Table 1 – Duct Size vs. Airflow at a Friction Rate of 0.1

Friction rate is is that number that you use on a duct calculator (aka, duct slide rule, ductulator, etc.) to figure out what size duct you need for a certain airflow.  It is one of the most complicated concepts to understand in the ACCA Manual D duct design methodology. It is even harder to explain to others (as you can tell by the length of this blog article). I have tried many times, with mixed success. I will try again now. At the very least you can laugh at my stupid analogies.

One of the biggest mistakes people make with friction rate is confusing it with static pressure. I’ve had people very confidently tell me that they “always use half inch on my ductulator because that’s what they design their total airflow to”. This makes me cringe. That means they are using 0.5 instead of something like 0.1, which means that they are expecting a bit more than twice as much air from a given size duct than what it will really deliver. Please, please, don’t do this. I’ve actually had people get quite angry with me for correcting them on this. Sorry. I’m just the messenger.

TESP is the total static pressure that your fan feels. Some of the pressure is on the return side and the fan has to pull against that pressure. Some of the pressure is on the supply side and the fan has to push against that pressure. The fan has no idea what is causing this pressure. It is just a dumb box with no eyes, ears or antennae. All it feels is negative pressure on one side and positive pressure on the other side. Those pressures could be caused by a box with a bunch of holes drilled in it, or a bouncy house, or a 3-mile-long sheet metal duct, or a tuba, or something else. It doesn’t matter. What matters is that you add those two pressures together, supply and return, (ignore the negative sign on the return pressure) and that’s the TESP. That’s how hard the fan is working. The harder it has to work, the less air will come out. Simple. Always, keep it simple.

The TESP is what the manufacturer’s airflow tables use to tell you how much airflow (cfm) the fan will deliver. The airflow table (chart, graph, whatever) tells you that at a certain TESP and a certain speed tap (high, med, low, etc. – let’s keep it simple and just focus on high speed), this fan will deliver a certain airflow, cfm. Friction rate is something completely different, related, but different. One is a total and one is a rate of something happening, like miles and miles per hour.

Another big mistake that “designers” make is that people assume that their systems will operate at a certain static pressure just by hooking up ducts and turning it on. They’ve heard a million times things like “ this fan delivers 400 cfm per ton at 0.5 inches of water column (iwc)”, so they just assume that’s what’s happening. They don’t realize that you have to design as system to those specifications for it to actually happen. In other words, if you want 400 cfm/ton, you have to design the ducts system so that the furnace only “feels” 0.5 iwc. Whether or not the furnace feels 0.5 iwc depends ultimately and completely on what kind of system it is attached to. It’s even worse for variable speed fans. People think they are magic and that no matter what kind of system they hook it up to, it will magically deliver the target airflow. Do not make this mistake.

So, how do you do design a duct system that will make the fan feel a certain static pressure? That’s where friction rate comes in. Friction rate allows you to pick a desired airflow, determine the static pressure needed to deliver that airflow (from the airflow tables) and then design the ducts around that static pressure to ensure that you get your desired airflow. These are the basic steps:

  1. Pick a desired cfm (e.g., 400 cfm/ton)
  2. Look at the airflow table and pick a TESP that will give you that cfm (e.g., 0.5 iwc).
  3. Calculate a friction rate based on that static pressure (e.g., 0.1 iwc/100’)
  4. Use that friction rate to size the ducts.
  5. Install the system, turn it on and you get the TESP you wanted (or less) and the cfm you wanted (or more).

Simple. Well, pretty simple.

Then why don’t more people do these steps? Because dummies like me keep continually throwing out numbers like 400 cfm/ton, 0.5 iwc, and 0.1 iwc/100’ in all their examples and everyone says, “If that’s what they always are, why do I need to bother calculating them?” Well, that’s a valid question. Those numbers are “typical” numbers, averages. If you were to design thousands of similar systems, they would all average out to about these numbers. I’ve done, it. It’s true.

Unfortunately, it’s not that simple. There’s enough variation between unique houses and individual equipment brands that you can’t just make these assumptions. That’s a recipe for trouble. Just like assuming that a 3 ton AC will give you exactly 36,000 Btuh. It won’t. Trust me. (Hopefully, those last few sentences don’t come as a surprise to you. If they do, you better read ACCA Manual S, quick.)

