If you are enjoying these posts and learning something, you may enjoy my class, “HVAC 1.0 – Introduction to Residential HVAC“. I will be adding more classes and dates regularly, please subscribe to my newsletter, The Sierra B.S. Newsletter. (B.S. stands for building science, of course) by sending an e-mail to info@sierrabuildingscience.com. If you know anyone who may benefit from this kind of information, please pass it along.
Today’s topic is Duct Siz vs. Airflow. This is Part 1 of a two or three part series on this topic.
One of the big misconceptions about airflow is how to determine how much air will flow through a certain size duct, or conversely, determining what size duct you need to deliver a certain airflow. You would not believe the range of flows I have heard as “rules of thumb”. This assumes that you have done the calculations necessary to determine how much air is needed in a room. That will be a different series of blog posts, to be sure.
Duct sizing is covered very well in ACCA Manual D and is fairly straightforward. For now just suffice it to say that there is a very important number called “Friction Rate” that determines the relationship between duct size and airflow. Friction rate describes the average pressure drop per 100 feet of duct in a system. Notice that this number is unique to a system, not just an individual duct run. For example, all things being equal, an 8” duct at the end of a long convoluted duct system will not deliver as much air as an 8” duct on a very short straight system. This is because everything that the air passes through has an impact on how much air comes out of the very end. Friction rate is a wonderful number because it takes into account how much static pressure you fan is providing, how much of that is left after you subtract out the big-ticket items like the coil, filter, supply registers and return grilles.
A common system configuration.
But, you say, most systems do not have runs that are 100 feet long! What use is that number that is “per 100 feet”? Actually, if you look at something called “equivalent lengths” a duct run can be well over 100 feet “long”. Equivalent lengths are numbers that can be looked up in an appendix of ACCA Manual D. This is where a fitting such as a t-wye or elbow is assigned a number that represents a length of straight duct that that has an equal pressure drop. For example a t-wye might have an equivalent length of 10 feet. A ninety degree elbow might have an equivalent length of 15 feet. A round start collar coming off of a sheet metal supply plenum can have equivalent lengths approaching 30 feet or more. When you add up the actual lengths and the equivalent lengths, it adds up quickly.
Even if the length of the run is very short, you can still use friction rate because the 100 feet is just a number they decided to use. They could have used pressure drop per 10 feet or even 1 foot. It just adds more decimal places. Don’t dwell on it. Move on. Get over it. Just don’t forget about it. One of the biggest mistakes I’ve seen contractors make is to confuse total operating static pressure (inches of water column) with friction rate (inches of water column lost per 100 feet).
The details of how to calculate friction rates are covered later, but a very common friction rate for a reasonably well-designed designed system is 0.1 iwc/100’. You can take that number and using a duct slide rule, duct calculator, or friction rate chart and determine duct size for a given airflow or determine how much air will come out of a given size duct.
Table 1 – Duct Size vs. Airflow at a Friction Rate of 0.1
Table 1 is an example of the airflow that you would get from various size vinyl flex ducts in a system with a friction rate of 0.1 iwc/100’.
Now, I’m taking a huge risk by putting this table out there and I will probably get a lot of grief for it, but here it is. The danger is using it on systems where the friction rate is something other than 0.1. (I use this table all of the time as a first guess, ball park number and it works fine. Of course, I fine-tune the calculations later, but it’s always pretty close. It’s a hundred times better than some of the numbers I’ve heard contractors rattling off.)
One of the first comments I used to get on my designs was that odd size ducts are not used. Did I mention that I have done about 2000 residential HVAC designs? Ninety-nine percent of them were for medium to large production home builders. What they meant to say was that odd size ducts are not normally stocked by their local wholesaler. That’s because none of the contractors used them. Supply, demand, etc., etc.
What if you did a detailed load calculation (ACCA Manual J), carefully selected equipment (Manual S), and knew exactly how much air each room needed. Now you are in the process of sizing ducts (Manual D). Let’s say that you had a room that needed 95 cfm. If you were a contractor who did not use odd size ducts, your choice would be between a 6″ duct, which does not give you enough air, or an 8″ duct with gives you almost twice what you need. Which would it be? Six inch, of course.
NO!
Suck it up and use 7″ duct, cheap skate!
Here’s some other interesting ways to use this table. If you have a room that needs 197 cfm and another right next to it that needs 72 cfm what kind of t-wye will you need to serve these two rooms? To deliver at least 72 cfm, you will need a 6″ duct. To deliver at least 197 cfm you will need at least a 9″ duct. The trunk that serves these two ducts needs to be able to deliver 72 + 197 = 269 cfm. Using Table 1, that means a 10″ trunk. By the way, a duct that is split into more than one duct is called a “trunk”, just like a tree. Ducts that are on the end of a trunk and terminate in a register are called . . . branches! How about that? And that’s why we call registers “leaves”. Just kidding. Nobody does that.
So, the t-wye will need to be a what is commonly referred to as a 10-9-6 sheet metal t-wye. Any contractor who complains about this not being and “off-the-shelf” fitting probably has not done many installs from a carefully designed plan. If they really complain, just tell them to round the odd sizes UP, Making this a 10-10-6 t-wye.
Next: Part 2 – Why two 6″ ducts will not deliver the same air as one 12″ duct. Seems obvious, doesn’t it. Stay tuned.
Again, all of these blog posts are based on the training materials and topics covered in my HVAC 1.0 Class. If you know anyone who might benefit from this kind of information, please refer them to my website. www.sierrabuildingscience.com.
Thanks!
Russ
Duc-Bloc register blocks for duct testing
HVAC 1.0 Book – Introduction to Residential HVAC Systems
Kwik Model 3D HVAC Design Software
Russell King, M.E. 
Aug 20, 2012 @ 15:52:44
Thanks Russ,
Always enjoy your tutorials!
