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Thread: Help me understand 2 stage furnace?

  1. #1
    DIY Senior Member Nate R's Avatar
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    Default Help me understand 2 stage furnace?

    Looking at a Goodman GMH8 to replace my GMS8. These are both 80% efficient furnaces. The GMS will go in the garage. The GMH is properly sized for my home (smaller than my existing GMS) and a 2 stage burner.
    Looked at all the manuals, install guides, etc I can find. Called Goodman, but consumer affairs was of no help, and they wouldn't let me talk to the technical side, since I'm not a licensed HVAC tech. The only mention of operation/settings for the 2 stages is this:


    What I'm trying to understand is this: When the DIP switches are set on 2 stage, and on the Auto setting of 1-12 minutes, what does the furnace use to decide whether to switch to the high stage? Since it's designed for a single stage Thermostat, it can't be the stat calling for 2nd stage.

    Does the furnace look at the time between the last cycle and current cycle? (Shorter time means more heat loss, so it runs less on the first stage)

    Hopefully someone here knows how it's done. Thanks!

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    Default

    I think it works like this...you can set the thing to automatically switch to high output after a fixed interval (if it needs to run that long), or in auto, it will decide how long to run in low and then switch to high based on history. If it is really cold, and it needs to restart often, but then runs only a moment on high to satisfy the needs, it then may decide if it ran a little longer on low the next time, it would be more efficient, and run longer on low.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    Plumber jimbo's Avatar
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    Default

    Here is some additional information cut and pasted from the Goodman Service Manual for that furnace:


    NOTE:
    A single-stage thermostat with only one heating
    stage can be used to control a two-stage furnace. The application
    of a single-stage thermostat does not offer "true"
    thermostat driven two-stage operation, but provides a
    timed

    transition from low to high fire. The furnace will run on low
    stage for a fixed period of time before stepping up to high
    stage to satisfy the thermostat's call for heat. The delay
    period prior to stepping up can be set at either 5 or 10 minutes
    through the DIP switch adjacent to the Heat Off delay
    DIP switches on the integrated control module. To use a
    single-stage thermostat, turn off power to the furnace, move
    the thermostat selection jumper on the integrated contro
    module from the "two-stage" position to the "single-stage"
    position, turn power back on. Refer to the following figures.
    3
    2
    1
    TWO
    SINGLE
    TSTAT
    OFF
    3
    2
    1

    T-Stat selection jumper in
    single-stage thermostat
    position.
    DIP switch position 3: ON
    Delay Period: 10 minutes.
    DIP switch position 3: OFF
    Delay Period: 5 minutes.
    TWO
    SINGLE
    TSTAT
    ON
    NOTE: The AMH, GCH, GDH, GMH and GME hybrid furnaces
    require
    a single stage thermostat. These furnaces
    utilize a control board with single stage or two stage capability.
    The second stage feature is established by setting a
    dip switch on the board, either one stage or two stage. All of
    the second stage timing is then controlled by dipswitch 3 on

    the circuit board.

    Single-Stage Thermostat Application - 80% and 90%
    Variable Speed Two-Stage Furnaces
    A single-stage thermostat with only one heating stage can
    be used to control this furnace. The application of a singlestage
    thermostat does not offer “true” thermostat-driven twostage
    operation, but provides a
    timed transition from low to
    high stage. The furnace will run on low stage for a fixed
    period of time before stepping up to high stage to satisfy the
    thermostat’s call for heat. The delay period prior to stepping
    up can be set at either 5 or 10 minutes through the DIP
    switch adjacent to the Heat Off delay DIP switches on the
    integrated control module. To use a single-stage thermostat,
    turn off power to the furnace, move the thermostat selection
    jumper on the integrated control module from the “twostage”
    position to the “single-stage” position, turn power back
    on. Refer to the following figures.

