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can a duct be too big?

ponytl | Posted in Energy, Heating & Insulation on April 19, 2006 08:14am

if a 16″ sq A/C duct is called for (main trunk line)…  would it hurt if it was 20″ x 20″ if all the openings remained the same?

I know it’s a wide open question… but any thoughts… or facts and i’d be greatful

tell me why i’m wrong… I’m think’n… same amount of air come’n out of the unit (volume wise)  and same size vent/grill openings as the smaller duct…. so i just have a larger volume of air under a little pressure?

what about return duct size?  do i need to speed of the air that the smaller duct would give?

will larger duct have less noise? 

thanks in advance

Pony

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  1. User avater
    Ricks503 | Apr 19, 2006 02:47pm | #1

    Go with correct size. Sizing is a function of the output, size of unit and other factors.  When all else is figured, going with a larger than required duct work will or can reduce the effectiveness and efficiency of the system

    1 - measure the board twice, 2 - cut it once, 3 - measure the space where it is supposed to go        4 - get a new board and go back to step 1
  2. User avater
    McDesign | Apr 19, 2006 02:51pm | #2

    I guess there may be a problem with stagnation - the flow velocity would be a lot less, and there may be issues associated with that.  Condensation, flow turbulence - I believe in fluids class a rule of thumb was that compressable flow moving into a smaller cross-sectional area had relatively less turbulence (lower Reynolds number) than flow expanding into a larger area. 

    Think traffic - people slow down, get in line to narrow to one lane, but when it opens up they go a variety of different speeds and directions to resume speed.

    I don't really know what all this would mean in the context of HVAC design, though!

    Forrest

    1. Tomrocks21212 | Apr 19, 2006 03:04pm | #3

      Yes, wouldn't it be nice if Bernoulli's Principle applied to traffic??

      1. User avater
        McDesign | Apr 19, 2006 09:36pm | #4

        I think it does, on an individual level - when I go faster, I feel less pressure!

        Forrest

  3. DanH | Apr 19, 2006 10:28pm | #5

    My take is that it all depends. Obviously, if you increase the size enormously the air in the duct will get cold, put on a sweater, and lay down for a nap before it gets to the register. But going from 16" to 20" is going from 256 to 400 sq in, not likely to make a difference in most cases.

    The other thing to consider is how the system is designed. Commercial systems are often designed for a constant air velocity throughout the system. You see the ductwork narrow down each time a register is dropped off of it. In such a case the momentum of the air is being used to help distribute it, and increasing the duct size reduces momentum per volume of air, while reducing duct size reduces pressure. Either of these can throw off the careful planning of the system.

    But most home systems aren't anywhere near so carefully designed. They basically rely on static pressure to distribute the air, with "fine tuning" being done by occupants adjusting register flappers. So increasing duct size (within reason) shouldn't upset things. OTOH, unless the duct is seriously undersized to begin with, increasing duct size isn't likely to do much good either.

    The size of the duct (again, within reason) isn't likely to make much difference in terms of noise. Most noise is generated either at the furnace itself (by the fan, etc) or at the registers. Or, on a zoned system, you may have some noise from the zone dampers.

    If Tyranny and Oppression come to this land, it will be in the guise of fighting a foreign enemy. --James Madison
    1. Tim | May 04, 2006 05:25pm | #10

      "Commercial systems are often designed for a constant air velocity .." This is somewhat true. There are two common duct design approaches used in commercial design. One is "static regain" where high velocity, low(er) pressure air is "used" near the air handler and slower, high(er) pressure air is "used" at the terminals. The other very common method is that of "constant friction" where duct sizes are selected to give a consistent pressure drop throughout the system, such as 0.08"/100ft. Typically a range of friction is used, with 0.1"/100 ft being the high end and 0.05"/100 ft being the low end, or the criteria used to step down the duct to a smaller size. These criteria are for supply duct design, which is design to a greater pressure drop than the return side, in good design. Exhaust systems, return systems, commercial kitchen exhaust all have different criteria.

