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Thread: Duct sizing

  1. #1
    Join Date
    Mar 2013
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    6

    Duct sizing

    My furnace lists the filter requirment as 20 x 20, but the filter is 14 x 20. I'm planning on having someone come in, but I'd like to become a little better educated beforehand.

    The return & exhaust trunks both start at 8 x 24 and gradually taper down in two smaller stages as they run along the floor joists away from the furnace. The system is both air & heat and for the most part the heat works great. The air on the other hand if I let the temperature rise during the mid-day and try to bring it down later in the afternoon by a couple of dergrees, the unit will run for hours (8-9 hrs) before it finally catches up. I'm in Lake Ontario region so we get to see as much snow as sun.

    The question I have is why would the trunk be tappered down, which I've seen in several homes? If the oil furnace is oversized for the ducting, what would I gain by having the ductsize increased? It would seem easier to fix the ducting than hauling out the furnace as I have easy access from the basement. I can't see any advantage to increase the filter size without upsizing the trunk yes/no?

    Thanks in advance.

  2. #2
    Join Date
    Aug 2001
    Location
    Pavilion, NY
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    Each section of duct needs to be able to handle the amount of air going through it. ( Simplified example) if a trunk served (3) ducts each at 200cfm then the first section needs 600cfm. After that duct the next section needs 400cfm and then the last section needs 200cfm. If the duct were all the same size then the air traveling at the end sections would be very slow velocity and lazy. Reductions in duct systems are To maintain a minimum velocity of air. There are other reasons but this should answer your question.
    ...

  3. #3
    Join Date
    Nov 2004
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    SW FL
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    6,271
    Quote Originally Posted by Quinte View Post
    My furnace lists the filter requirement as 20 x 20, but the filter is 14 x 20. I'm planning on having someone come in, but I'd like to become a little better educated beforehand.

    The return & exhaust trunks both start at 8 x 24 and gradually er down in two smaller stages as they run along the floor joists away from the furnace.
    The system is both air & heat and for the most part the heat works great.

    I'm in Lake Ontario region so we get to see as much snow as sun.
    Actually u get about 10 X snow as sun.

    2 square feet of filter might handle
    700 CFM. In other words, 2 tons of cooling, perhaps if using a MERV 7 filter or less.

    700 CFM at 50'F temperature rise for heating is about 39,000 BTU/HR
    Designer Dan
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  4. #4
    Join Date
    Mar 2013
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    6
    Quote Originally Posted by dan sw fl View Post
    Actually u get about 10 X snow as sun.

    2 square feet of filter might handle
    700 CFM. In other words, 2 tons of cooling, perhaps if using a MERV 7 filter or less.

    700 CFM at 50'F temperature rise for heating is about 39,000 BTU/HR
    Yes and it would be nice if the snow left. I'm ready for spring!

    I guess the better question to ask, would it be of any real significance to correct the filter size despite the possible limitations of the trunks?
    To me, 24 x 8 trunks seem fairly well sized for residential.
    Would I gain anything by having the trunck feed modified from 14 x 20 to 20 x20?

    I'm guessing that this would improve and reduce any filter resistance. I had to google MERV ratings. The filter I'm using is a low end off the shelf horse-hair style. I did have a pleated 3m filter at one time, but my oil service fellow told me it had a lot of resistance.

  5. #5
    Join Date
    Mar 2013
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    6
    Quote Originally Posted by kangaroogod View Post
    Each section of duct needs to be able to handle the amount of air going through it. ( Simplified example) if a trunk served (3) ducts each at 200cfm then the first section needs 600cfm. After that duct the next section needs 400cfm and then the last section needs 200cfm. If the duct were all the same size then the air traveling at the end sections would be very slow velocity and lazy. Reductions in duct systems are To maintain a minimum velocity of air. There are other reasons but this should answer your question.
    That makes perfect sense, thanks! One step I did take on my own, thanks to reading other posts, was to seal the trunk feeds with duct-sealer. I bought an el-cheapo flow meter, not so much for accuracy, but for comparisons of airflow around the house. On the one end I was getting a return flow rate of about 1.2m/s and on the other barely registering at .1m/s. After going through all the returns they are all now at roughly 1.2m/s. There are decent size registers distributed through every room on the main floor, with the exception of the bathroom and two returns in the basement. The exhaust ducts are reading 1.8m/s. If I open the fan cover on the furnace, they jump up by roughly .7m/s (2.5m/s). The house is only 5 years old and I'm sure there was some thought put into the HVAC design, I just wish I had know a little more before hand and paid more attention to what was installed. My major concern is with the cooling during the summer, during the winter the furnace seems to work as one would expect in relatively short cycles.

    I was told that I should keep the temperature set at one setting during the summer rather than allowing it to rise over the day and lowering in the later afternoon. Otherwise it takes hours to cool the house down. I'm trying to get myself up to speed on the concept of air-flow vs BTU capacity/requirements. Hence, is it a case of the furnace not pushing enough air flow or is the air conditioner undersized.

    Obviously I need to start with some type of load analysis, but I need to wrap my head a little around this so that I'm not led down the golden path. So any info/advice would certainly be appreciated. Thanks.

  6. #6
    Join Date
    Jun 2005
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    SW Wisconsin
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    Return Air Filter Sizing for Manual D, 300-fpm velocity

    My furnace lists the filter requirement as 20 x 20, but the filter is 14 x 20. I'm planning on having someone come in, but I'd like to become a little better educated beforehand.
    A 14X20 "cheap low resistance filter," according to Hart & Cooley Engineering, will only flow 401-cfm @ 300-fpm velocity through it. The "free-air-area," or (Ak) is only 1.335-sf. Therefore, it would require two 14X20 filter areas to flow 800-cfm @ 300-fpm through a news clean filter.

    Okay, the 20X20, only flows 575-cfm @ 300-fpm; the "free-air-area," (Ak) is only 1.917-sf; not nearly enough.

    Excessive Debris is blown through that type of filter above 500-fpm velocity through it.

    The RA filter area is extremely important to efficient airflow.

    When you opened the blower door, you bypassed the RA duct system & the restrictive RA filter area, resulting in much improved airflow to the rooms.

    Depending on the Btuh Output of the furnace, the temperature rise above the RA temp, may be too high!

    Additionally with filters in the RA duct with a 90-ell too close to the filter down stream & near the blower chamber, there ought to be turning vanes in the 90-ell, or the lower part of the filter will get much more air through it at a higher velocity than the upper portions of the filter.

    Few airflow systems are designed for optimal efficiency performance...most systems underperform...which causes heat & air equipment to underperform...

  7. #7
    Join Date
    Jun 2010
    Location
    Philadelphia PA
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    2,190
    If 20x8 is the right size for your duct then 20x14 standard filter sounds reasonqable.
    Need to know how much air you are moving to give better answer
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

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