Duct Sizing Explained - Why Ducts are undersized sometimes

Поділитися
Вставка
  • Опубліковано 26 лис 2024

КОМЕНТАРІ • 40

  • @AudiophileTommy
    @AudiophileTommy 11 місяців тому +9

    Can you do a video on return sizes ? Thanks !

    • @NewHVACGuide
      @NewHVACGuide  11 місяців тому +4

      I’ll add it to the list! 🙂

  • @waalwink
    @waalwink 5 місяців тому +2

    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.

  • @davedakroub5323
    @davedakroub5323 7 днів тому

    Hi, I have a very annoying issue with my homes duct work. I have rectangular duct work for my main trunk and return. In the winter, the ducts pop and bang extremely loud, the ducts are in rather poor condition and leaky. I’m receiving mixed opinions on whether the issue is from improper size or just expanding and contracting from the temperature swings and need to be insulated. They do not make noise in the summer with the AC. I found a large cutout in the return line and several other sections that need to be replaced. I have considered replacing at least the supply side with spiral duct as it seems a little easier to hang, but also received mixed opinions on doing that. Any suggestions? Im located in South East MI.

  • @Inkling777
    @Inkling777 11 місяців тому +2

    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?

    • @ironmatic1
      @ironmatic1 7 місяців тому +1

      The pumping energy required to overcome the increased friction loss would far offset any negligible thermal loss.

    • @Steven-mm7gb
      @Steven-mm7gb 4 місяці тому

      the higher the static pressure the noisier it gets also

  • @jordapen
    @jordapen 11 місяців тому

    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.

    • @ABChgdyhh
      @ABChgdyhh 8 місяців тому

      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

    • @jordapen
      @jordapen 8 місяців тому

      @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.

  • @allandavis6116
    @allandavis6116 3 місяці тому

    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?

  • @brianchadwick9405
    @brianchadwick9405 11 місяців тому

    I have always used the NCI cheat sheet. Until complicated systems need technical details.

  • @marksatalino2225
    @marksatalino2225 6 місяців тому

    Looking for a good hvac company in Nassau county forced air. Not getting enough air in the second floor bedrooms.

  • @enriqueramirez6199
    @enriqueramirez6199 11 місяців тому +1

    Why not size the ductwork to the system recommended static pressure? .05

    • @Steven-mm7gb
      @Steven-mm7gb 5 місяців тому

      rule of thumb 1 cfm per square foot

    • @JOIHIINI
      @JOIHIINI 4 місяці тому

      ​@@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

    • @JOIHIINI
      @JOIHIINI 4 місяці тому

      Coil + filter takes away about .15 before it leaves the gate

    • @Steven-mm7gb
      @Steven-mm7gb 4 місяці тому

      @@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.

    • @JOIHIINI
      @JOIHIINI 4 місяці тому

      @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

  • @trainsdawgz
    @trainsdawgz 6 місяців тому +1

    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!

    • @peterdarlington4117
      @peterdarlington4117 5 місяців тому +2

      Yes, ma’am, if it truly is undersized, it will destroy your system

    • @peterdarlington4117
      @peterdarlington4117 5 місяців тому +1

      And your electric bill

    • @Steven-mm7gb
      @Steven-mm7gb 5 місяців тому +1

      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

    • @Steven-mm7gb
      @Steven-mm7gb 5 місяців тому +1

      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

    • @raymondblakley3731
      @raymondblakley3731 2 місяці тому

      Yes true I went to one that had a dryer duct attached to the system

  • @jevells
    @jevells 5 місяців тому +1

    Now how do you k ow what cfm you need/vent/sqft?

    • @Steven-mm7gb
      @Steven-mm7gb 4 місяці тому +1

      rule of thumb 1 cfm per square foot of living space...can adjust for a full sun or no insulation etc..

    • @jevells
      @jevells 3 місяці тому

      @@Steven-mm7gb thank you!

  • @dnell6854
    @dnell6854 11 місяців тому

    Great video!!

  • @sensen3095
    @sensen3095 2 місяці тому

    Can someone explain this in electrician terms?

    • @convictednotconvinced
      @convictednotconvinced 2 місяці тому

      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.

  • @joew6107
    @joew6107 5 місяців тому +1

    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?

    • @mrteuy
      @mrteuy 4 місяці тому

      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.

    • @JOIHIINI
      @JOIHIINI 4 місяці тому

      ​@@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?

    • @jeffb6276
      @jeffb6276 3 місяці тому

      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)

    • @JOIHIINI
      @JOIHIINI 3 місяці тому

      @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.

    • @howardsmith5474
      @howardsmith5474 3 місяці тому

      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.

  • @kennethbubser9601
    @kennethbubser9601 11 місяців тому

    .07 supply .05 for return .07 now code in nj

    • @NewHVACGuide
      @NewHVACGuide  11 місяців тому +1

      Nice. That’s not a bad thing.