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  1. #1
    Join Date
    Sep 2008
    Location
    Portland
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    43

    Variable speed blower in inadequate ductwork

    Greetings all - I'm researching the replacement of my ancient forced-air oil furnace with a new high-efficiency gas model with variable speed blower and 2-stage burner. No AC.

    The house is a smallish cape cod style, 850sf main, 250sf single-room finished attic and another 250 semi-finished basement. Portland OR, mild climate.

    When the attic was finished, it was supplied with a single duct and no return (although there is an open stairwell which develops a good draft when the furnace is running). Even so, I have to keep the upstairs register partially closed throughout the winter to keep from overheating the attic room (where we sleep).

    The question is this - how might the change from the old blower to the new VS blower impact the conditioning of the attic room? If the present, presumably inadequate supply is "more than enough" heat, will the new, (presumably higher-volume? or longer run times at lower volume?) blower reduce or increase this effect?

    Anyone care to make educated predictions?

    And before you ask, adding additional ductwork to the attic doesn't seem like a viable option, there's just nowhere to put it without major reno work.

    Thanks for any input.

  2. #2
    Join Date
    Sep 2002
    Location
    I don't know
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    2,897
    Insufficient info provided.
    General public's attitude towards our energy predicament: "I reject the reality of finite resource depletion and substitute it with my own; energy is infinite, we just need an alternative storage medium to run the cars on. The economy can grow indefinitely - we just need to "green" everything! Technology is energy! Peak what?"

  3. #3
    Join Date
    Nov 2004
    Location
    SW FL
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    5,994
    Many in NW use heat pumps.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  4. #4
    Join Date
    Jul 2006
    Location
    Virginia
    Posts
    4,512
    a varible speed blower will not resolve the issue

  5. #5
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    A VS blower on undersized ductwork can actually even use MORE electricity than a standard blower motor. VS blowers don't solve ductwork problems. For what it matters, larger systems don't solve them either, they actually make them worse.

    You likely won't see a payback on a HE furnace in your climate and with such a small house. You'd better off wiht dual fuel and another 80% furnace from an economic standpoint. The only other advantage to a HE unit is the sealed combustion. But I'm guessing you probably have a natural draft hot water heater right now anyway.

    I'd put your money into trying to improve the ductwork and adding some insulation and sealing up doors, windows and other air leaks.

  6. #6
    Join Date
    Jun 2011
    Location
    Wake Forest, NC
    Posts
    352
    If you have under sized ductwork, changing the size of the unit will not help. It will damage the equipment and cost much more in the long run. Work on finding a cost effective solution on sizing the ductwork properly before replacing the system and possibly damaging it.
    It's not rocket-science...

    It's electromechanical thermodynamic engineering

  7. #7
    Join Date
    Dec 2002
    Location
    Houston,Tx.
    Posts
    15,506
    Quote Originally Posted by HVACTechNC View Post
    If you have under sized ductwork, changing the size of the unit will not help. It will damage the equipment and cost much more in the long run. Work on finding a cost effective solution on sizing the ductwork properly before replacing the system and possibly damaging it.
    Your are correct! I don't care if you have every manual A,B,C,D,E,F,G,H,I,J,K,L,M,N, etc. there is performed on your system, if the duct is not correct, your are wasting your hard earned $$$$$$. Who would build a 500 hp. engine and put a one barrel carb. on it? no one! This is what you would be doing investing many $$$$$$ in a system, and not addressing the duct properly.
    __________________________________________________ ____________________________
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  8. #8
    Join Date
    Jun 2011
    Location
    Wake Forest, NC
    Posts
    352
    Quote Originally Posted by Mr Bill View Post
    Your are correct! I don't care if you have every manual A,B,C,D,E,F,G,H,I,J,K,L,M,N, etc. there is performed on your system, if the duct is not correct, your are wasting your hard earned $$$$$$. Who would build a 500 hp. engine and put a one barrel carb. on it? no one! This is what you would be doing investing many $$$$$$ in a system, and not addressing the duct properly.
    Great analogy with the 500HP engine and a one barrel carb! I will have to use that sometime. Great stuff...
    It's not rocket-science...