An analogy is fuel economy in cars. Some cars are less than 10 mpg and some are more than 30. Let’s say the average mpg for all cars is around 20. If you have to buy the exact amount of gas it takes to go a certain distance and assume you will get 20 mpg without knowing what kind of car you will be driving, chances are about 50/50 that you will run out of gas.

When furnace manufacturers design their fans-in-a-box, they typically shoot for a certain target airflow because that’s what everyone is used to, usually 400 cfm/ton. Some are better (higher airflow), some are worse. Not only does it vary between manufacturers, but it varies within difference sizes of the same furnace line. A two-ton furnace (the furnace that they intend to be paired with a two ton condenser) might give you 860 cfm at 0.5 iwc (430 cfm/ton). But the five-ton furnace (the furnace that they intend to be paired with a 5 ton condenser) might only give you 1920 cfm at 0.5 iwc. (384 cfm/ton).

The vast majority of my designs were for large production home builders who sent our designs out to get bids. Some big production home builders had national accounts with certain HVAC equipment manufacturers, so we could use those specs. Some did not, which meant that our design had to be generic – not specific to one brand. To do a generic design, we typically had specs from 5 or 6 of the top manufacturers and averaged the all the specs and then used design criteria that was worse than the average. (We didn’t want to eliminate any manufacturers by using criteria they couldn’t meet.) When we did his averaging, guess what the static pressure, and airflow typically turned out to be – Right around 400 cfm/ton at 0.5 iwc!

Whatever airflow and TESP you choose, the key is that you design your duct system to that TESP, as opposed to assuming that it will just magically happen. Remember the 5 steps I mentioned above? We are on now onstep 3, calculate a friction rate. (Finally, he’s talking about friction rate!)

So, what is friction rate (FR) and how is it different than TESP? You may have noticed in the previous explanation that the units are different. TESP is inches of water column (iwc) and FR is inches of water column per 100 feet (iwc/100’). TESP is a static pressure and FR is pressure lost as you move down the ducts. If you start with 0.25 iwc and have a friction rate of 0.1 iwc/100’, that means you can go 250 feet before you run out of pressure to push the air. Keep in mind that the static pressure at the end of the ducts, once the air leaves the register, is ZERO, by definition. TESP is how much pressure you have to burn. FR is how fast you can burn it and have nothing left at the end. TESP is like your monthly expense allowance and FR is how much you want to spend per day so that you spend it all.

FR only applies to the pressure that pushes the air through the ducts. TESP is the pressure that pushes the air through everything, ducts, fittings, filters, etc. This means you have to remove from the TESP things other than the ducts that eat up static pressure. That includes things like the evaporator coil, filters, air cleaners, dehumidifiers, humidifiers, grilles, registers, dampers, etc. These are called component pressure loses (CPL). What you are left is something called “available static pressure” (ASP), which is essentially the static pressure available to just push the air through the ducts. If you know that number and how long your ducts are, you can figure out what your “pressure budget” is. In other words, how much pressure can the air use up as I go down the ducts from the fan to the register. Sound familiar? The units of this pressure budget is iwc per foot. The problem is that when looking at just one foot it is a very small number. Going back to the monthly expense budget analogy, it would be like how much you can spend per minute. This is too small and too precise to make sense. To make it more manageable (less decimal points) they multiply it by 100 and the units are iwc per 100 feet. More like dollars per month. As long as you keep track of those 100 feet by keeping them in the units (iwc/100’), it all works out.

So, maybe a better analogy (less stupid) for TESP and FR would be a road trip. I love road trips. Let’s say have to rent a car for a business business trip and your boss gives you a total budget. TESP is this budget. ASP is how much money you have left after you plan for meals, hotel, incidentals, etc. FR is like how much you can spend, per mile, on gas.

