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  1. #1
    Join Date
    Jan 2010

    Confused Modulating vs. 2-Stage Furnace

    I have problems with cold pockets -- primarily in the rooms furthest from the furnace (usually 3 degrees C lower than thermostat), and in the basement (usually 4-5 degrees C lower than thermostat) -- with my present single-stage, AC motor furnace. The furnace and AC are also past their service life and I have decide to replace given available government rebates.

    I have settled on Brand and sizing -- but just can't decide on whether to go for the 2-stage or modulating model:

    1. Will a modulating furnace significantly increase comfort?
    2. Will it be as reliable as a 2-stage furnace?
    3. Both the dealers I'm looking to work with are guiding me away from the modulating version .... why?

    Here is the American Standard based system I'm considering:

    • 80,000 BTU Comfort-R Variable Speed 2-Stage Furnace OR Comfort-R Variable Speed Modulating Furnace
    • 800 OR 900 Series Family Comfort Control Thermostat
    • 3.0 TON Allegiance 15 A/C and Coil
    • Generalaire MAC 2000 20x25x5 Media filter cabinet (I intend to use Lennox HC16 Filters)
    • Aprilaire 600 Series Humidifer with Automatic Control

    My home is approximately 3000 square feet in size, built 25 years ago.

    What would you do?


  2. #2
    Join Date
    Sep 2002
    I don't know
    Modulating units only run at 40% on low so the rooms which are far from the furnace may not recieve sufficient heat. (balancing may help though) I would personally go for the 2-stage for your application.
    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
    Apr 2009
    I think if the duct work is designed properly and the cold rooms are due to insulation (or the lack there of) then a modulating would only benefit you. Needless to say the system is only as good as the duct work, make sure that addressed beforehand....

  4. #4
    Join Date
    Jan 2010
    Quote Originally Posted by CynicX View Post
    I think if the duct work is designed properly and the cold rooms are due to insulation (or the lack there of) then a modulating would only benefit you. Needless to say the system is only as good as the duct work, make sure that addressed beforehand....
    I can definitely address any roof insulation issues.

    But duct work -- how can I address that? How can I even check if it is sufficient? The upstairs rooms are all finished, as is the basement.

    I will replace the drop and boot with the install -- but I really can't access anything else w/o ripping out drywall ...

  5. #5
    Join Date
    Feb 2009
    Tallahassee, FL
    Agree with above

    The two stage variable speed can be customized for your home to some degree by a good installer by fine tuning the air flow in low and hi speed. Of course this only corrects a small amount of duct undersizing. But its worth asking him what CFM (amount of air flow) he set it up with.

  6. #6
    Join Date
    Jul 2008
    Poestenkill, NY
    As above - if the duct work is problematic, a two stage or modulating furnace might still have the same or similar problems.

    Two points of interest, though:

    1) Modern multistage furnaces with variable speed blowers will likely have a higher airflow for the same btu input when compared to the old furnace you are taking out. Might be moot if you are pulling out a say 100 MBH furnace and putting in an 80...

    2) The new furnace with a variable speed blower will allow you to run the fan all the time at about 40% airflow to help continuously mix the air in the house and even temperatures. You can run the fan all the time with a standard blower, but it is usually pretty audible (annoying) and it does spin the electric meter. The new furnace with reduced "fan on" fan speed will usually be extremely quite (won't even know it's on) and it won't be using much power.

    As far as two-stage vs modulating...

    I have a two stage XV95 and love it. I can imagine a modulating furnace could only be better, but I can't say just how much better. I will offer from experience that a two stage is worlds better than single stage, as far as comfort. Depending on the up charge, you'll have to make the call.

  7. #7
    Join Date
    Jan 2004
    If the rooms that aren't warming up or cooling down the same as the rest of the house. Are because of not enough air due to under sized duct work.
    A 2 stage or mod can often help with that.
    The lower air flow in first stage or low modulation doesn't force as much air % wise out the supplies that are nearest to the furnace. So you get more air to the furtherest rooms.

    This is varied a bit by how under sized the duct system is also.
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