Jan 22, 2013 @ 23:16:09
Russ, you are the man!. Thank you so much for your tutorial, it’s something I have been looking for, can not be any simpler explanation.
Stay healthy.
Jan 15, 2014 @ 16:18:23
Can you provide any guidance on the percent reduction in CFM that one can expect with a 45 or 90 degree elbow placed in an otherwise straight path for a range hood ducting from kitchen. Example, if the straight line path using an 8″ diameter duct measures 1000 CFM what would be the expected result if a 45 degree elbow is placed in the path; what would be the expected result if a 90 degree elbow is placed in the path. Assume the elbow is at least 20 inches from the range hood vent and same diameter. Also, if possible a link to some on-line source material will be helpful
Jan 16, 2014 @ 16:36:14
Hi Donald,
Great question. The best resource that I know of for this kind of work is the Air Conditioning Contractors of America (ACCA.org) manuals. For this particular problem their Duct Calculation Slide Rule is very handy. It is based on the formula for friction rate. If you know any two of the following values, you can calculate the third: flow (cfm), duct diameter and friction rate. Also, Manual D would be useful. However, I’m not sure if Manual D works at the velocity you are talking about. If not, you may need to look in the ASHRAE Handbook for a more generic approach.
I saw a couple kitchen hoods on line that say they deliver 1000 cfm through an 8″ duct, but they don’t tell you on line how long that duct is or how many elbows. You need to know this and I’m sure it exists somewhere in the manufacturers literature. Better yet, a table showing flow (cfm) vs. static pressure would be even better.
The short answer to you basic question is: “It depends.” In Manual D, elbows, straight duct runs, and other fittings get accounted for as “equivalent lengths”. Equivalent lengths are the denominator of the equation for friction rate: FR = (available static pressure) x 100 / (total equivalent lengths), so adding equivalent length makes the friction rate go down, which makes airflow go down. The impact on airflow of adding one elbow depends on what all the other equivalent lengths are (how big the denominator is that you are starting with). In other words, are you adding an elbow to a very short, straight duct system or to a very long, complex duct system? One elbow will have a bigger impact on a short system.
I hope this helps. I’m sure it’s a lot more complicated that you were hoping for. Just remember that going to a bigger duct size will usually compensate easily for one or two bends. Also, most kitchen hood manufacturers will give you pretty easy to follow charts showing maximum duct length vs. # of elbows vs. duct diameter.
Mar 27, 2014 @ 17:26:46
Thank you for the information. It really help me a lot.
Al
Jun 13, 2014 @ 19:07:37
Hello Russ. Thanks for your tutorials. I am going to install bathroom exhaust fan rated 200 CFM. What diameter of hard duct should be used?
Jun 13, 2014 @ 21:21:42
Hi Russ,
Unfortunately, it’s not that simple. It depends on the pressure that the 200 cfm is rated at (probably 0.25iwg), how long the duct is and how many bends it will have. Usually, the fan manufacturer will provide a table that will help you. I’m guessing that 8″ should be safe, but the problem with too big of a duct is that the air slows down and condensation could increase. If you live in a humid climate and the duct will run through unconditioned space, you may want to think about insulating the duct and paying attention to where water drops on the inside surface will run to. If you have a low spot in the duct, water might collect there and rust out the duct. I’ve seen people use plastic pipe for bathroom fans too. I always dreaded designing bathroom exhaust fan systems. It’s almost impossible to design a perfect system, but very easy to design a horrible one. I even had one homeowner complain that a bathroom fan was too quite. Her comment: “I don’t want to hear what goes on in there when my husband is on the toilet.”
There are some pretty good fact sheets on bathroom fans out there. This zipped file has tons of info on ventilation: http://www.waptac.org/WAP-Standardized-Curricula/ASHRAE-62.2.aspx
Nov 02, 2014 @ 06:57:23
Russ,
I am putting in OTR microwave exhaust. Directions call for 6″ diameter, but I am having trouble finding all of the components…. Home Depot had a complete kit but 7″ diameter. Am I ok going to 7″. Duct will be straight up approx 60″ from microwave to roof.
Nov 04, 2014 @ 08:33:42
Hi Dave,
Sorry it took so long to respond. You’re probably done by now, but maybe not. Generally speaking, bigger ducts are better, in terms of airflow. HOWEVER, when you have a duct that moves something other than air, you might have to worry about velocity. In the case of a range hood, you are moving air laden with warm moisture, probably lots of grease/fats/oil, and perhaps some particulates (if you burn meals, like my kids and I do). Bad things happen when the air slows down too much. It it is more likely to cool off, which causes moisture to condense and the grease and fats to solidify. Also, the lower velocity can allow particulates (now including grease) to settle out. With that said, I do not think that going from a 6″ to a 7″ duct would cause a problem. Though, it ultimately depends on the the cfm rating of fan and length of the duct, how many bends/elbows/fittings, rise vs. run, etc.
ASHRAE 62.2 requires at least 100 cfm for a range hood, so you want to maintain that. Going to a bigger duct will improve cfm, so the question now is really about velocity. If the fan is rated for 100 cfm or higher and it’s not a super long run, you should be fine. If it’s rated for less than that, velocity might be an issue. Off the top of my head, I don’t know what a recommended minimum velocity should be for kitchen hood ducts. You should check with the manufacturer. It wouldn’t hurt to insulate the duct, too, as an extra precaution.
Dec 01, 2014 @ 09:35:17
Exhaust fan is rated at 110 cfm and has a 6 in. duct connector. The install directions say to use 6 in. duct. Approx length from fan to outside is 8 ft.with not much more than a total of 90 deg. bends. About how much flow will be lost if the contractor uses a reducer to 4 in. and 4 in. duct?
Dec 02, 2014 @ 07:50:43
Hi Joe,
There is not enough information to give a precise answer. We don’t know what kind of duct system the 110 cfm is rated at, but I would guess about a 40% – 60% reduction in flow.
