How does air move through a piece of duct? Try to imagine yourself as a molecule of air traveling through a piece of duct work with millions of your closest friends. It might feel like driving down I-25 at rush hour through the tech-center when everyone is staying in their lanes; five lanes of traffic moving in unison in one direction basically in one straight line. If the cars were air it would be called laminar flow; air moving smoothly though a straight piece of duct work without any obstructions, elbows, or eddies.
Now imagine an accident in the center lane of I-25. All the cars would have to slow and go around the accident disturbing the straight stream of traffic. If it were air, or any fluid, moving around the center lane accident turbulence in the flow air would occur. The resulting chaotic swirling lessons the amount of air that can flow through the duct just like the accident in the center lane restricts the number of cars that can pass and slows the traffic or the number of cars that can travel down the straight piece of highway.
Now imagine traveling down the 5 lane highway at rush hour and it changes from 5 lanes to two lanes to pass through a tunnel. The flow of traffic would slow and back up cueing for its turn to pass through the tunnel as the tunnel and the restriction of lanes is a resistance to the steady flow of traffic of the five lane highway. Sharp corners or curves in the road, passing though toll booths, and roundabouts are other examples of things that resist the flow of traffic. In the same way airflow can be restricted. Elbows, small diameter duct, poorly designed diffusers; all resist the flow of air in the duct and change the flow form efficient laminar flow to chaotic swirling turbulent flow. In duct work this is measured by the static pressure in the duct which is a measurement of the resistance to the flow of air, the greater the resistance the less air measured in cubic feet per minute can flow through the duct.
Now that we are measuring the flow of air through ducts on a regular basis, whether it be HVAC duct or bath fan duct, we are realizing that smooth straight properly sized duct is desired to meet the performance requirements of programs or code. This is especially true for bath fan duct as the termination is a vent cap instead of a register cover. It turns out that most vent caps act like the car accident in the center lane, dramatically restricting the flow air, while a duct register cover is like a car pulled over on the side of the road changing a tire, causing curiosity slowing but not impacting the flow of traffic dramatically.
As we move forward with EnergyStar V3 we are learning that the vent cap choice is as important as fan choice, duct layout, and upsizing the duct in order to ensure that fans are able to push the required amount of air out of the house. We are building tight so we need to ensure that moisture management systems are not only installed in homes but that they are performing as they have been designed. We know that the standard 50 CFM builder grade bath fan is not pushing nearly 50 CFM. That most often the louder the fan the less air it is actually moving. However, we were surprised when high quality Energy Star qualified fans that have been upsized in both CFM and ducting where not able to push 50 CFM. The culprit, it turns out, is the vent cap.
Below are pictures of a very typical vent cap that we see in our market. As you can see the screen portion surrounding the 4” diameter duct has about 1” of free space between the top of the duct and the top of the cap. The air crashes into the cap and cannot exit to the outdoors and flow through the fan is impinged increasing the static pressure or the resistance to the flow of air through the system.
Ventilation experts like Paul Raymer with Heyoka Solutions have studied the impact of different vent caps on the flow of air and more information can be found on his and others websites. In general search of a vent cap that allows air to smoothly leave the duct on its way to the outdoors. Just as it is difficult and dangerous to navigate a car traveling at 60 MPH around a 90° corner air can’t be expected to be able to flow smoothly into a wall (vent cap) and then exit to the outdoors. Remember it is there and does it work – all pieces of the building science puzzle are important when it comes to performance.