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  1. #1
    Join Date
    Jan 2005
    Location
    Bakersfield, CA
    Posts
    209
    Forum participant Panama stated on another post:

    "Manual S has a graph in Appendix 4 that shows negligible loss in efficiency if a modern furnace (AFUE > 75%) is oversized up to 5 times the heating load"

    Participant beenthere stated he did not believe the statement was reliable (or words to that effect).

    I'm faced with having to use a much larger than necessary furnace to handle the air on a 5 ton AC unit. If the above reference is true then I would feel better about having to oversize the furnace (on the other hand it really doesn't matter how I feel about it!)

    What are the opinions about the above Manual S reference given the professional participants' experience?

  2. #2
    Join Date
    Jan 2005
    Location
    Bakersfield, CA
    Posts
    209
    Let me clarify. If the Manual S reference is correct then an oversized furnace may not lose much efficiency, but the fact that it is too large will still mean that it will run for shorter run times, correct? Also, how would a variable speed furnace help to offset the oversizing?



    [Edited by nathan9999 on 06-14-2005 at 09:37 AM]

  3. #3
    Join Date
    May 2002
    Location
    Colorado
    Posts
    697
    Manual S, Appendix 4 states, "These curves are based on research that was published by the Department of Energy and the National Bureau of Standards."

    A furnace that is too large will have shorter run times even at design temperature. Short run times do affect comfort. A two-stage furnace helps to solve this problem.

    A furnace with a variable speed blower won't help unless it is linked to a two-stage gas valve.

  4. #4
    Join Date
    Jan 2005
    Location
    Bakersfield, CA
    Posts
    209
    Thanks, Panama. I am currently evaluating a Trane XV90 or XV80. Both are 2 stage and variable speed. If I have to go with a large unit to handle a 5 ton AC unit then either one will apparently be way too large (load calc shows heat loss of 33,464 BTU/H.

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