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  1. #1

    Furnace Order of Operations please help

    Well, I couldn't find the "right" section that was public to ask this question so I'm asking here. I'm not 100% on the order of operations in a regular high efficiency furnace, and I was asked the question on an interview, and I'm not sure if I got it right. I know its the most basic question, so if anyone can help me out here, it'll be greatly appreciated. Basically starting at the thermostat call for heat, gas valve energizes ect, I want to make sure I know it and understand it. Thanks in advance.

  2. #2
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    If you go to a mfg's web site and download a copy of an install manual, the sequence of that unit should be included towards the back, good luck.
    See an attached example
    Attached Images Attached Images

  3. #3
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    Quote Originally Posted by hvac_apprentice View Post
    Basically starting at the thermostat call for heat, gas valve energizes ect, I want to make sure I know it and understand it.
    1. Thermostat - calls for heat

    2. Control Board internal/external check (pressure switch open ?)

    3. Inducer motor - ON

    4. Control Board - proves pressure switch is closed, limits and rollout closed

    5. Hot Surface Igniter - ON (warm up timer starts)

    6. Gas Valve - OPEN

    7. Control Board - Proves Flame

    8. Blower timer starts (approx. 45 sec)

    9. Blower - ON

    10. Thermostat - satisfied

    11. Burner - OFF (gas valve closed)

    12. Inducer motor -post purge (time varies)

    13. Blower - OFF(after time delay)
    Last edited by rundawg; 03-01-2010 at 07:26 PM. Reason: added info per bigsky input
    Instead of learning the tricks of the trade, learn the trade.

  4. #4
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    Quote Originally Posted by rundawg View Post
    1. Thermostat - calls for heat

    2. Control Board internal/external check (pressure switch open ?)

    3. Inducer motor - ON

    4. Control Board - proves pressure switch is closed

    5. Hot Surface Igniter - ON (warm up timer starts)

    6. Gas Valve - OPEN

    7. Control Board - Proves Flame

    8. Blower timer starts (approx. 45 sec)

    9. Blower - ON

    10. Thermostat - satisfied

    11. Burner - OFF (gas valve closed)

    12. Inducer motor -post purge (time varies)

    13. Blower - OFF(after time delay)

    You forgot it has to go through the limits & rollouts before the ignitor glows, part of the safety circuit with the pressure switch.

  5. #5
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    Quote Originally Posted by big sky hvac View Post
    You forgot it has to go through the limits & rollouts before the ignitor glows, part of the safety circuit with the pressure switch.
    I did, thanks.
    Instead of learning the tricks of the trade, learn the trade.

  6. #6
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    Quote Originally Posted by rundawg View Post
    I did, thanks.
    No problem

  7. #7
    thank you, that's exactly what I was looking for.

  8. #8
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    Quote Originally Posted by rundawg View Post
    1. Thermostat - calls for heat

    2. Control Board internal/external check (pressure switch open ?)

    3. Inducer motor - ON

    4. Control Board - proves pressure switch is closed, limits and rollout closed

    5. Hot Surface Igniter - ON (warm up timer starts)

    6. Gas Valve - OPEN

    7. Control Board - Proves Flame

    8. Blower timer starts (approx. 45 sec)

    9. Blower - ON

    10. Thermostat - satisfied

    11. Burner - OFF (gas valve closed)

    12. Inducer motor -post purge (time varies)

    13. Blower - OFF(after time delay)


    Can you do the same thing for a modulate system, or they are the same?

  9. #9
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    Quote Originally Posted by 36183 View Post
    Can you do the same thing for a modulate system, or they are the same?
    Close, but not exactly the same.

    In Modulating Furnaces, the heating mode starts with the gas at 100% and the fan at 0%, the valve reducing and the blower increasing until maximum efficiency is reached. The fan then runs continuously, ramping up or down depending upon air delivery requirements. Since the fan and burner almost never run at full capacity, and energy-robbing on/off cycling is eliminated, the unit is highly fuel-efficient.


    Modulating Furnace Sequence of Operation

    1. Thermostat - calls for heat

    2. Control Board internal/external check (pressure switch open)

    3. Inducer motor - ON

    4. Control Board - proves pressure switch closed, limits switches & rollout closed

    5. Servo Valve on Gas Valve is energized-millamp current (NO GAS FLOW)

    6. DSI (Direct Spark Ignition)- Energized

    7. Main Gas Valve Energized (Gas flows at 100% thru warm up period)

    8. Control Board - Proves Flame

    9. Blower - energized (0% RPM)

    10. After warm-up period, the furnace will respond to the thermostat
    demand by altering the gas valve pressure and blower speeds anywhere
    between 40 -100 % capacity.

    11. Thermostat - satisfied

    12. Burner - OFF (gas valve closed)

    13. Inducer motor - post purge (time varies)

    14. Blower - OFF (after time delay)

    Chime in gentlemen if I missed something.
    Last edited by rundawg; 03-02-2010 at 05:31 PM.
    Instead of learning the tricks of the trade, learn the trade.

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