      "..the momentum of the air is being used to help distribute..." This on the other hand is an inventive guess or just plain bunk. Differential pressure drives the movement of air in all systems, commercial or otherwise.

      Something prevalent in commercial duct system that is not common in residential systems is duct liner. That and the fact that the ducts are substantially larger and installed in much greater quantity than in residences, requires a close eye on the costs be maintained. The cost of large, lined duct can add up real fast. A small branch bank that I recently provided all the duct for, cost the installing contractor $10k for the duct alone, and this was a small commercial building (~ 2500 sf).

      1. DonK | May 04, 2006 06:09pm | #11

        Apology to OP in advance for a minor hijack.

        Tim - Can you give me a simple formula for calculating the amount of hot water baseboard needed for a room (residential setting) ?

        I had one that translated off of cubic feet and don't know where I put it.

        Thanks.

        Don K.

        EJG Homes      Renovations - New Construction - Rentals

        1. Tim | May 05, 2006 12:21am | #13

          The best rule of thumb there is: "Don't hit yours with a hammer!".

          Guesses, rules of thumb and "simple formulas" will almost always get you in trouble, so I try to avoid them, using or sharing.

          However, a consevative, CYA, baseboard estimate would be as follows: given that the average run-of-the-mill residential baseboard will provide 500 btu/hr/linear foot at 1 gpm and 170 degF supply temp. In a semi-cold climate (like NY or MA, but not MN or WI) with average construction (2x4 with fg batts), you will need 6 to 8 feet of baseboard for every 100 square feet of house.

          1. DonK | May 05, 2006 06:47am | #14

            Tim  - Thanks for that "rule of thumb". I'm working 9" ceilings with 1" polyiso on top of old 2x6 wall studs and 2x8 ceiling beams (1885 main structure) with fg batts. Windows are new replacements with low-e, double paned. I can do a little plus or minus and it will get me where I need to go.

            If you are at Tipifest, I'll buy you a beer. (BTW, for whatever it's worth, when I was practicing law, I spent a good chunk of 15 years representing design professionals who were unlucky enough to get sued. ) 

            Don K.

  4. Tim | May 04, 2006 12:19am | #6

    1. "..would it hurt if it.."? No, not at all. If I were calling for a 16x16 supply trunk, it would be for 1600 cfm, at 950 ft/min. The same volume in the 20x20 would be 600 ft/minute. The difference to travel 100 ft is 4 seconds (6 vs 10). Slightly (and in this case, slightly does apply) oversizing of duct work causes no detriment to the system. Consistency is important, though, as I would not recommend oversizing one of two trunk lines. Then you would (potentially) be causing some balance issues.

    2. "..do i need to speed of the air that the smaller duct would give." No, velocity inside of ducts (commercial kitchen exhaust ducts/grease ducts are a very specific exception) need not meet any "minimum". Cost and size constraints dictate that ducts be as small as possible. Pressure drop and noise dictate that ducts be as large as possible. Design, is partially a process that takes the two conflicting "needs" and makes suitable compromises.

    3. "..will larger duct have less noise.."? Yes and no. During constant, continuous air flow, the lower the velocity, the less friction and noise will be experienced. However, during transients, especially fan start-up, the larger the surface area of the inside of the duct, the greater the chance of "oil canning", that "tink" noise when air pressure causes the duct to flex a little. Also, duct noise is usually small compared to the other noises in a forced air system: fan/blower noise, inducer noise, burner noise and noise at registes and grilles.



    Edited 5/4/2006 9:30 am by Tim

    1. migraine | May 04, 2006 01:58am | #7

      This sure post sure has the right timing for me.  I'm sort of in the same situation as to duct sizing as the original poster.   Maybe I'm even quite a bit deeper into it.   I'm dealing with an existing FAU stem that needs to be replaced with a split AC/gas fired furnace.

      The existing rigid supply ducts will remain and the 16" flex return will too(I hope) which brings me to the question.

