Duct Sizing Explained - Why Ducts are undersized sometimes
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- Опубліковано 3 гру 2023
- 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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#hvac #ductwork #ductsizing - Навчання та стиль
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.
Great video!!
Can you do a video on return sizes ? Thanks !
I’ll add it to the list! 🙂
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?
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.
Now how do you k ow what cfm you need/vent/sqft?
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
Why not size the ductwork to the system recommended static pressure? .05
rule of thumb 1 cfm per square foot
.07 supply .05 for return .07 now code in nj
Nice. That’s not a bad thing.