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  1. #1
    Join Date
    Aug 2005
    Posts
    40
    I need some guidance on what to do with existing HVAC. I don't know if I should get the existing duct system sized correctly to support my current system or put in properly sized equipment. Some background.. I have run the HVAC-CALC load calculation program for my house and from my results the equipment is way over (especially heat) sized. Results were:
    sensible gain - 22,116
    latent gain - 3109
    heat loss - 34,737

    My existing system is the following (all Amana installed in '97):
    GDC090x40b (87K BTUH output)
    RCC36 (3 Ton 12 SEER)
    CHH36TCD (horiz coil)

    From my previous post and member responses, I belive my duct work is minimum at best for supporting my 3 ton. Note: my original system was a 2.5 ton heat pump. Currently in heat mode, the fan speed is set to medium and from looking at the Amana spec sheet (assuming 0.5 ESP) thats around 1657 CFMs. There is a lower speed setting that could be used that keeps the temperature rise in spec and that would be around 1386 CFMs (medium-low). The low speed which is where my AC runs doesn't look like that would be safe because of the temperature rise would be close to maximum.

    Questions/Concerns:
    1. Should I take out the existing equipment and downsize or upgrade the duct work to support existing equipment?
    2. How can I check and verify the temperature rise to ensure its within spec?

    Comment: this is a great site and I wish I had known about it back in 1997 when my system was replace. No the contractor didn't do any load calculation or Manual D and that why I'm trying to learn as much as I can now.

    I do have one other question and this has to do with the HVAC-CALC program. Why doesn't this program add any latent gain for duct loss? When I use the Excel MJ8 spreadsheet that comes with the book, it added about 620.

  2. #2
    Join Date
    Jul 2001
    Location
    Gold Coast of Connecticut
    Posts
    4,656
    How does the cooling perform? Does it cycle on the warmest days?

    I think i would upgrade the ductwork to get the most you can out of your current system. If the ductwork is undersized you should increase your blower speed.
    Aire Serv of SW Connecticut- Gas heat, dual fuel and central a/c systems installed and serviced

  3. #3
    Join Date
    Aug 2005
    Posts
    40
    Freezeking

    Thanks for reponding. Yes, the AC unit does cycle even when we had 95 degree days. Although it was cycling too often when I had that cheap digital round stat from Honeywell. I had a Honeywell Visionpro installed and it has the adjustable cycle rate for cooling which the other stat didn't have, and this make thing much better. Looks to me like increasing the blower speed would make the heat cycles shorter because one would be delivery higher CFMs.

  4. #4
    Join Date
    Apr 2004
    Location
    Bartlett, IL
    Posts
    6,619
    Wow, you sure you did the calculation right?

    That seems like a wayyy oversized unit, hard to believe. It looks like it's more than double what you need.

    Where do you live? The only time I remember installing a 40K furnace was in a small apartment.

  5. #5
    Join Date
    Aug 2005
    Posts
    40
    2hot2coolme

    I'm pretty sure my load calculations are right. I live in the Charlotte area, single story brick home, 1714 sq ft, double pane windows, R19 on the floor and about 10" blown insulation in attic. If my last system was a 2.5 ton heat pump, what would the KW of the backup emergency heat strips? My guess would be 15KW but not sure? 15KW is about 51K BTUH.

  6. #6
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304

    What is limit of cycling

    >>How does the cooling perform? Does it cycle on the warmest days?

    FreezeKing, would you clarify? How much continuous run-time in your professional opinion, before one says "not cycling"? One full hour without stopping? Two? Three? Just trying to get my understanding in sync with the pros.

    Thanks much -- P.Student

  7. #7
    Join Date
    Sep 2001
    Location
    Vancouver Canada
    Posts
    996
    seems to be moving an awful lot of air 1657 cfm, for 2.5 ton should be down around 1000cfm. 1657 is enough air for a 4 ton. At that rate of CFM you are definetly going to have cool air coming out of that heat pump.
    "Go big or Go Home"

  8. #8
    Join Date
    Aug 2005
    Posts
    40
    Black Adder >> seems to be moving an awful lot of air 1657 cfm, for 2.5 ton should be down around 1000cfm. 1657 is enough air for a 4 ton. At that rate of CFM you are definetly going to have cool air coming out of that heat pump.

    My system is not a heat pump, its a gas furnace with a 3 ton AC. The Amana gas furnace supports up to a 4 ton AC unit and so there's 4 fan adjustment settings. Right now the fan setting for heat is set to medium (1657 @ 0.5 ESP)which I believe to be high to support my duct work. There are two lower fan settings (med-lw 1386 cfm @ 0.5 ESP and low 1173 @ 0.5 ESP). The problem with the low setting is the temperature rise would exceed the 35-65 deg spec. Unless the ESP is lower than 0.5. There are acceptable temp drops on the low fan speed if ESP is 0.3 or less.

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