    3
    2
    1
    TWO
    SINGLE
    TSTAT
    OFF
    3
    2
    1

    T-Stat selection jumper in
    single-stage thermostat
    position.
    DIP switch position 3: ON
    Delay Period: 10 minutes.
    DIP switch position 3: OFF
    Delay Period: 5 minutes.




  4. #4
    DIY Senior Member Nate R's Avatar
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    Default

    This is for the GMH8, jimbo? Have they changed the board w/ a newer model maybe? What you posted looks different than what I've seen on the Goodman website as far as what the options are for the two-stage setup.

  5. #5
    Plumber jimbo's Avatar
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    Yes, but perhaps I muddied the waters by just clipping a few paragraphs. The service manual covers a multitude of models, including all GMH. There are several different operating sequences depending on what model of ignition module you have. The manual is 130 pages, so I can't post the whole thing. It is on a secure website, so I can't link it. If you can read the model # off the ignition module, I could post or scan a few more specific pages. Perhaps Goodman would send it to you. If you know a service tech, he will have it, or his company will have access to it on the website.

    The igintion modules mentioned are White Rogers models: 50A55,50A65,50V51,50V61,50M56
    Last edited by jimbo; 02-27-2008 at 07:02 AM. Reason: add

  6. #6
    DIY Senior Member Nate R's Avatar
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    Ah, searched for the service manual and got a hold of it.
    It's a 50M56 module in the furnace I'd like, I think.

    Unless I missed something, the service manual tells me nothing more than when the high heat delay is reached, it switches to high heat. No explanation of how it works. I suppose that's because there's no need for a tech to know/deal with it?

    Either way, can anyone confirm jadnashua's explanation?

    The White-Rodgers site doesn't give consumers any real info, either.

  7. #7
    Plumber jimbo's Avatar
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    PRODUCT DESIGN
    73
    NORMAL SEQUENCE OF OPERATION
    Goodman Two-Stage 80% or 90% Furnaces
    White-Rodgers
    50M56 Integrated Ignition Control

    P
    OWER UP

    The normal power up sequence is as follows:
    •
    115 VAC power applied to furnace.

    •
    Integrated control module performs internal checks.

    •
    Integrated control module LED will light.

    •
    Integrated control module monitors safety circuits continuously.

    •
    Furnace awaits call from thermostat.

    H
    EATING MODE

    (M
    ODE DIP SWITCH IS SET TO “1 STG” POSITION)

    The normal operational sequence in heating mode is as follows:
    •
    R and W thermostat contacts close, initiating a call for
    heat.

    •
    Integrated control module performs safety circuit checks.

    •
    Induced draft blower is energized for 15 second prepurge
    period causing pressure switch contacts to close.

    •
    Igniter warm up begins after 15 second prepurge expires.

    •
    Low and high stage gas valves open at end of igniter
    warm up period, delivering gas to burners and establishing
    flame.

    •
    Integrated control module monitors flame presence. Gas
    valve will remain open only if flame is detected.

    •
    Circulator blower is energized on high heat speed following
    a fixed thirty second blower on delay. Electronic
    air cleaner terminals are energized with circulator blower.

    •
    Furnace operates; integrated control module monitors
    safety circuits continuously.

    •
    R and W thermostat contacts open, completing the call
    for heat.

    •
    Gas valve closes, extinguishing flame.

    •
    Induced draft blower is de-energized following a fifteen
    second post purge.

    •
    The circulator blower remains at high heat speed for thirty
    seconds. The circulator blower then switches to low heat
    speed for the remainder of the selected heat off delay
    period. For example, the selected heat off delay period
    is 150 seconds. The circulator blower operates at high
    heat for 30 seconds and at low speed for 150 - 30 = 120
    seconds.

    •
    Furnace awaits the next call from thermostat.

    (M
    ODE DIP SWITCH IS SET TO “2 STG” POSITION)

    The normal operational sequence in sequence is as follows:
    •
    R and W thermostat contacts close, initiating a call
    for heat.

