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  1. #1
    Join Date
    Nov 2005
    Kansas City (zip 66204)
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    Evening all,

    I understand what a 2 stage furnace is, but what does the variable speed blower get you? Why would you want a 2 stage variable speed furnace over a 2 stage furnace? I assumed that you would want the air moving to match the high or low output of the furnace or heatpump.


  2. #2
    Join Date
    Nov 2001
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    There are many here that can tell you in a way that might be more high tech, and/or simple to understand.

    But a VS motor to start with runs off of DC voltage, so it uses less electric to operate.
    It will use some say a 1/10th of the power that a typical motor might.
    By doing that it makes it much more likely to operate the blower 24/7 which will help with the total comfort of your house.

    During the heat cycle it will run at a determend CFM (determed at the set up of the install) to match the heat output of the furnace. And deliver the proper cfm for that amount of heat.

    It will do the same for the cooling mode, but will run a bit faster, due to the need for more force to move that air.

    In the "fan on" mode it will circulate air at a slower cfm, just to move air and exchange the room air.

    They do all this by starting from a stopped position, it will slowly ramp up to the running speed of each mode of operation, then when the stat is satisfied it will ramp down to a stop. So you don't have the sudden start and stop like the normal motor.

    Giving you an almost un-noticed fan operation.
    Since the blower runs at a slower cfm for much of the time it helps with dehumidification, and greater comfort, for less operational costs.

    Others will jump in now, I'm sure,
    I just thought I'd break the ice.
    If you try to fail, and succeed.
    Which have you done ?

  3. #3
    Join Date
    Oct 2005
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    crunch ... splash ...

    that broke the ice all right.

    DC voltage: well, kinda. The variable speed motor is a GE critter called an ECM (for electronically commutated motor) and it looks like a round box of electronics stuck on the end of what's actually a 3-phase AC motor. The power connection on the box is fed with the perfectly ordinary 120 volt 60 cycle AC (in the US) serving the furnace. The electronics convert that into DC temporarily; what they do next is reinvert the DC into 3-phase AC at just the right frequency to run the 3-phase motor at a selected speed.

    What makes it energy efficient? Not that it's DC per se; power is power; there's no magic about getting it in AC or DC form. But the thing is, a standard old-fashioned multispeed blower (a permanent-split-capacitor, PSC, motor for 1-phase AC) is pretty efficient on the high-speed tap, but its efficiency drops off on the lower speed taps, so its low speeds use a little less power but not much less than high speed. That can't be helped because the incoming AC is at a fixed frequency and the motor is designed to match it at full speed; at lower speeds it doesn't.

    So the power advantage of the ECM is that its low speeds are just as efficient as its high speeds because it tailors the drive frequency to match. You wouldn't want to run a PSC motor 24x7 even on lowest speed because its lowest speed is inefficient and still uses a few hundred watts; the ECM on a very low speed really might be using well under a hundred.

    The ECM feature that marketing types seem to drool over is that it ramps up to speed slowly and ramps down slowly so you don't notice drastic on/off changes. I sort of picture guys interrupting cocktail parties to point out to their guests how the furnace is slowly ramping up. Big deal.

    No, the thing that's really cool is that the ECM blower senses and actively controls airflow. With a PSC blower you measure some static pressures or flows, look in a chart, and pick the speed tap that's supposed to give you the right CFM in those conditions. After that the motor runs at that fixed power and you cross your fingers. If the filter loads up or the HO closes off some registers, the flow may drop below spec; if somebody adds another supply takeoff and pressure drops, the flow may rise above spec.

    With the ECM blower you will actually program it to the proper CFM for heating and cooling in your application. It will constantly adjust its speed to make that flow rate.
    If you pull out the clean filter and put in a loaded one while the blower is running, you'll hear the blower immediately ramp up to compensate. Put the clean one back in and it ramps back down. There are limits - it can't make up for a completely misdesigned duct system - but give it anything within its range and it will take care of delivering the correct flow.

    That leads to the surprise some people get with a variable speed blower: sales person talks about how much quieter it'll be than the old furnace and how much less juice it'll use, and then maybe it turns out louder and uses more power! If the old blower wasn't correctly set up for the duct system and wasn't moving enough air, the new ECM is gonna move more air, and use more juice to do it. If the filters aren't changed often, the ECM will actively use more power to move the right flow through them anyway, and that'll show up on the electric bill. So it's not always quieter or always cheaper; the best thing about it is that it's usally right.

    And the fact that it can just be programmed to any number of speeds makes it convenient for use in a newer system that might have two stages of heat (or even modulate like the Rheem), two stages of cool, plus dehumidify speeds a little bit lower than the normal speeds for both cooling stages, plus a very low speed continuous setting; that'd be a lot of relays with a, say, seven-speed-tap PSC motor. With the ECM, the control board just tells the blower to make a certain flow, and the blower goes and does it.

  4. #4
    Join Date
    Oct 2005
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    Wink Cool speech!

    Thanks for explaining that in laymans terms. I've read about them while researching what I wanted to buy but had not seen it explained like that. It makes even more sense to me now. I have one in my new Lennox G61 & for our house with its cold downstairs & hot upstairs, this is the perfect application. We leave it run all the time & no more hot & cold spots.
    I read somewhere about a study that was done in Wisconson & a furnace with an ECM saved an average of $200 a year on electric. I'll take it but I sure didn't buy it for that.

  5. #5
    Join Date
    Oct 2005
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    Re: Cool speech!


    I read somewhere about a study that was done in Wisconson & a furnace with an ECM saved an average of $200 a year on electric. I'll take it but I sure didn't buy it for that.
    The funny part is, in the heating season, the difference between a more and less efficient blower motor sort of disappears in the end. Any wasted energy in a motor comes out as heat, and the blower motor's in the airstream and you want the heat anyway; the GAMA efficiency ratings explicitly figure that if you have a more efficient motor you'll use that much less electricity and just that much more gas for the total heat. You wind up a little bit ahead as long as your gas is cheaper per btu than your electricity, but that's all.

    Now in the cooling season the more efficient motor is a definite win, because then the waste heat is working against you.

    Also some states or provinces will give extra incentives for the more efficient motors for environmental reasons, for instance if the state gets most of its electricity from coal, then more efficient motors in the heating season are effectively a way to shift some of the coal demand to natural gas and improve air quality. It's a small difference for one household but across a state it can add up.

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