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  1. #1
    Join Date
    Nov 2012
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    18

    Modulating vs 2-stage furnace (and 2-stage AC)? Benefits of each.

    Background: valve is shot on an old gas furnace/AC system, mixed components (24 years old). Oversized, 9-SEER.

    Goals: 1) Reduce Noise (ductless return air at base of unit). 2) Cost: our condo is for sale (but may not sell for awhile).

    The last contractor I talked with is steering me away from a modulating furnace, and towards a 2-stage furnace, 2-stage AC, slight reduction in size from 5 tons to 4 tons (Carrier or York). Last winter I obtained a proposal for a Coleman modulating 80% furnace with a single stage AC. I hadn't considered the implications of the single stage AC until now.

    Our unit covers three floors, ground level is mostly a garage. The recent contractor said 2-stage systems work well in split level units.

    1) Is his advice sound? What are the benefits of a modulating furnace in terms of noise reduction, if any? Is this only a matter of comfort or would the modulating furnace reduce noise? (Essentially, I would be swapping a 2-stage AC for the Modulating furnace--but still paying more.)

    2) Would a single stage AC be either on or off, or would the fan/blower still be variable?

    3) Is there that big a difference between that quality of brands such as Coleman or York/Carrier that I should be concerned with it--and willing to pay more?

  2. #2
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    3,948
    Since you are selling it, I'd install a basic single stage. Most buyers will see the value of a new unit, but wont pay extra for a place that has a high end unit.

  3. #3
    Join Date
    Feb 2009
    Location
    Portland OR
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    1,845
    Not many times do I recommend single stage units but if you are selling it the new homeowner will certainly enjoy the benefit of the modulating furnace and two stage AC but you won't see any extra money from it unless you can sell the new owner on the features which I don't think is going to happen.
    There is a reason why new homes come with horrible HVAC systems but top of the line cabinets and countertops, HVAC does not sell, its something you add for yourself.
    Check out my YouTube channel - http://www.youtube.com/user/skyheating1 We have customer testimonials, product reviews and more!
    Like us on FACEBOOK if you like our advice here!

  4. #4
    Join Date
    Jul 2008
    Location
    Rochester NY
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    4,633
    Quote Originally Posted by 54regcab View Post
    Since you are selling it, I'd install a basic single stage. Most buyers will see the value of a new unit, but wont pay extra for a place that has a high end unit.
    While this is fingernails on a chalkboard from an energy perspective, from an economic perspective I think it's dead on.

    I don't know how you reconcile the two, but good luck with it. (Econ trumps zero return EE for me - put the increment into EE on your NEW home.)

    The new owners won't realize how noisy or uncomfortable your house is until it's their problem, and smaller equipment will likely mean an improvement (possibly significant) over what you are living with.

    Noise will be solved by putting in the smallest unit you can. 4 tons requires a LOT of airflow. Seems most homes have about 2x the size they need. During worst case conditions if your equipment doesn't run continuously, how long does it run? You can attempt to extrapolate load from that, and reconcile accuracy of your load calculation.
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    DO THE NUMBERS! Step on a HOMESCALE.
    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to BPI.org, or RESNET, and find an auditor near you.

  5. #5
    Join Date
    Nov 2012
    Posts
    18
    We have to live here until it sells. Anyone care to ignore the fact that it is for sale and talk briefly about how 2-stage vs modulating would affect noise? Any difference? Anything regarding the type of ECM fans or equipment that can help? (It's 5 ton now, so assume that 4 ton, maybe 3.5 ton could be workable.) Thanks.

  6. #6
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    3,948
    Modulating units can go from about 40% to 100% variable. The key is to size them correctly so that it actually goes to 100% on the coldest nights of the year. 2 stage is typically 70%/100% ratio. Most furnaces are 2x the size needed to begin with, so the high stage is rarely needed.

    For cooling the key is to install the smallest unit that will do the job. If a 2 stage unit is selected it's VERY important that it's sized correctly or the 2nd stage will never kick on. The key to a 2 stage's comfort is it runs a LOT on low speed.

    Economically, high end equipment will rarely pay for itself in energy savings alone. You buy high end for comfort.