    •
    Integrated control module performs safety circuit
    checks.

    •
    Induced draft blower is energized for 15 second
    prepurge period causing pressure switch contacts to
    close.

    •
    Igniter warm up begins after 15 second prepurge expires.

    •
    Low and high-stage gas valves open at end of igniter
    warm up period, delivering gas to burners and establishing
    flame.

    •
    High-stage gas valve closes after five seconds; lowstage
    gas valve remains open.

    •
    Integrated control module monitors flame presence.
    Gas valve will remain open only if flame is detected.

    •
    Circulator blower is energized on low heat speed following
    a fixed thirty second blower on delay. Electronic
    air cleaner terminals are energized with circulator
    blower.

    •
    Furnace is now operating in low-stage heating mode.

    •
    Furnace operates; integrated control module monitors
    safety circuits continuously.

    •
    If low-stage delay period expires, control will shift operation
    from low-stage heating mode operation to highstage
    heating mode operation. Control will energize circulator
    blower high heat speed and high stage gas valve.

    •
    Furnace is now operating in high-stage heating mode.

    •
    R and W thermostat contacts open, completing the
    call for heat.

    •
    Induced draft blower is de-energized following a fifteen
    second post purge.

    •
    Circulator blower is de-energized following a heat off
    delay period (selectable 100 or 150 seconds; factory
    set at 150 seconds).
    If the furnace is operating in the low-stage heating
    mode when thermostat contacts open, circulator remains
    at low heat speed for the selected delay off period.
    If the furnace is operating in high-stage heating mode
    when the thermostat contacts open, the circulator blower
    remains at high heat speed for thirty seconds. The circulator
    blower then switches to low heat speed for the
    remainder of the selected heat off delay period. For
    example, the selected heat off delay period is 150 seconds.
    The circulator blower operates at high heat for 30
    seconds and at low speed for 150 - 30 = 120 seconds.

    • Furnace awaits the next call from thermostat.

  8. #8
    DIY Senior Member Nate R's Avatar
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    Sure, but it still doesn't explain what determines when the low-stage delay period expires if it's set on Auto 1-12 minutes instead of the fixed 5 minute setting.

  9. #9
    DIY Junior Member therinnaiguy's Avatar
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    Default Are you sure?

    Goodman furnaces are OK for a cheap furnace. Prior to purchasing, research all the help sites that deal with HVAC questions. I do and have seen a greater number of Goodman questions and issues compared to American Sandard, Trane and Carrier brand furnaced. 10 to 1 or better in fact. Also, it is not recommended to put a furnace in an unconditioned space. This is a condensing furnace and one of the products of combustion is water. No one else picked up on this? Do a little more research on furnaces to see which ones can be put in a cold location.

  10. #10
    DIY Senior Member Nate R's Avatar
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    Both the new and old furnace are 80% efficient. I don't think the new one is condensing. My old furnace, an 80% efficient unit that is DEFINITELY not a condensing unit is going in the unconditioned space (my garage)

    Also, I would think Goodman would get tons more questions because it's one of the few brands a consumer can get themselves. I haven't seen other brands for sale available to me.

    And as you said, they're OK for a cheap furnace. I'm fine w/ that. I have a cheap house. If my house was/could be worth something, I *might* consider spending more.

  11. #11
    Plumber jimbo's Avatar
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    Goodman does not allow their distributors to sell directly to consumers. There are ways to get around that, as you found out. But that is why Goodman tech only talks to distributors. Depending on where you bought it, those folks have the ability to get a hold of a tech at Goodman and ask questions.

  12. #12

    Default Nate R,

    The lack of description of the 'rules' for shifting to high speed maybe due to changes in software after the manual was written. Or someone may have decided that by not publishing the "rules" there will be fewer warranty calls on the subject.
    Carrier has some "based on it's algorytm" clauses. If they get complaints they may change the software on future releases.

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