Among the biggest misconceptions in the HVAC industry is the determination of the required duct size to support a given volume of airflow. If you do your research widely, you will come across a variety of rules of thumb. The assumption is that you have done the necessary calculations to determine the much air you need in your target room.
The Friction Rate
The friction rate is a very critical number which determines the relationship that exists between airflow and duct size. Friction rate is a description of the average pressure drop for every 100 feet of duct length in a system. This number is not only unique to an individual duct run, but also unique to a system.
For instance, other factors held constant, an 8-inch duct at the very end of a long and winding duct system doesn’t deliver as much air as an 8-inch duct that has a short straight system. The reason behind this variation is that as air passes through spaces and objects, it has an impact that affects the much air that comes out at the end.
The beauty with friction rate is that it factors in the static pressure provided by your fan and how much of it is left after you less the big-ticket items such as the supply registers, filter, coil, and return grilles.
Equivalent Length
While it may be argued that a majority of duct systems do not have a 100 feet long runs, whichever the length of your duct system, you can always equate it per 100 feet. Equivalent lengths are numbers that you can look up directly at the appendix of ACCA Manual D.
For instance, a fitting such as an elbow or t-wye will be assigned a number which is equivalent to a length of straight ducts with the same pressure drop. You may see a 90-degree elbow being equated to a length of 15 feet or a t-wye given an equivalent length of 10 feet.
Even if the length of the run is short, you may still use the friction rate because the 100-feet number can be expressed in different forms. A common mistake in duct installation and measurements is to confuse friction rate which is basically the inches of water column lost for every 100 feet with static pressure which is simply the inches of water column.
There are tables computed to help you in picking the right duct size for a particular air flow. Alternatively, you can do a detailed load calculation using the ACCA Manual J using carefully selected equipment as listed on Manual S to help you exactly figure out the much air you need in each room.
At times, using the table blindly may lead you to a destination where the said duct size doesn’t deliver the corresponding air flow. The duct system has a trunk and it splits in two more trunks that end up in registers. These also affect the air flow hence the need for professional duct installation services.
