’s also 250 cfm coming into the building — moving into the kitchen and towards the exhaust fan. It’s important to remember that air is lazy and will take the path of least resistance. It’s much easier for air to flow through a large opening close to the exhaust fan than a smaller one that’s farther away.

Air temperature differences also change building pressures and influence airflow. A simple way to remember this is that “hot air rises, cold air falls.” Heated air moves towards upper levels in a building, while cooler air moves to the lower levels.

There are a lot of interactions that influence building pressure and airflow. Some occur naturally, while others are set in motion mechanically.

NATURAL BUILDING PRESSURE INFLUENCERS

Natural building pressure influencers typically fall into two categories: wind-driven and stack effect.

Wind-driven pressures occur when the wind is blowing. This natural force creates a pressure difference across any opening that drives airflow. Imagine your home on a windy day. Unless you live in a submarine or refrigerator, anytime the wind blows, air flows into the side of the building the wind blows against, and air leaves from the opposite side.

Stack effect is another naturally driven airflow that occurs due to temperature and air density differences between the indoors and outdoors. Airflow is tied to warmer air rising and cooler air dropping down to displace it. In the winter, warm air rises to the building’s upper levels and exits, while cooler air pulls into the lower levels. The building mimics a chimney, or “stack.”

You’ll find this interaction is the source of many issues your customers want solutions for. They include uncontrollable humidity, uncomfortable rooms, large temperature swings, and cold floors. HVAC equipment upgrades alone typically don’t fix these issues, so don’t fool yourself.

MECHANICAL BUILDING PRESSURE INFLUENCERS

Fans are mechanical devices that also change building pressure and airflow. Hopefully, the influence is intentional and controlled such as ventilation from a make-up air duct, ERV (Energy Recovery Ventilator)/HRV (heat recovery ventilator), and a balanced HVAC system.

Installation defects such as duct leakage cause unintentional interactions. Any air lost through these leaks impacts building pressure and could lead to uncomfortable and unhealthy conditions when the HVAC system is running.

Poor design also leads to unintentional interactions. Consider how an interior door acts like a manual damper in a duct system. If you have a central return grille installation, door position affects how much air moves from the supply registers back to the return grille.

When the doors are open, it’s easy for supply air to move back to the return. If the doors are closed, the return grille is starved for air from those rooms and will pull it from wherever it can. If you catch yourself chasing random comfort issues, this might be the source.

NEXT STEPS

I hope this brief introduction on building pressure and airflow inspires you to look at buildings differently. Become a student of building pressures and work to better understand how it affects your customers and HVAC installations.

It’s easy to guess and use silver bullets to address comfort, indoor air quality, and carbon monoxide issues. What if building pressures are causing these issues? It will take a higher level of dedication and understanding to expand your view and consider the part of the duct system that your customers live in.

Next month, we’ll dig deeper into this topic as we look at how to pressure test the building side of the duct system and diagnose some of the most common problems.

If you’re an HVAC contractor or technician interested in learning more about building pressures, contact me at davidr@ncihvac.com. NCI’s website www.nationalcomfortinstitute.com is full of free technical articles and downloads to help you improve your professionalism and strengthen your company.

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