Russ
Dec 24, 2014 @ 05:21:56
is the a way i could email you a question regarding an install i am laying out?
Dec 24, 2014 @ 07:57:45
Hi Jeremy,
Sure. Go to http://www.sierrabuildingscience.com and use the e-mail in the bottom right corner.
Russ
Jan 21, 2015 @ 19:46:57
There is somebody in india who is reading your stuff. Starting out as an ventilation guy myself.
Jan 21, 2015 @ 19:58:40
Awesome. Thanks for checking in.
Jan 31, 2015 @ 11:16:24
im renovating our 2 story house thats 100 years old, after reading your blog im a bit bummed as ive installed all 6 ” round duct to the 2nd floor.5 runs total. the 2 largest rooms are 170 sqft..the longest run is 40′ with 2 90deg. bends and 1 45 deg… have i made a major mistake and should i redo what ive done? going to larger pipe?..would adding 1 additional run to the 2 larger rooms help..i havent done the 1st floor yet and am wondering if i should use 8″ on the 1st floor.the upstairs is 620 sq. ft. 3 bdr. bath and laundry.the downstairs is all open floor plan and is about 800 sq.ft…wish i had come across your blog sooner!!..thx
Jan 31, 2015 @ 22:59:42
Hi Paul,
If you still have the opportunity to add runs to some of the rooms all is not lost. Rather than thinking about each room as a separate entity, think of each room as some fraction of the total. Rather than saying this room needs an 6″ duct because it is XXX sf, say this room needs a 6″ duct because a 6″ duct delivers the amount of air that is that rooms fair share of airflow relative to ALL the other rooms in the house. Always make sure that you accommodate the total airflow of the system first.
Start with the airflow that the system puts out. Here is a really simple ball park example: If you have cooling, round up to the nearest ton and multiply it by 400 cfm (if you are really adventurous, multiply it by 500) That’s your hypothetical total system fan flow. If you don’t have cooling, multiply your heating output in MBtu times 22-25 (92,000 Btu = 92 MBtu x 22 = 2024, use 2000 cfm to keep math easy)
That’s what you want your system to deliver so you need to make sure you provide the ductwork to handle that much air. If you only had three 200 sf rooms (same windows, walls etc.) then you would need three ducts to handle 1/3 of the air (667 cfm each). If you put the same system on a house that had ten 200 sf rooms (same windows, walls etc.) then you would need ten ducts, each able to handle 200 cfm. The rooms are the same in both cases, but in the first case they need 14″ or 16″ inch ducts, but in the second case they need 9 or 10 inch ducts.
So, it’s not a function of the size of each room, but a function of the size of each room relative to all the other rooms in the house. To be even more precise, it’s not the size of each room, but the LOAD of each room (how many heating or cooling btus each room needs to stay warm, or cool) that matters.
The airflow to each room should be proportional to each room’s load. In other words, if one room accounts for 10 percent of the total load, then it should get 10 percent of the airflow. Sorry for the shameless plug, but my book covers this pretty well.
I always catch a lot of grief for doing this, but here are some safe duct sizes for you: 4″:20 cfm, 5″:40 cfm, 6″:60cfm, 7″:90 cfm, 8″:135 cfm, 9″:185 cfm, 10″:250 cfm, 12″:400 cfm, 14″:600 cfm, 16″:850 cfm, 18″:1200 cfm, 20″:1600 cfm. The reason I catch a lot of grief is that TRUE duct size depends on available static pressure, total equivalent lengths, friction rate, etc. etc, The people who get uptight about it want you to think that it is really difficult and complex to size ducts. It’s a job security thing. 🙂 The hardest thing about designing a duct system is making it fit the space you’ve been provided, which is not the ducts’ problem, but the houses’ problem. It’s the architects’ fault, always! 🙂 The vast majority of ducts are undersized because smaller ducts are easier to install. Too many contractors then compensate by upsizing the equipment. Bad idea. OK. I’m off my soap box for today. I hope this helps.
Russ
Feb 08, 2015 @ 08:57:51
thx very much for your response, that helps alot
Feb 02, 2015 @ 09:24:14
great information! Wish I had found it before I had completed most of my basement finishing project. I have one part left to figure out…. I need to add a run to the area under my stairs where the sump basket is. By code I need either a register or a mechanical fan with humidistat. I can very easily put a t-wye into the 6″ branch feeding my entryway with a 4″ line to feed the area under the stairs but I’m not sure if this would be allowed. If I need to I can run it all the way from the trunk but it seems overkill for the space i’m trying to only get a small amount of conditioned air to.. Any suggestions?
Feb 04, 2015 @ 08:18:04
Hi Kurt,
I’m not familiar with that code requirement. We don’t have many finished basements out here. I suspect that the intent is to prevent moisture from building up in that space. In these situations, I always prefer an exhaust fan. Putting a supply register in there would simply push the moisture (and musty smells, gases, etc.) out into the living space. There are plenty of bathroom fans out available with humidistats in them. I recommend investing in a good quality, energy star rated fan and start it on a very low humidity setting. If it seems to run too much, you can adjust it, but will also help ventilate your basement. If you have gas burning appliances in your basement that are not sealed combustion, always have someone do a good combustion safety test (CAZ/CAS). Find someone who is BPI certified. bpi.org
Good Luck!
Russ
Feb 25, 2015 @ 12:44:28
Russ,
I’m remodeling a kitchen and currently have one 10×6 duct coming straight out of the wall. I am looking at cutting the metal duct work back a few feet and adapting to a flex hose that will run about 8 feet long over the ceiling and install a ceiling register. My question is what size flex duct would be required to maintain the same airflow as i currently have?
