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  1. #1
    Join Date
    Feb 2004
    Posts
    110

    Plenum Sizing with Variable Speed Furnace

    Hello,

    Sorry in advance for the lengthy post.

    Currently have Nat Gas 90% AFU furnace that is 12+ years old and was way oversized for the house (120k BTUh vs 75k BTUH with 20% oversizing) so I had to derate the furnace to keep if from tripping on hi heat all the time.

    Based on age (I know not that old) and look of the heat exchanger I am planning on replacing with new variable modulating variable speed furnace in the spring with 2 zones, one for the first floor and one for the second floor

    Did manual J and duct calculations and determined proper size of furnace and A/C. All branch lines are properly sized but the existing trunk lines, including return are wrong.

    This is what I have determined. First floor combined CFM (heating and cooling) is 800. Second Floor combined CFM is 520.

    Based on the above and about 800 FPM design on the trunk lines I would need a 8x20 for the first floor and a 8x14 for the second floor at 0.08 static

    The return at 700 FPM design on the trunk line would be 10x25 or 12x25

    My question is being a multizone unit, I believe I would need to increase each trunk line by 1/3 (less the return) to compensate for only a single zone calling for heat and install a static pressure by-pass damper?

    Can you pls provide some guidance on the trunk line sizing with multizone units with variable speed blowers?

    Thanks

  2. #2
    Join Date
    Feb 2011
    Location
    North East Ohio
    Posts
    772
    YES on the by-pass damper. I would not oversize the trunks too much as the FPM will drop as the ESP goes down. Good luck

  3. #3
    Join Date
    Dec 2010
    Location
    The Quad-Cities area (midwest).
    Posts
    2,626
    Quote Originally Posted by lkaufman View Post
    Hello,

    Sorry in advance for the lengthy post.

    Currently have Nat Gas 90% AFU furnace that is 12+ years old and was way oversized for the house (120k BTUh vs 75k BTUH with 20% oversizing) so I had to derate the furnace to keep if from tripping on hi heat all the time.

    Based on age (I know not that old) and look of the heat exchanger I am planning on replacing with new variable modulating variable speed furnace in the spring with 2 zones, one for the first floor and one for the second floor

    Did manual J and duct calculations and determined proper size of furnace and A/C. All branch lines are properly sized but the existing trunk lines, including return are wrong.

    This is what I have determined. First floor combined CFM (heating and cooling) is 800. Second Floor combined CFM is 520.

    Based on the above and about 800 FPM design on the trunk lines I would need a 8x20 for the first floor and a 8x14 for the second floor at 0.08 static

    The return at 700 FPM design on the trunk line would be 10x25 or 12x25

    My question is being a multizone unit, I believe I would need to increase each trunk line by 1/3 (less the return) to compensate for only a single zone calling for heat and install a static pressure by-pass damper?

    Can you pls provide some guidance on the trunk line sizing with multizone units with variable speed blowers?

    Thanks
    So you're changing the ductwork? What about the existing supplies? What size is the new A/C going to be? You may not need a by-pass damper at all if designed properly. If a by-pass is used, particularly with the VS blower, a modulating by-pass if preferred. Oversizing the ductwork by 25% is what I was taught. However, zoning can be done on the existing ductwork with no problem. It'll just be a little noisier.

  4. #4
    Join Date
    Feb 2004
    Posts
    110
    Thank you for the information. So it sounds like the preferred method is a motorized SP bypass damper tied to a SP switch rather than the 25-30% oversizing of the ductwork? My contractor says the same thing but I wanted to be sure as some folks I had over said "just oversize by 1/3" and you will be fine.
    Just 1 question in terms of the SP bypass. As the air will be returned to the return duct (dont have a dump zone) will this create an issue with temperature rise across the coil or should it balance itself out?

    BTW the supply, branch, lines are all ok with the exception of one 8" line to the family room that is a bit small. As it serves the first floor and is in the basement, a simple upgrade to 9" with a new floor box will do the trick. The rest of the lines are all 5", 6" and 7" and are correctly sized. The A/C came in at 37.5 KBTUH so I will go with a 3 Ton unit and not 4.

    Any other input would be appreciated.

    Thanks

  5. #5
    Join Date
    Jun 2007
    Location
    Greenwood Indiana (Indianapolis)
    Posts
    420
    As the air will be returned to the return duct (dont have a dump zone) will this create an issue with temperature rise across the coil or should it balance itself out?



    Any other input would be appreciated.

    What I was taught was that the largest zone damper, undersize that by 2 inchs. Example, 20x8 duct, insert a 18x8 damper, there is your by-pass without dumping hot or cold air back into return which will overheat the furnace and could freeze up the a/c coil. Also less likely the damper will whistle.

    Thanks[/QUOTE]
    As iron sharpens iron so one man sharpens another Proverbs 27:17 NIV84

  6. #6
    Join Date
    Feb 2004
    Posts
    110
    I am sorry but I dont understand your response. My largest zone is the first floor with 8x20. So you are saying insert an 8x18 instead and connect it where?

  7. #7
    Join Date
    Jun 2007
    Location
    Greenwood Indiana (Indianapolis)
    Posts
    420
    Quote Originally Posted by lkaufman View Post
    I am sorry but I dont understand your response. My largest zone is the first floor with 8x20. So you are saying insert an 8x18 instead and connect it where?
    My example was the largest zone is 20x8. Instead of installing a 20x8 damper into the 20x8 duct, insert a 18x8 damper. Then you will have a 2x8 bypass when the damper is closed. So when the smaller 14x8 duct is open and the 18x8 damper is closed, a small amount of air will go past the 18x8 damper, but not enough to feel from a register. Theres your bypass without dumping air back into the return air. Or, a heat run or two that are dump ducts, which means these ducts are upstream of the dampers and air will blow out no matter which zone is calling.
    As iron sharpens iron so one man sharpens another Proverbs 27:17 NIV84

  8. #8
    Join Date
    Oct 2004
    Location
    So Cal
    Posts
    3,489
    Are you hiring a pro to design & perform this work or you looking to DIY this?

  9. #9
    Join Date
    Feb 2004
    Posts
    110
    I am hiring a pro, however I have had several to the house for a quote and each one has a different approach and they are not 100% sure. This is why I am trying get a consensus on the best approach.

  10. #10
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    With vairable speed, I' wouldnt use a bypass. I'd instead either make sure the smallest zone can handle at least 300CFM/ton or the lowest airflow the modulating furnace will run at use a system that will slow the blower when only 1 stage is calling and/or leave the othe zone damper cracked slightly open maybe even just 10% to bleed some airflow into those zones.

    Careful using a modulating furnace with a zone system. Not ever mod is created equal and some have certain operating quirks in terms of controlling low fire with zoning. I wouldn't use a mod with a zone system unless it was communicating with that furnace, not just using W or W2 calls for heat.

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