What friction rate should we use when designing a residential duct system?

Question:

What friction rate should we use when designing a residential duct system? is 0.1” of water column safe?

Answer:

When it comes to understanding friction rate in residential ductwork design, Allison Bailes III, Ph.D., of Energy Vanguard, recently explained on the HVAC School podcast that friction rate may be a little of a misnomer. Here’s what he says:

The pressure that is exerted in all directions against the walls of the ducts and equipment by a forced air blower is called static pressure, and it’s often measured in inches of water column.

Before we can understand how friction and static pressure is used in design, let’s back up and talk about the blower.

If you’ve got a forced air system — a central air conditioning system, a furnace, whatever it is that’s blowing air into the house through a duct system — that blower creates a pressure difference. Some of that pressure is “used up” when it crosses things in the air stream outside the unit itself.

The air handler or a furnace is essentially a box with air moving through it and the blower rated for a certain amount of static pressure. When the air is blown out of the system — when it’s pulling air into the return side and blowing air out the supply side — let’s say it’s rated for .5 inches a water column.

Some of that pressure is used up on things like the filter and balancing dampers, registers and grilles, anything you put into the air stream is going to “use up” some of that allowable pressure. You are going to have pressure drops over these things. And what’s left after that is called the available static pressure.

After you subtract those things off, you have the available static pressure. That’s what’s available to move the air through the whole duct system and the fittings.

When you do a duct design, you lay out the duct system. You put your vents in different rooms. You figure out what you are doing about return air, is it going to be a central return or is it going to be ducted returns to each bedroom. And you set that up.

Then you look at (the software looks at) what is the most restrictive run from return all the way through to the most restrictive supply. What it is doing is calculating pressure drops for the duct layout. And it does pressure drops using something called the equivalent length and the total effective length. And here’s a little sidebar for you, if you read Manual D they use both of those terms.

Equivalent length applies to fittings only, so if you have say a 90 degree elbow, it might have an equivalent length of 25 feet. That means it has the same pressure drop of 25 feet of straight duct of that type, or 25 feet of straight duct of either rigid metal or really well-installed flex duct.

The other term, total effective length, is the sum of the equivalent lengths and the straight lengths of the duct. This is not the total length of all the straight duct and all the fittings for the whole duct system. It is the most restrictive run. So on the return side, if you’ve got multiple runs, you find the one that adds up to the highest effective length. And on the supply side, you do the same thing. You’ve got your most restrictive run there, the one with the longest total effective length.

Now you have available static pressure and total effective length of your layout. But your total effective length hasn’t looked at the diameter of the ducts you are using yet, how big or round are the ducts, how much volume of air can they hold. And that’s where you get into friction rate.

So you take those two other numbers, the available static pressure and the total effective length and you come up with a rate. So what you are doing is your available static pressure is going to be spread out over that total effective length, and that tells you the rate. How much pressure you can use per foot. The rate is usually put per hundred feet to make the numbers easier to look at and you have fewer zeros after the decimal.

So you come up with that number, and it’s typically called a friction rate, which I think is a terrible name for that quantity when you are doing this side of the design process. When you think about friction, if you are talking about moving air through a duct system, you want low friction, right? You want to minimize the amount of friction. And in this case, what we want when we are doing this calculation, we want that number to be higher and not lower.

So when you first learn this, your mind has a little bit of cognitive dissonance there because it’s like, wait a minute? I want friction to be low. I don’t want friction to be high.

So I say friction rate is really the wrong name there. It’s really the available static pressure rate. It is how that available static pressure spread out over the total effective length.

The other thing is that total effective length, if you are doing this in the software, you can look at what the total effective length is, and you can see how much of it is coming from the runouts, the straight runs of ducts, and how much of it is coming from the fittings.

You will see that usually 80 percent of your total effective length, sometimes higher, is just from the fittings. So fittings matter enormously when you are doing this and choosing the right kind of fittings can make a big difference.

So there is no one “friction rate” you can use across the board. You really need to use proper design find your “available static pressure” based on the equipment, fittings, locations of the vents and available space.

Manual D

The ANSI-recognized national standard for residential HVAC duct design will help you design a duct system in less time.

The ANSI-recognized national standard for residential HVAC duct design will help you design a duct system in less time. The procedure has been revised to include updated and expanded VAV guidance, with detailed examples; impacts of excess length, sag and compression in flexible ducts; and new equivalent length values for flex duct junction boxes. Manual D, from the Air Conditioning Contractors of America (ACCA), provides a single set of ANSI-recognized duct sizing principles and calculations that apply to all duct materials.

The newly released Manual D has been significantly revised to include:

Updated and expanded VAV guidance, with detailed examples.
Impacts of excess length, sag and compression in flexible ducts.
New equivalent length values for flex duct junction boxes.
A single set of ANSI-recognized duct sizing principles and calculations that apply to all duct materials
System operating point (supply Cfm and external static pressure), and airway sizing for single-speed and multi-speed (ECM) blowers
A method for determining the impact of duct friction and fitting pressure drop on blower performance and air delivery
The most comprehensive equivalent length data ever published.

You can apply the Manual D procedure to constant volume systems and zoned variable air volume systems, over a full range of duct construction materials.

Got an HVACR question you want answered in SNIPS magazine? Send your questions to adolphuse@bnpmedia.com.

This story originally appeared in the July 2019 issue of SNIPS magazine.