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  1. #1
    Join Date
    Jul 2008

    How to size a 2-stage furnace?

    I need experienced opinion on how to size a 2-stage gas furnace.

    The concept of a 2-stage furnace seems very efficient, if properly sized. To me, it seems logical to target the high heat stage to be slightly above the heat loss when considering extreme design conditions. The low heat stage would target the more typical winter conditions. Is this stream of logic not valid?

    We contacted 4 heating contractors and obtained proposals. All 4 briefly inspected the existing furnace and asked a few questions about the duct system layout. None of the contractors performed a detailed load calc - Manual J or other. All 4 contractors proposed units that are much greater capacity than the original unit. (proposed units ranged 60 - 75k BTU). The low heat stage on these proposed units ranged from the same as the original furnace to notable more than the original furnace.

    The original (existing) furnace is a Goodman 80% unit. It's gross input rating is 45,000 BTU/Hr. The furnace seems adequate. Temperatures throughout the house have been reasonably consistent. Even on the coldest days (overnight below 20 deg F), the thermostat setpoint is maintained. The system seems to cycle ON-OFF a lot, with the OFF phase being longer than the ON phase.
    As a point of reference, I performed a load calc based mostly on ASHRAE Fundamentals Handbook. I estimated a heat loss load of about 35,000 BTU/Hr. This supports the original equipment rated at 36,000 BTU/Hr output.

    Two questions:
    Is my line of reasoning valid for sizing a 2-stage furnace?
    When referring to an over-sized heat output, what multiplier would be consider "definitely over-sized" vs possibly acceptable?
    (one salesman suggested that 70% over the design heating load is within the normal range.)

  2. #2
    Join Date
    Sep 2002
    I don't know
    Your contractors are dead wrong; keep on looking. (Run!!)

    If the existing furnace was sufficient, the new unit should be of equal or lower output.

    Oversizing by 70% defeats the purpose of getting a 2-stage furnace (long runtime, even heat, quiet; you'll effectively end up with a very expensive furnace which never hits second stage.

    Furthermore, your existing ductwork most likely can not handle the volume of air required for a 60-75k furnace.
    General public's attitude towards our energy predicament: "I reject the reality of finite resource depletion and substitute it with my own; energy is infinite, we just need an alternative storage medium to run the cars on. The economy can grow indefinitely - we just need to "green" everything! Technology is energy! Peak what?"

  3. #3
    Join Date
    May 2008
    A 40,000 BTUH, 2-stage 90+ efficiency furnace will do the job. I size my 2-stage furnaces as you have described. Keep getting estimates until you find someone who understands the 2-stage concept.

  4. #4
    Join Date
    Jan 2004
    2 stage is sized the same as single stage. It is sized to handle the load of the normal design temp. NOT for the extreme coldest temp ever reached.

    Your contractors are doing you no favors increasing size.
    Good chance you will use more gas. 2 stage gas furnaces are NOT more efficiecnt in first stage then second stage.

    Doubtful your duct system can handle the extra air a 75,000 BTU needs to move. Good chance the HX would fail, and leave you without heat down the road.
    Contractor locator map


    How many times must one fix something before it is fixed?

  5. #5
    Join Date
    Jul 2008


    Thank you all for your advice and comments. I greatly appreciate the help.

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