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  1. #1
    Join Date
    Sep 2005
    Location
    NJ
    Posts
    416
    My question is how does the furnace know when to go to the 2nd stage.
    I seen on the trane website if the temp. drops 3 degrees below the thermostat setting the furnace goes to 2nd stage heat. Is that the same for all furnaces or is everyone different.


    Also do outdoor temp. sensors have to be installed with a 2 stage furnace.

  2. #2
    The answer is ... it all depends.
    It depends on how it is installed. Many times a two stage furnace will be installed with a single stage thermostat. When that is done a timer in the board (or on some furnaces exterior to the board) cliks off about ten minutes between the stages as long as there is a continous call for heat. Sometimes a two stage thermostat is used and then when the temperature in the house drops a degree or two from set point the second stage will kick in.
    My favorite way is to use a two stage thermostat and run it through an outdoor thermostat to keep the second stage from kicking in above say 45 - 50 degrees.

    Any way that it is set up, proper performance depends on proper furnace sizing and proper installation

  3. #3
    Join Date
    Jul 2005
    Location
    Wilmington, DE
    Posts
    355
    its not the furnace its the stat unless someone uses a singel stage stat on a two stage furnace. what you have said is usually one of the cases you have listed but also some if the furnace doesnt satisfy in a certain time period to the set temp then it also will fire in the second stage because that wouldnt be very efficiant if the furnace ran untill it dropped 3 degrees in the house then decided to to fire up the 2nd stage then some drop down to low fire right before the setpoint is reached

  4. #4
    Join Date
    Feb 2006
    Location
    Columbus, Ohio
    Posts
    132
    Some furnaces have dip switches that allow the furnace to operate as two-stage with single stage stat. Outdoor stat is not required with 2-stage unless it is dual fuel or thermidistat.

  5. #5
    Join Date
    Dec 2002
    Location
    Rochester, MN
    Posts
    5,304
    3 degree is pretty extrem. I have a Trane XV90 w/ a VP t-stat, and it cycled 2nd stage every now and then, but the temp did not drop 3 degrees.


    I think the 3 degrees depends on what t-stat is used.

    If you do go two stage, DO NOT let the dealer talk you useing the single stage stat.

  6. #6
    Join Date
    Aug 2002
    Posts
    1,647
    I believe most digital 2-stage thermostats look at the temp in 10ths of a degree and the time it takes to move each 10th of a degree and if it is loosing ground with differant algorithms to determine when 2-stage is needed.. That is how my thermidastat operates and also how my dad's honeywell operates, only may see a drop of one degree sometimes but most times they kick into high stage without seeing a drop..

    Old mercury t-stats had to have a temp drop before 2-stage would initiate.. But with these high tech digitals and the use of algorithms they can operate to a closer margin..

    JMHO
    J

  7. #7
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,273
    Originally posted by k c iceman
    The answer is ... it all depends.

    My favorite way is to use a two stage thermostat and run it through an outdoor thermostat to keep the second stage from kicking in above say 45 - 50 degrees.
    It seems like one may have significant sizing issues,
    if the second stage is actually needed > 45'F
    (~40% of design capacity @ design temp of 10'F).

    Set point-Current O.A. Temp/(Set Point- Design Temp)
    70'-45'/(70'-10')=
    25'/60' = ~40%

    Or
    70'-45'/(70'-20')=
    25'/50' = 50%
    makes 45'F lock-out seem a bit more reasonable
    in a more mild climate.

    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

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