Duct Sizing Explained - Why Ducts are undersized sometimes
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- เผยแพร่เมื่อ 5 ต.ค. 2024
- In this video, Joshua Griffin explains a few theories and misconceptions when it comes to duct sizing. The hope is with more understanding of how ducts are sized, homeowners and contractors alike will not have any issues when sizing ductwork and understanding why certain sizes are significantly larger, or only somewhat larger. Joshua uses a graphic to show the differences between a 6, 8, 10, and 12-inch duct. He also explains the CFM levels of each size and the understanding of a 6 vs a 12-inch round duct.
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Can you do a video on return sizes ? Thanks !
I’ll add it to the list! 🙂
Flow is directly related to cross sectional area. A little bit of geometry says the cross sectional area is pi times the radius squared. This is why air flow is not linear related to the diameter.
And static pressure.
So for 4 ton system - it generates a flow of 1600 cfm - so off the main box I should have, for example 4 10'' ducts = 4x400 cfm, or 2 12'' ducts and 1 10'' duct = 2*600 + 1*400 = 1600 cfm. Then at the room vents there could be 8 8'' ducts = 8*200 = 1600 cfm. Correct?
I have always used the NCI cheat sheet. Until complicated systems need technical details.
For ductwork in unconditioned space wouldn't a small duct with a powerful enough fan push the air through quickly enough to reduce the loss of heating or cooling and thus save money?
The pumping energy required to overcome the increased friction loss would far offset any negligible thermal loss.
the higher the static pressure the noisier it gets also
My new Daikin mini split sydtem has been working well, but there has been times when it is set to 22 C, it can only make it 21 C. The temperature had been -6 C, and it could hold 22C, then a week later when it it was - 6 C again, it could only hold 21 C. The contractor has checked it out, and found things in order. He will be back with his diagonistic device to check out the circuitry.
Open your manual and check minimal temperatures for this unit. Also check how much square footage you have in that room , multiple it by 30 and compare it to unit BTUs. Probably it would be lower than your unit
@ABChgdyhh I've made a lot of headway since then. It involved moving things to give the downstairs unit unrestricted airflow for both intake and output. Also, if it is going to get very cold, I set the temperature up to 24 C, and it now will maintain 22 C right down to -12 C. It hasn't gotten any colder to try it at a lower temperature.
Looking for a good hvac company in Nassau county forced air. Not getting enough air in the second floor bedrooms.
I was told my duct work is too small for the tonnage of my system and that this would cause damage to the system. They recommend new duct work. Does this sound right? Thank you!
Yes, ma’am, if it truly is undersized, it will destroy your system
And your electric bill
you wont get enough flow of air across the coil and it will gather ice, freeze up and then no air will flow can set fan speed to high but your unit will only produce so much static pressure and may not be able to push the air. If you increase static pressure more air will flow through a smaller duct
rule of thumb is 1 cfm per sq foot so can figure out if your tonnage of cooling and duct correct size and can use duculater to confirm duct size per room...400 cfm per ton of cooling
Yes true I went to one that had a dryer duct attached to the system
Great video!!
Why not size the ductwork to the system recommended static pressure? .05
rule of thumb 1 cfm per square foot
@@Steven-mm7gbfrom what I've seen the friction rate to have 1cfm per square foot would have to be well below .05 for supply which would be on longer ductwork applications. In shorter ducts friction rate can be around the 1 range which would make those ducts way oversized. It'd be the difference between a 6 and an 8 round
Coil + filter takes away about .15 before it leaves the gate
@@JOIHIINI I think my original post left out what i meant to say , "rule of thumb 1 cfm per square foot" I should have said ....rule of thumb 1 cfm per square foot of room size, duct should be sized to the room served. This info given to me 35 years ago when I started the trade in Commercial Sheet metal from multiple HVAC engineers as a rule of thumb for my home system. 1cfm per SQ foot of area in a room. These are professional cal poly graduates with engineering stamps.
