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  1. #1
    Join Date
    Jan 2005
    Location
    Bakersfield, CA
    Posts
    209
    I am building a new home and have decided on the HVAC setup (based significantly on information gleaned from this forum). Either a Trane XL19i or XL16i (5 ton) and either a VS90 or VS80 in a zoned system.

    It looks as though the furnace will be oversized in order to match the capacity of the AC unit. I have been advised that I will probably need a 5 ton furnace to go with the 5 ton AC unit. I have not yet decided on 80+ or 90+% but will definitely have VS.

    My question has to do with the benefit of VS with an oversized furnace. How much will VS help out on the oversizing issue? How low (RPM) will the VS operate? Does this have to do with the set up by the installer?

    My last question has to do with furnace efficiency. Since the furnace will be oversized would it be better to go with the 80% furnace so that it will not heat the home too quickly, thus leading to overcycling?

    Thanks very much.

  2. #2
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,743
    Get a 2 stage 90+ furnace.

    Weather your blower is VS, or PSC, the furnace still needs the same amount of air.

    You can't go wrong with a VS blower though.


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  3. #3
    Join Date
    Nov 2002
    Location
    VA
    Posts
    296
    nathan9999;

    Glad to hear that this forum has proven to be a resource for you. beenthere has shared meaningful input... "You can't go wrong with a VS blower though” Other than higher initial investment, a variable speed blower helps lower operating cost (DC motor pulls less current), provides greater ability of remove latent heat and yields quieter operation with better temperature leveling throughout the structure. That said you mentioned a possibilities of a Zone system, which could have even greater impact on your total comfort level (highly dependent on the unique requirements of your home). BTW: ACCA’s Manual S quotes studies by the US Department of Energy which show clearly that fossil fuel furnaces can be oversized up to 100% without a major lose in efficiency (that doesn’t however mean they should since there are other variables in the equation besides efficiency)

    The industry “best practices” answers to your questions are actually found (and very thoughtfully documented) in the Manual S publication from ACCA. To truly appreciate Manual S, and its implications you may also need to review ACCA’s Manual J and Manual D. This trilogy once mastered will put all the complexities of your query in clear and proper perspective. The good news is these publications are available through your local Library system via inter-library loan. Please do invest the several weeks necessary to come up the curve in the aforementioned publications. Failing to do this places you at risk of falling prey to undocumented and miss-guided opinion, which unfortunately is all to present in the residential arena.

    Good luck and God Bless

  4. #4
    Join Date
    Jun 2001
    Location
    Louisville, KY
    Posts
    12,145
    Originally posted by nathan9999
    It looks as though the furnace will be oversized in order to match the capacity of the AC unit. I have been advised that I will probably need a 5 ton furnace to go with the 5 ton AC unit. I have not yet decided on 80+ or 90+% but will definitely have VS.
    The XV90 4-ton drive 100 KBTUH (TUY100R9V4W) will deliver 1770 CFM @ 0.5 IWC ESP. In your terms, it will deliver enough air for a 5-ton AC. Additionally, with a zoned system and 2-stage cooling, you will likely only see 2nd stage if recovering from a setback period or a really, really hot day. That furnace will deliver plenty of air.

    Your contractor needs to read and understand the product data that is readily available from his distributor. There's not a need to grossly oversize the furnace to achieve the desired cooling CFM.

  5. #5
    Join Date
    Jan 2005
    Location
    Bakersfield, CA
    Posts
    209
    Thanks for all the responses.

    How about the 80 vs. 90 furnace issue? My understanding is that the 90 furnace will put out more heat at the same stage than an 80 furnace. If this is the case, then it seems as though a 90 furnace will run for less time per cycle, thus leading to more cycling and less filtration (through the Aprilaire 2400 in my case).

    If this is the case would I be better off with an 80 instead of a 90 furnace, just as far as comfort and air quality are concerned (not necessarily energy efficiency)?

    Just as an aside the Trane website application that configures optimum systems selected an 80% furnace for my location (mild winters) even with their most expensive option. So given my location a 90% furnace may not be warranted.

    Faith, I have performed a Manual J with the software linked from this site. I found that process very educational and informative. I am fortunate that I have a good local contractor who is willing to discuss options and alternatives.

    jrbenny, I spoke with another contractor who mentioned using a 4 ton furnace but warned me about problems of mismatched equipment. Would the ARI website confirm that a 4 ton furnace would be a match for a 5 ton AC unit? Would a 4 ton XV80 also work?

    Thanks again for all the responses.

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