Let’s say you start with $200 dollars total and what you have left for gas is $100. And you have to go 500 miles. This means you can spend $0.20 per mile ($100/500 miles). If gas costs $3 per gallon, you better have a car that gets at least 15 miles per gallon ($3 per gal / $0.20 per mile). No problem. If you have to go 800 miles, you can only spend $0.125 per mile ($100/800 miles), so you better have a car that gets at least 24 mpg ($3 per gal / $0.125 per mile). If you spend more on meals and you only have $80 left for gas and you have to go 800 miles, you can only spend $0.10 per mile ($80/800 miles), so your car better get 30 mpg ($3 per gal / $0.10 per mile). And so on.

So, in this road trip analogy the $200 you start with is your TESP. The $100 or $80 you have left for gas is your ASP. The distance you can travel is the length of the ducts. The amount you can spend per mile is your FR. So what does the fuel efficiency of the car represent? Well, that would be how big your ducts are. Bigger ducts are more efficient. They burn less pressure for each foot the air travels (a more efficinent car burns less dollars for each mile it travels) Note: in the analogy dollars can be converted directly to fuel ($3 per gallon), so they are basically the same thing.

Let’s see how well this analogy works.

All else being equal, if you start with a higher expense budget (higher TESP), after usual expenses you will have more gas money (higher ASP) and you can afford more dollars per mile (higher FR) so you vehicle doesn’t have to be as efficient (smaller ducts).

Another way to look at this is if you don’t want to have to drive a Prius (huge ducts), you have the following options:

  1. Start with more money (higher TESP)
  2. Not have as many expenses other than gas (less static pressure losses, resulting in higher ASP)
  3. Not drive as far (shorter ducts)

If you think about a real duct system and how far the air has to travel, the distance varies from register to register. Some registers are very close to the supply plenum and some can be very far away. Taking the road trip analogy to the next level, a duct system is like a series of roads to and from a town. The furnace is downtown. They always design roads so you have to pass through downtown. The roads traveling business people typically come in on are the return ducts. Roads that they leave town on are supply ducts. Some routes in and out of town to their final destination are shorter and some routes are longer. If you look at option 3, above, this implies that for shorter runs, you don’t have to have as big of a duct. This makes perfect sense. If you have a fixed expense budget and don’t have to drive as far, you can drive a less efficient vehicle (small ducts). If you have to drive farther, you have to have a more efficient vehicle (bigger ducts). It varies for different routes through town.   Some routes require a Prius and some can be Ferrari. (Interestingly, the analogy holds up well here because the less efficient car goes faster which is analogous to smaller ducts having a higher velocity, which is true! hmmm)

What this is really saying is that you can have a different friction rate for different paths that the air can take through a duct system. Some HVAC duct design software programs have the option to toggle between using the worst-case FR or using a variable (custom) friction rates for each run. With computers doing all the work, I don’t see any good reason to base your entire duct system on the longest run. Back in the day, when we had to do all the calculations by hand with a calculator and a pencil, it was a pain to calculate and keep track of so many friction rates. We just found the worst case and based everything on that, knowing that it would oversize the ducts on the shorter runs. But this can actually cause some serious balancing problems if there was a big difference between the longest run and the shortest run. I’ve seen it happen. To this day, I still prefer a duct layout where all the runs have about the same length because it took away, or at least lessened this difference between the worst case and best case runs. The ability to make all of you duct runs the same length of course depends on the location of the air handler relative to all the supply registers and return grilles.

By the way, using variable friction rates to size ducts will result in what is sometimes referred to as better “self balancing”. If longer ducts are relatively bigger and shorter ducts are relatively smaller, when you turn the system on the airflow delivered to each register will be closer to what you were shooting for. If however, you use the worst case FR for all of the ducts, the shorter ducts will get much more air than they need. I remember one time we did this and there was a laundry room very close to the furnace. We sized the duct using a friction rate based on much longer run and the duct was oversized. Oversized, plus being super close to the furnace meant that the laundry room got WAY more air than it needed. Unfortunately, guess what was just outside of the laundry room . . . the thermostat. This one oversized duct caused the thermostat to shut off the system way too early. The entire 2200 square foot first floor of a very expensive 4000 square foot tract home was not comfortable because of this one duct. Fortunately, this was discovered in the sales model and the fix was to just damper down the airflow in the laundry room. We adjusted the design after that and they all worked wonderfully.