@Steven-mm7gb yeah I figured u meant room size in the first place. Just seems like a lot, I don't have that experience tho. I was just sizing a central heat pump unit and they rate a 4 ton for 1500-2000 square feet but 1cfm per square foot would only work on the very low end of that spectrum
Can someone explain this in electrician terms?
There are certainly parallels between fluid mechanics and electrical theory, and I'll do my best to make some of those connections, however I don't think that's the best way to explain what's at the core of this. First off, the relationship of duct size to CFM is the fluid mechanics version of Olms law. A pressure is exerted on some fluid (air in this case) which is the equivalent of Volts., the system has some intrinsic resistance to the motion of the fluid, the Olms equivalent, and the proportion of these two will determine the current, CFM in this example, and Amperes in electrical... thank you captain obvious! This is all about selecting the appropriate conductor AWG based on current requirements, voltage drop, environmental factors, blah blah blah.
Just like with different wire gages, duct size inversely changes the resistance component of the equation, and while total resistance to air flow is a sum of many factors (impedance in electrical), friction caused by air molecules rubbing against the interior sides of the duct is the major source of the resistance. The reason the change is nonlinear has to do with a simple concept in geometry, specifically the ratio between the circumference of the duct and its cross sectional area. Interestingly this same concept, but in three dimensions, is the reason the cells in our bodies have to be small.
So as the duct diameter increases, it's cross sectional surface area increases at a faster rate than the length of circumference, and since surface area dictates CFM while circumference/ perimeter dictates resistance, not only is the CFM increasing but so is the efficiency. Your buying your CFM in bulk and therefore getting a better price. In the case of our cells, the ratio of cell surface area to volume is important because water and nutrients are absorbed through the cell walls in order to feed the cell volume. For this purpose smaller is better such that there is ever increasing area for absorption while having to sustain an ever decreasing amount of volume. You can prove this yourself by calculating the the surface area and circumference of a circle given a random diameter, and then take the ratio of the two. You see how that ratio changes with its radius and that's all that's happening here.
- Whew fuck! Time for work.
Now how do you k ow what cfm you need/vent/sqft?
rule of thumb 1 cfm per square foot of living space...can adjust for a full sun or no insulation etc..
@@Steven-mm7gb thank you!
I don't see how a 6 in, carrying 100 cfm, can transition a 8 in, and then there is 200 cfm. Wouldn't the 6 in restrict the flow to 100 cfm? Where did the additional 100 cfm come from?
You’re increasing the cross section which cfm increases due to the volumetric increase. Velocity changes (slows) that will affect the time of moving air giving you the cubic feet per minute.
@@mrteuyit just seems odd. By that logic I could run a 6 inch, increase that to 8, then branch to 2 6 inches a foot later and there would be 100cfm at each 6 inch?
Keep in mind, I don't know a lot, but his measurement is with the same static pressure in thr 6 and 8 inch supply, which to my under educated guess means the 8 inch gets a bigger blower to create the same pressure within the system as the smaller blower with the 6 inch. He's not saying just swap the duct diameter, he's saying bumping up to an 8 inch system, which brings the volume from 28 square inches to 50 square inches (I'm writing this just as much to retain it as to help)
@jeffb6276 in a standard system though the ducts branch off a large trunk so I don't really get the blower size thing. The trunk is sized based on the required cfm the unit can provide and then the duct runs are distributed based on the cfm of the trunk. From what I can tell this logic is pretty much saying you could have a 6 inch get the same air flow as an 8 inch which just doesn't make sense. Then you may as well have a 6 by 6 trunk and branch off of it. The static pressure is obviously important but it doesn't make a difference if the duct is too small, and the unit itself has a max static pressure that it can operate at so you can't really increase it that much to make up for loss in sizing to deliver that cfm.
To answer your question it doesn’t. He is only talking about maximum capacity of cfm of each size pipe. I am not aware of a situation where you would start with a 6” pipe and then increase the size downstream of the air handler, in fact its the opposite- it gets smaller because with each branch off your main trunk the cfm requirement goes down.
.07 supply .05 for return .07 now code in nj
Nice. That’s not a bad thing.