Mar 02, 2015 @ 08:44:02
Hi Ron, That calculates out to be right around an 8″ duct, but the problem is that no matter what you do, you will be adding resistance compared to what you currently have. If you can possibly leave it as a wall register, you will probably be better off. If not, I would go a size or two larger to minimize the resistance of the newly added duct. Maybe from the 10×6 to a 10″ round duct. Whatever you do, be sure to use a bar-faced register rather than a cheaper, common stamped-face register. An example of a bar type:
An example of a stamp-faced register:
May 16, 2015 @ 13:57:15
Russ, This is a great blog. As I am trying to remodel l my basement I will use this info. The biggest thing I have to do is relocate my duct work so it does not take up so much of the overhead, which is only 7ft before the ducting. Tbe installer used an off the shelf product that is 8″ tall. I’d like to either have custom ducts made that are only 4″ tall but I understand the cost and aspect ratio issues with that, or I’d like to run the trunk along the floor on the back wall (he is 50′ long 25′ wide) and run branches up and over through the overhead to ser ve the far side of the house. My concern with the second option is whether a 6″ or 8″ branch line running ~31′ is too long. If you have any thoughts or recommendations they would be appreciated.
May 18, 2015 @ 07:21:47
Hi Justin, as usual, it’s an unsatisfying answer: It depends. It’s hard to isolate one just part of a duct system. One duct affects the entire system. Each duct run has it’s own friction rate which accounts for available static pressure from the fan and the total equivalent length of the run (actual length plus fittings and bends). The higher the friction rate is the smaller the duct can be to deliver a certain amount of air. That sounds backwards, but it’s a quirk of the how friction rate is calculated. A really long 8″ duct run will deliver less air than a short 8″ duct run. Getting the appropriate amount of air to each room is an exercise in proportion. Each room needs the same percentage of the total airflow as the percentage of heating or cooling demand that the room accounts for. I describe this in my book. You’d have to evaluate the entire system to figure out 1. how much air you need and 2. how big the duct should be to deliver it. I hope this helps.
Russ
May 24, 2015 @ 19:23:11
I will be using two 8″ duct fans going into a 12″ duct fan to help move hot air from my upstairs bedroom. I will be using a 8x8x12 on the suction. My question is about the discharge.
I can only exhaust into two other 8″ ducts and was wondering if I should
1. Use a second wye on the discharge of the fan and run (2) 8″ branches or
2. Run (1) 12″ duct until I need to split into the other 8″ and then use the wye.
Seems like I would want to do option 2 as there would be less friction due to the 12″ duct having a larger duct area.
Jun 05, 2015 @ 10:49:09
Hi, Sorry for the slow reply. Yes, the 12″ duct will have far less resistance than the two 8″ ducts, so the longer the air can run in the 12″ the better.
Russ
Jun 03, 2015 @ 11:43:19
Our HVAC unit is being replaced, and like you stated, we should get multiple estimates and we have. One guy — looked at our unit, asked me the number of registers, and looked at our fuse box. He said we should upgrade to a 3.5 ton, 14 ser vs the 3 ton 10 ser we currently have. The second guy, crawled under our house, and took measurements of the size of the house and told us that the duct size was too small. Our current duct work is 5 in( we live in an older house- probably only had heat at the time) He wants to upgrade to 8 inch duct work, upgrade to a 3.5 ton 13 ser, increase the size of our return air thing, and add a register to our large living room. (the living room was an addition 18×30 and only had two registers). The quotes were very close in price. (The first was through a large named home improvement store). Is replacing the duct work something that should be done? He also quoted me a price to just put in the a/c with no upgrades in the house. Just wanted a third unbiased opinion. And what is the difference between a 13 ser and a 14 ser?
Jun 05, 2015 @ 11:10:35
I would discourage going to a larger system unless the 3 ton was simply not keeping up at all. If the 3-ton was working reasonably well, a new 3-ton with properly sized (and less leaky) ducts will work much better. If the new one doesn’t quite keep up, then consider improving the building shell to reduce its load, like more ceiling insulation, better windows, sealing leaks, etc. Unless your house is already very energy efficient, putting a bigger system on it before improving the shell is like putting a bigger engine in an old pickup rather than putting air in the tires and taking all the bricks and scrap iron out of the back. A 3.5 ton system will need a larger air handler and much larger ducts than a 3 ton system.
I would be very leery of a contractor who would put larger equipment in without touching the duct work. I highly recommend that you use the ACCA checklist found in this document: checklist At the very least get the contractor to guarantee a leakage rate (in CA it’s 6% for new systems) and a minimum airflow rate of 350-400 cfm/ton.
SEER is an efficiency rating. A 14 SEER will use approximately 8% less electricity to deliver the same amount of cooling that a 13 SEER of the same size would. (13/14=0.92)
Good luck.
Rus
Jul 29, 2015 @ 11:41:27
The airflow coming out of some of my registers is very little to almost none… Any idea what I need to check for?
Jul 29, 2015 @ 11:42:16
…and it’s not very cold air
Jul 30, 2015 @ 14:55:08
That’s unusual. Typically, the slower the air is moving across the coil, the colder it gets. However, if the ducts are passing slowly through poorly insulated ducts that are passing through a hot attic, the air in the ducts can warm back up before it reaches the rooms. A disconnected major duct can also cause similar symptoms. Air will take the path of least resistance, which is to escape through the leak, leaving very little pressure to push the air into the rooms.
Let me know what you find out.
Russ
Jul 30, 2015 @ 14:50:54
If it has always been low, it is probably a design or installation problem such as undersized, crushed or pinched ducts. Undersized filters and return ducts can also be a big problem. If the problem did not use to be as bad, it could be a very dirty coil, dirty filter, a major duct has come loose, or the fan motor is failing. An expert can diagnose the problem by taking pressure readings at different places and figuring out where the main blockage is.
The California Energy Commission recommends the following:
(a) Check to determine that the air filter media is clean. If the air filter media is dirty, then replace it with
clean filter media.