  7. #7
    Join Date
    Jul 2008
    Location
    Rochester NY
    Posts
    4,633
    Noise depends upon how the system matches your duct and registers. If your equipment is grossly oversized to your ducts you will have a lot of noise, like riding in a semi with the windows down at 80. Drop a stage and it might be like putting the windows up. Still very noisy.

    If your equipment is not over sized to you duct you may have the 35 mph in a Mercedes with the windows down experience. Drop a stage and it might be like putting the windows up. Both are pretty quiet.

    Multi-stage is definitely much much nicer. But you've been living without it for years, what's it worth to have it for 6 months?

    And once you've had it it will seem a necessity (Power windows and heated seats are no longer an option for me). The experience of not having it (your new home may not have it) will be that much more unpleasant. I find people think they want nicer stuff until they see the cost. If I were going to keep a car 6 months, doubt I'd get heated seats, particularly if they added no value at sale.
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    DO THE NUMBERS! Step on a HOMESCALE.
    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to BPI.org, or RESNET, and find an auditor near you.

  8. #8
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    I always find it interesting that you cna have a heat pump system with only 34k or 51k BTU's and that's plenty, but the same home with gas would have a 60k or 80k BTU furnace respectively. I guess electric BTU's heat better than Gas BTU's.

  9. #9
    Join Date
    Jul 2008
    Location
    Rochester NY
    Posts
    4,633
    uraff. (You are a funny... friend.)

    And if they're magic electric heater built by Amish they don't heat 2x better they heat 3x better.
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    DO THE NUMBERS! Step on a HOMESCALE.
    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to BPI.org, or RESNET, and find an auditor near you.

  10. #10
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    3,948
    Quote Originally Posted by motoguy128 View Post
    I always find it interesting that you cna have a heat pump system with only 34k or 51k BTU's and that's plenty, but the same home with gas would have a 60k or 80k BTU furnace respectively. I guess electric BTU's heat better than Gas BTU's.
    You can thank the relatively low difference in wholesale pricing between gas furnaces for that.. Also that higher blower capacities only come with big furnaces..

  11. #11
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Well, it's less of an issue if you don't oversize the AC. .


    Seems like usually only the 80k BTU furnace comes in 2 blower sizes.

    The badly need a 45k BTU with a 4 Ton drive and a 30k BTU with a 3 ton drive.

    I suspect the problem is that they want to use the ame modular heat exchanger and burners. The cabinet width needs to match the burner size, other wise yuo'd have to increase the airflow to accomidate the bypassed air, then live with a lower temp rise. A simple solution would be partial dampers on either side of hte heat exchanger that would close when the furnace is running, but open up with in fan only or cooling.

  12. #12
    Join Date
    Jul 2008
    Location
    Rochester NY
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    Quote Originally Posted by motoguy128 View Post
    Well, it's less of an issue if you don't oversize the AC. .
    yep. And that's going to happen as long as people are told they can "setback and save".

    Quote Originally Posted by motoguy128 View Post
    Seems like usually only the 80k BTU furnace comes in 2 blower sizes.

    The badly need a 45k BTU with a 4 Ton drive and a 30k BTU with a 3 ton drive.

    ... live with a lower temp rise. A simple solution would be ...
    Educate about proper operation, then 4 ton blowers are only needed for big houses.

    Saving energy is keeping it in the house, not NOT running the equipment.
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    DO THE NUMBERS! Step on a HOMESCALE.
    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to BPI.org, or RESNET, and find an auditor near you.

  13. #13
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    3,948
    Quote Originally Posted by motoguy128 View Post
    Well, it's less of an issue if you don't oversize the AC. .


    Seems like usually only the 80k BTU furnace comes in 2 blower sizes.

    The badly need a 45k BTU with a 4 Ton drive and a 30k BTU with a 3 ton drive.

    I suspect the problem is that they want to use the ame modular heat exchanger and burners. The cabinet width needs to match the burner size, other wise yuo'd have to increase the airflow to accomidate the bypassed air, then live with a lower temp rise. A simple solution would be partial dampers on either side of hte heat exchanger that would close when the furnace is running, but open up with in fan only or cooling.
    I don't see an issue with lower temp rise, gas heat output is too hot anyways for most homes. It would be better just to redesign the heat exchanger for low BTU/high airflow furnaces.

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