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Thread: BTU Question

  1. #14
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    Thread Starter
    Great, that is an important consideration. Thank you.

    Now I will have to determine the capacity(s) for the existing ducts to see if they will allow for this.

  2. #15
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    Well you are sturbon!

  3. #16
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    Thread Starter
    I really do not mind if it is "inefficient" as long as it will do what I want and not damage anything in the process.

  4. #17
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    The vast majority of duct systems are undersized and the vast majority of equipment is oversized.
    Homeowners frequently post here asking why their new furnace is so noisy compared to the old one.
    Do not indiscriminately upsize your furnace without first determining how much air flow your duct system can deliver. Then decide wether you will upsize the furnace or upgrade the ducts.
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    Free online load calculator: http://www.loadcalc.net/


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  5. #18
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    Negatives of oversizing:
    (No matter if system is single stage, two stage or modulating)
    1. Noise
    2. Loss of efficiency
    3. Loss of typical life of components
    4. Uneven temperatures thru out house
    5. Cost of resizing ductwork to accommodate the 33% increase in size, your unit is 60k next size is 80k which will require a minimum of 33% larger ductwork assuming it is even sized for your current size unit. Most duct systems are undersized by a minimum of 10% for even current sized systems.

    Pros of oversizing:
    1. Quick warm up of house from setback
    2. Know when system is running from the wind tunnel noise
    3. Get regular repairs on your system, so you always have newer parts in your unit
    4. Always know your contractor by his first name and company on speed dial.
    5. Mandatory all new shiny ductwork
    6. Contribute more money to this economy recovery due to the increase in cost of new system and ductwork

    The proper size is the smallest size that load calculation states will maintain your indoor temp at average lowest temperature outside. This promotes lowest noise, highest comfort, best energy costs, less likely the need for all new ductwork modifications, and the longest life of the system components.

    But go ahead and request a larger than needed system, your contractor and the economy will thank you.

    J

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  7. #19
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    Quote Originally Posted by Gray01 View Post
    Our house (2300 square foot, built 2007) is currently heated with a Carrier 58MVB. The placard shows input of 60k High and 39K Low BTU, and output of 56K High and 36K low BTU. (I am assuming that this means that the furnace was configured as a 60K BTU model.)

    The house is well insulated, and has double-pane vinyl-framed windows.

    We live right at a Climate Zone 4 and 5 line. (Last night was 15F).

    We are upgrading our equipment to a Bryant 987M configured suite, including full communicating controls.

    My question is:

    1. Is there any negative attribute to specifying an increase in BTU rating for the furnace?

    During heating season, we turn the furnace down about 8 degrees F at night/sleeping), and I was (possibly quite erroneously) wondering if additional BTU's would speed morning temperature recovery time.
    Increasing furnace size to 10,000 BTUs more output will require a 20% increase in air flow. Increasing furnace size to 20,000 BTUs more output will require 40% more air flow.

    A larger furnace on your current duct system will cause the furnace to lock out on its high limit during recovery from night set back. Causing it to take longer to heat the house back up. The higher air flow requirement will also cause your current duct system to be loud at the registers.
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  8. #20
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    Quote Originally Posted by Gray01 View Post
    Great, that is an important consideration. Thank you.

    Now I will have to determine the capacity(s) for the existing ducts to see if they will allow for this.
    What size is Your A/C system?
    Designer Dan
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    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  9. #21
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    No, not the secondary, the primary. The secondary is designed for the moisture, the primary is not because the unit should run long enough to reach a temperature where it is exhausted. Over sized units don’t run long enough especially in moderate weather. To make a primary to handle the conditions of an over sized unit would be prohibitive from a cost standpoint. Why would a manufacturer make something cost prohibitive that is working outside their design parameters.

    Quote Originally Posted by Gray01 View Post
    So what you are alluding to is that there is an increase in carbonic acid residue in the secondary heat exchanger?

    If so, I though that the condensate (containing carbonic acid, ie. hydrolyzed carbon dioxide) drained out via the condensate drains. Does that not happen because of higher heat production?

  10. #22
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    Nov 2009
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    Moscow, ID
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    Thread Starter
    beenthere,

    Thank you for the information regarding CFM/duct sizing. I will be checking our existing ductwork. (Based upon the existing poor workmanship I will be extremely surprised if any calcs were performed to design any sizing for the system.)

    dan sw fl,

    The A/C is 2.5 tons single speed. I am considering increasing that to 3 ton double or variable speed to obtain some surge capacity. We have zero humidity issues in this locale, and it is not "hot" in comparison to the east and south. We do have times when it will run near 100f for a few weeks. During that time period, and also in the 90's with guests attending, the current system falls behind until outside temps moderate around 11-12pm.

    I do understand the need for proper duct sizing and CFM to accomplish our desired performance.

  11. #23
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    Nov 2009
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    Moscow, ID
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    Thread Starter
    Advanced Response,

    So what is seems that you are saying that there is no practical way with the present system to provide the type of comfort that we would like: rapid heating (and/or cooling) to a specified temp, and then unobtrusive maintenance of that temperature unless the user determines otherwise. (Sort of like a rapid climb on takeoff until cruising altitude is achieved, and then a comfortable ride for the flight duration.)

    Loss of efficiency is immaterial to me as a primary attribute; comfort upon demand is the goal. Based upon the other comments, I might have to have the entire duct system redesigned and retrofit if it cannot accommodate my desired performance characteristics. If that is what is required, I will probably use an architect to retain an ASHRAE engineer to properly design and supervise the job.

  12. #24
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    Anything can be done if that is what you want for a cost.

    With what you want to achieve, it will require all the ductwork to be enlarged and possibly additional duct run and outlet to each room with an enlarged return and possibly additional returns inlets installed.

    I am not sure if this is a 1 story with equipment in basement with open basement where all ductwork can be as easily as possible replaced and enlarged.

    Or

    Is the home a 2 story with equipment in finished basement where walls leading to second story and the basement ceiling will need to be opened up to enlarge and add ductwork to achieve your goal.

    I hope cost is of no object for you also, what you want can be done but will most likely at least cost you 2 times the cost of a normal replacement system as all the ductwork will need to be replaced. And if walls and ceilings need to be R&R by the time the job is said and done could cost 4-5 tome the cost of a normal replacement system.

    It all depends on how much you are willing to spend on your goal. Heed this warning, do not go cheap on this project if this is what you want, I see systems all the time that can’t even perform the normal duties required, let alone imagining a system that needs to be designed for the unusual goal that you want to achieve.

    Be careful on the can of worms that you may be opening with this adventure, especially if your current system seems to keep you comfortable except for the long warmup after an aggressive setback.

    Please post pictures of your progress and the end result, we enjoy success stories and need a good laugh when the $h!? Hits the fan.

    Good luck,
    J

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