    It's electromechanical thermodynamic engineering

  9. #9
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,086
    Quote Originally Posted by Mr Bill View Post
    Your are correct! I don't care if you have every manual A,B,C,D,E,F,G,H,I,J,K,L,M,N, etc. there is performed on your system, if the duct is not correct, your are wasting your hard earned $$$$$$.
    Manual D would be handy for knowing whether the ducting is correct or not. Just sayin'...
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  10. #10
    Join Date
    Nov 2006
    Location
    Southeastern Pa
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    17,014
    That poor vs motor will try and try and try to overcome the insufficient ducts, and it will kill the motor after it uses a lot of kwh in the attempt.

    Fix the ducts first, my friend.
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  11. #11
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    I think some can be astonished at what jsut a small change in ductwork to increase airlfow or reduce static pressure can do for a system. Reduced noise is the most immediate change in many cases. I recently added an additonal supply to our master bedroom along with a couple dampers and I was finally able to properly balance the smaller rooms and overall noise levels dropped in half.

    Now I'll be in great shape for the future when I get the equipment downsized to it's correct capcity for the the part of the home.


    Yup, start with the basics (airflow) after that, figure out what size you need then just pick whatever contractor you think will do the best installtion and proide the equipment that suits you.

    VS motors aren't a :magic bullet". Just a better motor.

  12. #12
    Join Date
    Feb 2009
    Location
    Portland OR
    Posts
    1,845
    I am pretty sure I went to your house to give you a proposal on the furnace so I have actually seen this one. Here is a little more information and what my suggestions were...

    I gave proposals for Ruud and Trane 95% efficient two stage variable speed gas furnaces and a Trane XC95Modulating 60 stage communicating furnace. The house currently has oil so we are making some ductwork changes in the basement but there is enough ductwork to accomodate the blower according to my CFM's and the ductulator I used. However the problem is the duct to the usptairs, it is 2.5 floors above the basement and NO closets line up to get additional ductwork or an RA up there. The OP is currently using a 12K BTU air conditioner for the summer time and I could not find where the trunk is to be able to damper it down without cutting into ductwork(which if we are selected to do the install we will do).

    The OP's questions is how will the VS blower affect the overheated room and somehow you have all gone on a tangent of the VS blower working to hard... that is not true as there is ductwork that is capable of the blower capacity on the other floors of the house.

    IMO the VS blower is the only way to go, getting longer run times and since the RA is at the bottom of the stairs may get some pull from the hot room upstairs and if we do the installation we will damper off or slow down that ductwork. Hope this helps and to the Tad, If you have any additional questions we can discuss the options on the phone, you should have my number

    -Travis

    We also looked at ductless but it was not cost effective.
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  13. #13
    Join Date
    Sep 2008
    Location
    Portland
    Posts
    43
    Hi all - thanks for the replies. Yes Travis you got me pegged (probably not too many Tad's on your roster, and I did admit to finding you here to begin with!). No offense intended to you or your expertise but I was hoping to tap into the usual variety of opinions found here on the forum.

    I apologize if I unclear earlier. As far as I know (and Travis above confirms) that the overall ductwork capacity is fine... the question is specifically about this one attic room with the single duct, and how the VS blower will impact it's behavior.

    I'm not trying to address any particular problem (other than getting rid of the old crappy oil furnace) with the VS. However I was under the impression that modern HE blowers move more air than the older units, so I wondered how that plus the probable extended run times (especially in comparison to the hot-and-short runtimes of my probably-oversized oil furnace) might change the overall dynamic.

    To address some of the comments - I'd love to have a heat pump, but the upfront costs are quite a bit higher, payback time is long, and one of the primary advantages (central air) is negated by the lack of adequate ductwork to the room that really needs it(the attic room).

    Motoguy128 - I'm interested in your comment about lack of payback from a HE furnace in this climate. You're right that I like the idea of sealed combustion, and I do not have a natural draft water heater at the moment (electric), although I'm considering changing to a power-vent gas unit when we bring gas into the house.

    We've already changed windows and doors and will be adding insulation in the roof and walls before winter.

    So... assuming that the overall duct capacity is okay, how does that change everyone's answers? What additional info could I provide to get better answers?

    Thanks again for your thoughts.

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