(b) Open all registers and dampers and remove any obstructions.
(c) Replace crushed, blocked or restricted ducts if possible.
(d) Check to determine that the evaporator coil is clean, or that there are no obstructions to airflow through
the evaporator coil. If the evaporator coil is dirty or blocked with debris, if possible, clean the evaporator
coil using a method approved by the manufacturer.
(e) Set the air handler fan to high speed for cooling, and ensure that the blower wheel and motor are
operating properly, within manufacturers specifications.
(f) Check to determine whether the return duct system or return filter grille is sized too small for the
installed system. If the return duct or return grille is sized too small, if possible, perform applicable
alterations work on the return duct system or return grille in order to improve the system airflow rate.
When performing these remedial actions determines that there is a fault, a corrective action shall be performed
if possible. In many cases, airflow can be improved by adding a return duct and filter grille, or enlarging the
existing return duct or filter grille. Alteration of the return duct system is an alternative that shall be considered if
applicable to the existing system, and if other remedial actions do not improve the airflow. Alteration of the
return duct system to bring the system airflow rate into compliance is expected to be attainable for systems with
ducts in an attic space with sufficient clearances for accommodating improvements to the return duct system.
Good luck.
Russ
Oct 01, 2015 @ 16:51:50
I am installing an ventilation system in my home to help disperse heat from the room my wood heater is in. My plan is to install registers on each side of my home, where air would pull from. The air would go to the “hot” room through a 780 cfm inline fan. I am hoping the air pulling from the end rooms and into the room that is warm will help heat the house. My idea is to use the 780 cfm fan to blow into the hot room through 15 foot of flexible insulated duct. I would have a splitter on the intake side of the motor where 1 8 inch duct would travel 40 foot to one side, and another 8 inch would travel 25 foot to the other side. Is this sufficient size ducting to use? Also is it okay to use insulated flexible ducting on the negative side as I am planning? Any ideas or advise would be very greatly appreciated!! Thank you very much!
Oct 05, 2015 @ 08:22:28
Hello, Sorry for the slow reply. 8″ doesn’t sound big enough for 780 CFM. Is that what the manufacturer of the fan recommends? That will be very high velocity and likely noisy. There is a chance that the 8″ is intended for metal ducts. I’d recommend larger, up to 14″ or 2 tens, if you have room. Two eights on the return side is better, but one 8 on the supply side seems very small. You can put a larger-to-8″ and 8″-to-larger transition before and after the fan. If you do run 8″ ducts, it’s not the end of the world, you just won’t get anywhere near 780 cfm. But who’s to say it won’t still work? Use a good quality bar type register on both ends and you are less likely to have noise issues (e.g. Shoemaker 950 series). Insulated duct should be used on both sides.
My main concern with your proposed design is how well heat will make it back to your cold rooms. You are creating a negative pressure in the cold rooms and a positive pressure in the hot room. As long as there is a clear path between the positive and negative pressures and not other ways for air to be drawn into the cold rooms, air should move just fine. If the cold rooms are leaky, you will just pull outside air into them. If the hot room is leaky, you will just push hot air to the outside. I highly recommend that you have a good home performance contractor test and seal your house up.
The other option would be to reverse the direction of the airflow, but without knowing more details I can’t recommend this. Even though this would pump hot air directly into the cold rooms, creating a negative pressure in a room with a wood stove can be very dangerous. You can potentially backdraft combustion gases from the flue which contain carbon monoxide. Even from wood burning appliances. Again, a good home performance contractor would be able to performance a combustion safety test on you home and determine if this is a problem or not.
Good luck,
Russ
Oct 15, 2015 @ 10:18:18
Hi Russ, Im so glad I found this tutorial so I can pick your brains on this!
My husband and I just had a wood stove installed in our basement. We have been tasked with running hard pipe or flex pipe or both. I am disagreeing with my father-in-law on what he installed, and works very well for him, vs what I have done research on.
My 2 blowers put out 800 cfm’s each. My research is telling me you need 2-14″ ducts coming off of the plenum to maximize the airflow. One side of the house, 1800’s farm house, will use one duct that can be split off into 4 different rooms. This duct has to run about 40ft and bend twice before it splits off. Then I believe we split off between a 5″ and/or 6″ duct depending on room sizes. The 2nd part of the house I would split 3 ducts to the room sizes I needed.
My father-in-law has 2-8″ pipes coming out of his stove. His house is very warm but I don’t know what his cfm’s are. He suggests running 10-15′ of hard pipe 1st to create a steady airflow to get to the longer side of the house then use flex pipe.
I don’t want to upset him by not taking his advice but I want to ensure I get the most out of my vents through our long cold winter months. Do you think you can steer me in the right direction?
Thanks a bunch!
Cynthia
Oct 17, 2015 @ 16:38:05
Hi Cynthia,
It really depends on the fans’ ratings. They should have a chart or table showing airflow at a certain resistance (in inches of water column). As the resistance goes up, the flow goes down. Assuming that 800 cfm is a reasonable assumption, 14″ ducts are good if you have room for them. 12″ ducts are probably good too, but you might lose a little air flow. The main difference between sheet metal and flex duct is that size for size, metal ducts deliver more air and are less susceptible to kinks and crushing. All else being equal, going up one size in flex duct will accomplish about the same thing as a sheet metal duct. The benefit of flex duct is lower cost, very airtight and easy to install. Also, it comes pre insulated with up to R-8 insulation, maybe more. Sheet metal ducts have to be sealed and insulated. I would recommend vinyl flex ducts, but not aluma-flex ducts because they are very fragile.
One very important word of caution is to be very careful about sucking lots of air out of a room with any kind of combustion appliance or fire place. You can inadvertently create a negative pressure in the room that can cause combustion products, including carbon monoxide to back draft into the house. I highly recommend that you have a good combustion safety analyst or home performance contractor test your house before you do anything like this. Try http://www.bpi.org
Good luck.