      On a 1700sqft, 14" thick  brick home with the main floor as one zone, is a 16" flex return sufficient for a return line?.  If the  Cfm is used on the blower of 2000cfm(4 ton ac unit), my mind is telling me that the air is moving too fast through the return, or does it not matter as much as it would through the supply lines?  Still think that air moving that fast would be nuisance  

      1. Tim | May 04, 2006 04:51pm | #9

        A 4 ton AC would be nominally sized at 1600 cfm (400 cfm/ton is the standard). In an ideal system, there would be no flex in the return side and there would be more than one. However, fast moving air through flex or any other duct, within the ability of the system to provide the required flow, is not a problem unless it creates too much noise or pressure drop. Flex ducts tend to be quiet. IF you take the pressure loss of the entire system (from the inlet of the return(s), RA ducting, unit filter (dirty), heat exchanger, wet/dirty AC coil, SA ducting and dampers, and supply registers) into account and the air handler can provide the necessary flow (or reasonably close to it) with the system resistence as is, then it is OK. 1600 cfm through a 16" round duct is beyond the range that I would consider good practice for low pressure system design and 2000 cfm is further beyond that range. IF I was looking into a system that was not working well, the return as you describe would be the prime culprit.

        1. migraine | May 04, 2006 06:21pm | #12

          The reason why I'm asking is that we are replacing our system and the three companies that have come out here have all looked at the existing ducts and none of them have stated that any of them need any type of rework.  I have even pointed out the RA line and it it is easily visible from the main floor and from the furnace.   Seems that they are more concerned with the sale of the equiptment than addressing the correct application.  

          Is it my understanding that trying to put in a system that is too large for my duct work(as in return air line) is less effiecient than putting in a smaller or even larger system?   or I am not understanding the princinples?

          Let alone that each one of these companies has quoted a different size AC units.  3 ton, 3 1/2 ton, 4 ton.  The two of the companies have agreed on a 2 1/2 ton on the second floor zone (1100 sqft). 

          I should probably add that the RA run is almost directly above the furnace.  Nine foot run and 2) 90* bends

          Edited 5/4/2006 3:06 pm by migraine

          1. Tim | May 05, 2006 03:57pm | #15

            The contractors have learned from experience that proposing to rework the system's ducting is 1) a potential can of worm, 2) usually an expensive option that most will not pay for and 3) if accepted, removes all excuses if everything doen't work perfectly. So, unless the customer (you) specifically told them of problems related to the ducting, they are taking the "if it ain't broke..." approach, and reasonably so. OTH, such a short run of return duct, undersized or not, will not be overly restrictive.

            As far a the unit(s) capacities are concerned, each competent contractor can and will show you their load determination for their selection(s). If not, consider that a big red flag, and a deal killer (IMPO).

            A properly sized AC system, in say Seattle for instance (design temp is 85 degF) should run about 18 to 20 hours on a 90+ degree day. However, when an AC unit does run that much on a design day, contractors invarialbly get complaints about the system being too small. So, they pad it a little adding an extra 1/2 ton or 1 ton, to CYA. That is most likely the reason for the range in sizes proposed.

          2. junkhound | May 06, 2006 06:38pm | #16

            properly sized AC system, in say Seattle for instance (design temp is 85 degF) should run about 18 to 20 hours on a 90+ degree day

            Ha, would that it ever got warm here. 

            We have trees, HP for winter -- have NOT even run the system as AC for 15 minutes in the last 4 years.

            as to the original question, ducting can only be too big if it takes up room that could be used otherwise (or adds weight past break even point, as in aircraft)

          3. Tim | May 08, 2006 03:54pm | #17

            "..would that it ever got warm here"

            Not my neck of the woods, by any means. IF I lived where cooling design conditions were that low, I wouldn't bother. I don't run mine in N. Il until it gets into the nineties.

    2. ponytl | May 04, 2006 02:08am | #8

      thanks for take'n the time to educate me... where i am and the building construction AC is more an issue than heat...

      in one area i have 2  5 ton gas-packs  that have a 20ft drop (roof to ceiling of 1st floor)   one serves the kitchen, butlers pantry, d/r and part of the entry... the other serves the L/r, bar common area and entry... 70% is exposed duct...

      thanks again

      p

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