I came across a good post on how a 6 inch duct will not always deliver 100cfm. The writer brings up a good point with regards to how to size ducts and in particular the careful and correct use of a ductulator.

In an ideal world, a duct sized at say 0.1 inches per 100 feet should give you 100cfm if the duct size is 6″. However, there is a big difference between a theoretical calculation that you can do on  a ductulator vs the real world application. The way the duct is applied has a lot to do with the delivered air flow. If its a residential application and the duct is a spiral that is laid down say in the attic, then there is great chance that kinks and other obstructions can impede the airflow reducing the airflow.

To make matters worse, most residential fans are typically fractional horsepower fans, meaning that their power or “oomph”  to deliver air does not react well to these obstructions. You end up having quite low airflows. In addition, the inability to measure airflows on small jobs makes it harder to tell how well the duct is performing.

So what is the best way to deal with this. A couple of things should help,

1. When sizing your ducts on a ductulator, try using a lower friction rate to allow for bigger duct size.
2. Keep in mind that when installing that the less obstructions you have on your duct the better you will be able to achieve your design airflow.

A ductulator is a design tool that helps designers quickly size HVAC ducts without resorting to a calculator. Originally, duct sizing what based off either tables or charts. Over time as a need to improve productivity a ductulator was invented and is currently in wide use.

A ductulator is composed of four(4) sections: the air volume, velocity, rectangular dimensions, and round duct sections:

In addition, most ductulators also have both a metric and imperial side. The thing to note is that the placement of the  sections is different between the two sides and after regular use, most people are able to determine which units that they are working in.

A typical process to use the ductulator is to use the following process:

1. Determine the amount of air you are moving, in either CFM or L/S
2. Decide on the driver for your selection – either pressure drop usually given in in per 100ft or Pa or velocity(m/s or fpm)
   • There is a relationship between velocity/pressure drop and your duct size and energy use
   • Generally the higher the pressure drop, the smaller the duct size, but the higher the energy use.
   • Most designers design to 0.8″/100ft for small volumes and for high volumes, maintain the velocity between 1,800 fpm and 4,000 fpm.
3. Once any combination of the two variables mentioned above is set, all other readings can be made.
4. The round duct diameter or the rectangular ducts can now be read.

I hope the quick introduction to the ductulator allows you to get a high level view on how to use it. In other posts, I will get into more details on other intricacies on how to use this tool properly.

On the smartphone, there are many apps that offer the ability to use a ductulator on the go. One of them is Ductulator – Duct Sizer