Russ
Nov 28, 2015 @ 18:41:43
Hi Russ!
I am installing a new 400 cfm range hood. The new hood requires a 7″ round duct, the hood I am replacing used a 5″ round duct. It appears to be a straight run. What am I sacrificing should I use a 7in to 5in duct reducer? Thanks for your help!
Nov 28, 2015 @ 20:11:45
Hi Ryan,
It really depends on the performance curve of the fan, but all things being equal in terms of length, bends, etc., using a 5″ duct rather than a 7″ duct you could lose 30% to 50% of your airflow.
Russ
Nov 29, 2015 @ 11:13:24
Russ,
Thanks for the reply. The 30-50% loss in airflow would be at the 400 cfm setting I am assuming? If the first 2 settings are lower, then would that result in less reduction of airflow?
Nov 30, 2015 @ 09:05:19
Probably a little less, yes, but still substantial. A 5″ duct has half the cross sectional area of a 7″ duct.
Feb 01, 2016 @ 20:58:50
Hello Russ. I am going to install centralized air conditioning system. My question is how to design a duct of a two story building having an area of 270sq.m in 1st floor same area in 2nd floor, in 2nd floor we have an area of 52 sq.m each room equally and the 1st floor we have different area in each room, room 1. the area is 36 sq.m, room 2. the area is 52 sq.m , room 3.the area is 36 sq.m, room 4. the area is 18 sq.m, room 5.the area is 37.5 sq.m, room 6.the area is 34.5 sq.m, room 7. the area is 54 sq.m. hat will be the required TONS of air condition unit that I used? rectangular duct to be use. plss help! waiting for your respond.. thx!
Feb 02, 2016 @ 21:13:58
Hello, Sorry to be the bearer of bad news, but it’s not that simple. Tons is the total amount of cooling required to keep the house at a certain inside temperature when it is a certain outside temperature. those two temperatures determine how fast heat is coming into the house as much as the size of the house matters. Also, window area and orientation are more important than floor area. To determine total cooling load, the best standard out their is ACCA Manual J, but it is not for the amateur (it’s even too much for a lot of so called “professionals” – and that is not a knock on Manual J). Maybe a better starter book would be one called “Bob’s House” also by ACCA and available here: http://www.acca.org/store. Another book that was written for the “non-technician” is my book: HVAC 1.0 – Introduction to Residential HVAC Systems, available at http://www.sierrabuildingscience.com or on Amazon. Things that determine how much cooling you will need are: inside and outside design temps (determined by location), wall area and U-factor, ceiling area and u_factor, floor area and U-factorr, window area, orientation, solar heat gain coefficient, u-factor, leakage of the house, duct location, r-value and leakage, and internal gains (people, lights, appliances, etc.) Once you know all those, you pick an air conditioner that will meet that load at design conditions (ACCA Manual S), this will determine how much air you have to deliver to all the rooms. Knowing the load of each room will tell you how much air to deliver to each room. All this will help you size your ducts (ACCA Manual D).
Sorry to have no better suggestion than to go buy some stuff, but the more you know, the better off you will be.
Good luck! Check back in and let us know how it goes.
Russ
Feb 26, 2016 @ 10:31:49
Great article
Jul 21, 2016 @ 09:57:38
Hi Russ, Please help me this this question, will a 3 ton unit be overpowering for a full dormer cape house. I was told that is why they put a 2 ton in because our forced air ducts were to small. I believe the duct work is 6 inch. My first floor some days cannot get cooler than 75.
thank you
Jul 21, 2016 @ 10:42:48
Hi Ellen,
It really depends on so many things, but I can tell you that over-sized equipment is a huge problem if the ducts are too small. You can’t force a house to cool better by putting bigger equipment on it. A house with oversized equipment and undersized ducts will be just as uncomfortable as a house with undersized equipment and undersized ducts (while using more energy) Note the common problem: undersized ducts. A house with slightly undersized equipment and properly sized ducts will perform BETTER than a house with bigger equipment and undersized ducts. So, assuming a 2 ton is close to meeting the loads of your house, I would recommend improving airflow long before I would recommend increasing the size of the condenser. Also, I would recommend making the house more efficient first, by sealing, insulating, etc.
Good Luck!
Russ
Sep 13, 2016 @ 15:30:41
hello i have a wood furnace and my longest run is about 30 ft by the time it gets to the end the air is barley coming out would using a smaller duct pipe give more pressure thus more heat or does it not work that way thank you for any help
Sep 13, 2016 @ 15:41:21
Hi Dave,
I guess you could think of it as there being two kinds of pressure fighting each other: The good pressure comes from the fan and increases airflow. The bad pressure is resistance to the airflow and goes up with smaller ducts, more bends, fittings, etc. Making the ducts smaller reduces airflow. To increase airflow, you either have to increase the good kind (put on a bigger, stronger fan) and/or decrease the bad kind (make the ducts bigger and straighter). I hope that helps.
Russ
Feb 13, 2017 @ 21:03:40
Hey there, will i get more air in a 30×8 return air grill then a 30×6? Or does it really depened on the hole in the wall thru the floor which is a 2×4 wall 3 1/2″ x 30″
May 11, 2017 @ 08:38:28
Hi Jared,
Sorry for the slow reply. For some reason, I’m not getting notifications when I have a question waiting.
The larger return will have less resistance, therefore it will allow more air to pass through. 3.5 x 30 is about equivalent to a 10″ round duct. In a well designed system (friction rate = 0.1 iwc/100′) a 10″ round duct will deliver about 300 cfm. A 30×8 return grill should be adequate for that, but it depends on the grill. The manufacturer should provide a table showing how much air it will handle at a given pressure drop. A reasonable pressure drop to use is 0.03 iwc.