Another thing to think about is that even though two lengths of ducts could be the same in terms of feet, one could have a lot more resistance than the other if one is straight and the other has a bunch of bends and turns. The road trip analogy actually explains this pretty well. Driving one mile down a perfectly straight road will result in better gas mileage than driving one mile where you have to make a bunch of 90 degree turns every block. So, the route with turns will require a more efficient vehicle (bigger ducts). But the distance traveled is the same, so how to we account for that? Manual D assigns something called equivalent lengths to bends and turns and other fittings. An equivalent length of 20 feet is like saying this bend has the same resistance as 20 feet of straight duct. Very interesting.

So, instead of just using actual duct lengths we use actual lengths plus equivalent lengths, we can account for the resistance (loss of static pressure) for turns and bends and fittings. Nice! This is called Total Equivalent Lengths (TEL)

The equation for determining gas budget per mile, let’s call that the money burn rate, on a road trip is:

                                         MBR = (TEB – UE) / MTG


                                        MBR = Money Burn Rate ($ / mile)

                                        TEB = Total Expense Budget ($)

                                        UE = Usual expenses, other than gas ($)

                                        MTG = Miles To Go (miles)


The equation for FR is:

                                        fr = (TESP – CPL) / TEL


                                        TESP = Total External Static Pressure

                                        CPL = Component Pressure Losses

                                        TEL = Total Equivalent Lengths

Notice the small ”fr”? That’s because we haven’t multiplied it by 100 feet to make it a more easy to use number. So, the actual equation is:

                                        FR = [(TESP – CPL) x 100] / TEL

Remember that (TESP – CPL) is sometimes referred to as ASP, available static pressure.

I highly suggest you memorize this equation, because it is very useful. You can shorten it to:

                                        FR = ASP x 100 / TEL

Here is the BOTTOM LINE:

Higher friction rate = smaller ducts. I have the hardest time remembering this. It comes out backwards in my brain for some reason. Just think of it like this. Smaller ducts eat up more pressure, which means more friction, which means a higher friction rate.  Here’s where it is really confusing: A system with higher friction rate will deliver more air for a given duct size.  Wait, higher friction rate is better? That’s totally counter-intuitive.  Maybe the way to think of friction rate is not as a description of the ducts but as a description of what the system can handle.  If there is plenty of ASP and not a lot of TEL then the system can handle a higher burn rate.  It’s able to tolerate smaller ducts.  It can overcome more resistance to airflow.  I hope I unconfused you.  If not, just be like me: Go back and look at something, like this blog post, and get it straight before saying something dumb.

In the diagram above or to the right, depending on your screen size, notice that at a friction rate of 0.11, a 6″ duct (yellow) will deliver up to 77 cfm.  At a friction rate of 0.08 it will only deliver 61 cfm.  In other words, if you needed to deliver 70 cfm to a room, you could use a 6″ duct if the run was shorter and the friction rate was less than 0.10, but if the friction rate was 0.09 or lower, you would need a 7″ duct (green).  Also notice that at 80-90 cfm the friction rate can be anywhere between 0.11 and 0.08 and a 7″ duct would work.  It’s only in those border regions where it matters, but it REALLY matters.  The difference between a 7″ duct and a 6″ duct at a FR of 0.1 is 105 cfm vs 70 cfm.  That’s a decrease in airflow of 1/3 by going down just one duct size!

Looking at the equation.  Things that make the friction rate smaller (ducts bigger) are

  1. Less ASP
  2. Less TESP (results in less ASP)
  3. More CPS (results in less ASP)
  4. More TEL

Putting this in plain words, bigger ducts are needed if you:

  1. Have less pressure available to push the air through the ducts
  2. Have less total pressure from the fan
  3. Have more components eating up static pressure
  4. Have longer ducts and/or more bends or fittings that the air has to travel through

Whew. Well, there you go. Friction rate can be explained as a road trip, after all. Maybe. I hope that helps explain friction rate. If not, let me know and I will try again. Maybe more graphs would help. I like graphs.



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