Equivalent Length Merry-Go-Round

I recently read a tech tip by Matt Bruner about using the Manual D process. In the article, he designed a duct system for a small job using a ducted mini-split. One thing I noticed about his design was that the supply and return duct velocity was fairly low compared to the arbitrary 900-feet-per-minute supply and 700-feet-per-minute return that Manual D uses as limits and a basis for the rated equivalent lengths.

Being that I recently spent a week in Punta Cana, laying on the beach and reading the ACCA manuals front to back (you can tell I’m the life of the party), his article made me think about a small section towards the back of the book that I ended up revisiting in Appendix 3 on page 108. This appendix is titled “The basis for equivalent lengths.” I remembered there was a formula that made it possible to correct equivalent lengths based on actual velocity.

I sent Matt a message, saying, “You know, in reality, the equivalent lengths on that job are much less than what you have shown in the article.”  Matt sounded interested—but probably more annoyed with me randomly quoting a tiny excerpt from the residential air flow Bible that is Manual D.

I thought it would be an exciting project to do the math on his duct system to determine the equivalent lengths. I expected to find a reduction in almost all of his duct sizes and a slightly different friction rate. If you’re going to do the math, this is not a one-and-done type of problem. It is an iterative process that needs to be repeated until you are satisfied with the results.

This process might make little sense to you—why would the math have to be done over and over again? Think of it like riding a merry-go-round as a child; do you think you would be satisfied with just one time around? I compared it to a merry-go-round because when my kids were young, they could ride all day long and still be disappointed when they had to get off.

Let’s look at the math so you can understand my comparison. So, my first time around, the equivalent lengths had a drastic reduction, wildly increasing my friction rate way outside of the ACCA wedge. The second time around, the opposite happened, and I ended up with an insanely low friction rate.

At this point, I started calling the people I knew that are far more intelligent than me on these topics. My first call was to Ed Janowiak. Ed is the manager of HVAC design education at ACCA. I explained my conundrum to him. He said that if you are going to do the math, you have to keep doing it until you are satisfied with the results.

I ran through one more iteration and then became further discouraged. So, I decided to call Alex Meaney. Alex is a Design Consultant at Mean HVAC and a former Wrightsoft trainer. I explained the situation and asked for his guidance. Alex told me that specific programs have a feature that will perform this calculation for you when performing a Manual D. He agreed that it is an iterative process that might take ten tries—or maybe even 500 tries—to get it right. Alex cautioned me about the journey down this path; what he said makes a lot of sense: Manual D has a bit of cushion built into it. This fact can be handy in a real-life situation when a plumber installs a pipe in your way or the builder forgot to mention the steel beam that is right in the path of your duct run.

Even the most skilled designers will only anticipate some things that happen, and you have to end up going around things—in other words, altering the original design. Minor tweaks to a duct design made me anxious that something wouldn’t work right. The cushion in Manual D can often protect you from these types of situations.

I’m not saying it’s okay to loop a duct around the house three times randomly, but if you have an extra elbow here or there, it probably isn’t the end of the world. On the other hand, if you actually correct for velocity, you are removing the cushion in the design—this means you have no margin for error.

So, how do you know if and when your design has a cushion? The tip Alex gave me was to look at the velocity of your system. Suppose the actual velocity based on the calculated friction rate is close to the 900 FPM supply and 700 FPM return (in Manual D). In that case, the equivalent lengths in your design are close to reality—this means taking heed when randomly making field changes.

Doing this math by hand has made me not want to do it again. I realized that there are items in Manual D that are there to protect you and ensure that your customer will have a system that they are happy with. If you’ve followed Manual D and tested your designs after installation, I’m sure you found that many systems run at a lower external static pressure than designed. I scratch this up as a win. This means that if we follow the process, we will provide our customers with an efficient and comfortable home while reducing future equipment issues. I’m good with that, and it makes sense why this is a small little blip in the back of the book.