Russ
May 09, 2017 @ 18:15:03
Hi Russ,
I just did a kitchen remodel and now for the first time will have a outdoor vented professional range hood. The exhaust opening on top of the hood is 8″ round and the rigid duct work is approximately 4 feet straight up to the roof.
Now the question, since I do not already have an opening in the roof and I’d like the vent cap to be as small as possible for aesthetic reasons, I was thinking of a 6″ roof cap and using a 8″ to 6″ reducer at the very end of the run, just under the roof. My question is will this reduction have any significant impact on airflow/efficiency since it’s at the very end? Thanks for any feedback.
May 11, 2017 @ 08:25:31
Hi Scott,
It will have an effect, but given that the duct is so short and straight, I think you will still get adequate airflow. See if the exhaust hood comes with a table to show how lengths of straight duct, elbows and other fittings affect airflow. I suspect that they allow a lot more than 4 feet and probably also allow one or two elbows. You should be fine. You can also reduce the impact by picking a style of cap that has better airflow than others.
Russ
Jul 26, 2017 @ 12:05:23
Hi i recently swapped units but used the supply plennum box wich was 3 inches smaller. Now it sweats really bad and the condencsate water is over flowing.i thpught the drain line was clogged but wasnt and the system is leveled. could using a smaller plennum box be causing a restriction that causes the iver flow. The old unit was 3 ton and this one is 3.5.thanks in advance
Jul 26, 2017 @ 13:41:11
Hi Albert, Very interesting question!
My guess is that the smaller supply plenum is slowing down the airflow across the coil. Condensation is increased when you have a greater temperature drop across the coil (“delta T” = air temperature in minus air temperature out). When you have a bigger condenser and coil and the air spends more time in contact with the coil (because of reduced airflow) you will get a much greater temperature drop and more condensation. Most technicians think that a high delta t is a good thing, but it’s usually a symptom of low airflow, which is a very bad thing. My recommendation is to do whatever you can to improve airflow, which may not necessarily require that you replace the supply plenum, but that certainly wouldn’t hurt. Almost every system in the US would benefit from improved return air (add a return grill/duct, increase the size of the existing grill/duct, etc.). On the supply side, add or enlarge supply ducts/registers. Also, consider replacing all supply grilles with one with better airflow (replace stamp-faced ceiling registers with bar-type ceiling registers (e.g., Shoemaker 900 series).
Good luck. Let me know how it goes!
Russ
Aug 21, 2017 @ 20:55:07
Hi Russ-
Thanks for a great site!
I’m going to be installing a 90 CFM bathroom exhaust fan in a new bathroom where there is currently no duct. This fan comes with both a 4″ and 6″ adapters. My question is which one would be the best to exhaust the air the most efficiently? The side wall is about 6 feet away, but it needs to turn 180 degrees to terminate. I know that elbows affect airflow, so another question would be what length of duct would be the best between the final elbow and the termination hood?
Thanks in advance. I appreciate any feedback.
Aug 22, 2017 @ 08:18:32
Hi Jim,
With very, very few exceptions, bigger is always better. Some fans are designed for a minimum amount of resistance. If they don’t have enough resistance, they can free-spin without moving any air, but I seriously doubt that you have that issue. Your fan should come with a table that shows impact on airflow compared to duct size, length and number of bends. I’ll wager that you’d be fine going with 6″ all the way and should have substantially improved airflow. As long as a 6″ will easily fit, the cost difference should be minor compared to a 4″.
good luck!
Aug 22, 2017 @ 17:31:02
Thanks Russ. I’ll go with the 6″.
Aug 28, 2018 @ 12:02:32
Finally! Another HVAC contractor who understands duct sizing. I moved to another section of the country where all they install are 6″ ducts run to undersized registers (12×4)
And no one’s even heard of manual J load calcs. Thank you for your postings
Aug 28, 2018 @ 19:43:07
Sorry. I’m not a contractor. Just a lowly engineer, but thanks just the same. 🙂
Nov 24, 2020 @ 12:32:57
Hope everyone is safe and well.
It’s always a treat finding someone with such specialized knowledge! I was wondering if you can find the time to provide some help with a specialized issue I have come across.
During a remodel of an upstairs bedroom, I came across an air return that was not attached to the air return ducting in the basement leading back to the forced air system. The stud bay they picked ended up at the side of the basement stairwell so they couldn’t get it across the stairs to join into the ducting. And, because they couldn’t do that, they left it. Yeah, I know… The return ducting is in the next joist bay.
I was thinking the most straightforward solution would be to knock some holes in the joist between the air return and the return air ducting. I will research that one further but I was wondering if you could provide some insight into how much joist I would have to remove to make sure an adequate amount of air would be taken from the upstairs bedroom. For example, would a 2 3/8″ diameter hole let enough air through? Double that, triple that, etc.?
My air returns are the kind built into the stud and joist bays and I have 2×8″ joists. I also noticed that the joists are doubled and they are connected to a header at the stairs. Not sure if there is tie there or not, if that matters. Thanks
Nov 24, 2020 @ 13:41:37
Hi Eddie,
Thanks for your question. Very interesting situation.
First, a big caveat before I answer your question. One of the mechanical engineering departments I ran was owned by a very large structural engineering firm. I had an agreement with the structural engineers: I promised not to pretend like I knew anything about structural engineering if they promised not to pretend that they knew anything about mechanical engineering. So, anything I tell you is totally dependent on getting any modifications preapproved by a structural engineer. Not having your house fall down is way more important than proper airflow!
It sounds like any airflow will be an improvement. Perhaps we you can avoid any structural modifications. Would it be horrible to just put a new return grille right where the ducts terminate at the joist that you want to drill holes in? If the original return grille that it was supposed to connect to is not on the other side of a closable door, the location is not that big of a deal.
If that is not an option, it would help to know how much return capacity you currently have. Please read this blog post about evaluating your system. You can just do the return side, if you want. https://russellking.me/2017/09/06/how-to-quickly-evaluate-a-residential-hvac-duct-layout/
Let me know how it comes out and we can go from there.
Russ
May 22, 2022 @ 14:44:38
I’m trying to find part 2 of this article but can’t find it.
May 23, 2022 @ 09:06:17
Hi Billy,
I never never got around to posting Part 2. lol. Part 1 turned out to be a teaser to people to buy the book I wrote “HVAC 1.0 – Introduction to Residential HVAC Systems”, available on Amazon. To understand airflow in ducts you really need to understand what Friction Rate is. I highly recommend that you read this blog post next:
https://wordpress.com/post/russellking.me/333
Thanks!
Russ
Dec 13, 2022 @ 10:02:05
Any thoughts about take-off locations on return and supply plenums when there are multiple take-offs in each plenum? Should they be on the sides, perpendicular to airflow through the furnace, on the ends of the plenums, in line with the furnace airflow, or doesn’t it really make a difference?
I’ve really appreciated your approach in describing HVAC system design. Thanks.
Dec 15, 2022 @ 07:50:21
Hi Tom,
Sorry for the slow reply. I have lots of thoughts on take offs from supply plenums. I’m not saying these are the best way to do it, just that I’ve had really good success following these basic rules of thumb. (not all rules of thumb are evil, btw) Keep in mind that air has momentum. The air crashing into the end of a supply plenum creates a lot of pressure. The pressure is higher at the end and drops off upstream toward the air handler. The closer a take off is to the end, the more air it will get. Using custom friction rates for each run, rather than using a worst case friction rate for all ducts, is important for these to work.
– Air likes symmetry. Try to have equal take offs on either side of the supply plenum.
– because most of my designs were for tract homes that were built in any orientation, I arranged my trunks so they would serve parts of the house that would be similarly affected by rotating the house. I usually divided a 1 story hours into four “quadrants”, each one getting a trunk. 2 story houses I would use no less than 6 trunks. Each trunk would get a balancing damper (which turns out, rarely were used – a good overall design does not need to be adjusted as much).
– try to give the master suite its own trunk
– try not to put upstairs and downstairs rooms on the same trunk, unless you can put an easily accessible balancing damper on the upstairs portion
– put take offs for trunks serving downstairs (airhandler in the attic) closer to the end of the plenum, put upstairs trunks farther away from the end
– avoid putting a take off on the end of the plenum, unless it is the only take off, or in the very rare case where one trunk has to go MUCH farther than the rest.
That’s all I can think of right now.
Russ
Dec 15, 2022 @ 10:17:24
Great insight. In my single story case I was looking at 3 fairly balanced 12″ supply trunks, easy to put one on each side of the plenum, but as you said the one on the end gets air forced down it at the expense of the others. So a longer plenum would let you put two on one side and one on the other…or redistribute and create 4 trunks, putting two on each side.
On the return plenum with multiple returns it sounds like side or end location has less impact.
Aug 14, 2023 @ 06:39:26
I could not find an answer to this next question on the internet. Suppose we have a 6 inch duct. If we are on the end of a 20 ft branch 1) reduce to 4” to increase room velocity and it will fill the room faster or 2) the 4” will act as a restriction and reduce cfm
The question really comes down to when will reducing size to increase velocity (same cfm) get to the point of being a restriction and reduce cfm?
Aug 14, 2023 @ 09:47:04
Hi Grant,
Thanks for your question. Your option #2 is correct. Your option #1 is a common misconception. Velocity (feet per minute) and flow (cubic feet per minute) are very different things. Velocity is just how fast the air particles are moving. Any increase in velocity by reducing diameter will come at the cost of available pressure (the pressure left at the end of the duct where the air comes out). Available pressure is what moves the air. Increasing velocity this way will reduce flow. Flow is what heats and cools the room. The only reason you need to increase velocity is to improve throw and mixing in the room and this is best done by the supply register, not the duct. To understand the details of this, you should study ACCA Manual D and the concept of equivalent lengths, available static pressure and friction rate. Manual T covers register selection and sizing. Also, see my blog article about friction rate. I hope this helps.
Russ
Aug 15, 2023 @ 11:31:54
Is this site still active?
Aug 15, 2023 @ 11:41:10
Somewhat. I still monitor questions, but I don’t post as often as I should. I’m too busy training (arguing with) people on Facebook these days. 🙂
Aug 15, 2023 @ 17:10:34
Thx for your immediate response Russ!
Hope you can help……..
I have a damp basement and I want to run an exhaust fan 24/7. My concern is having the outside vent higher than our snowfall depth so the vent won’t be blocked.
The fan I’m using is an AC Infinity CLOUDLINE PRO T6, Quiet 6” Inline Duct Fan with Temperature Humidity VPD Controller, Airflow: 402 CFM.
My plan is to reduce the airflow from the 6″ fan down to a 2″ PVC pipe, have the pipe go through the wall, to an elbow, then up 42″, into an inverted 2″ “U” fitting, so the opening faces downward.
Every site I’ve seen says this will reduce airflow. You say it won’t – and you’re the one I want to believe !
I just want to be sure your theory would apply to this setup?
Thank you,
JVista
Aug 15, 2023 @ 19:00:28
Hello,
I’m not sure why you think I said it won’t, but I did not. If you reduce duct diameter, you will absolutely restrict air flow. I’m also not sure that an exhaust fan is your best option to reduce moisture. You may want to research that a little more. Where is the replacement air coming from? Is it really that much dryer and do you have to heat it? A dehumidifier might be a better option especially for a conditioned basement.
Russ
Aug 16, 2023 @ 05:05:11
Thank you again Russ. Is it possible to reply directly to you without posting my reply on this public forum?