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  1. #1
    Join Date
    Oct 2005
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    5
    I am trying to decide between a variable speed and multispeed fan for an 80% gas furnace. I don't care about comfort (for the purpose of this post anyway), I just want to know whether I'll save enough in electricity costs over 10 years to make up the substantial difference in cost.

    Problem is, I can't find published specs for the blowers to figure out how much electricity they use ... and besides, since I don't run my blower 24/7, the best I could do is a rough estimate even if I had the specs.

    I'm specifically looking at Trane XL80 vs. XV80. I use 300 therms of gas per heating season.

  2. #2
    Join Date
    Apr 2004
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    7,680
    Your ductowrk decides which motor is les costly to operate. Ask your dealer what your current airflow is and what the total static is. If its correct airflow and your static is below .5", then the ECM will be a great savings, if you got high static, then forget it.

  3. #3
    Join Date
    Oct 2005
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    88
    If its correct airflow and your static is below .5", then the ECM will be a great savings, if you got high static, then forget it.
    There's been some work at Lawrence Berkeley National Lab to look at that. http://repositories.cdlib.org/lbnl/LBNL-53947/

    From the graph on p. 25 (p. 27 of the pdf), it looks like the ECM blower is more efficient (higher CFM per watt) across all static pressures in the test. At .5" the PSC motor gave about 2.25 CFM/W while the ECM 2.3 was above 3 CFM/W. Pushing up to 1.0" static, the ECM efficiency has dropped to about 2.25--but the PSC has dropped to about 1.8 by that point so the ECM is still ahead.

    Now part of what could be going into docholiday's comment could be that if you're underducted, with high static, there's a good chance you're not getting correct airflow with the PSC motor. Put in an ECM and that motor is going to sense the problem and ramp up to give you correct flow--and then it might be drawing more power than the old one even though it is more efficient--but that's drawing more power to move more air, to give you the correct flow you didn't have before. That's worth something anyway.

  4. #4
    Join Date
    Oct 2005
    Posts
    88
    By the way, the CFM/W figures from the graph ought to be enough to get to the OP's question from the data given.

    The size of the XL80 and XV80 being considered should be divided into the 300 therms per season to get operating hours per season. (If the 300 therms figure came from the usage of an existing furnace, multiply by that furnace's efficiency to get output therms for the season, and divide by the output rating of the new Trane. Divide a furnace rating in btu/hr by 100 000 to get therms/hr.)

    Look up the proper heating CFM for each furnace, divide by the CFM per watt figure from the graph at the static pressure of the ductwork, to get watts, and multiply by the operating hours per season to get watthours. From your utility's price per kilowatthour you can figure the costs and the difference. It'll be an estimate, but something to go on.

  5. #5
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,248

    Wink No Comfort HEATing Solution

    Originally posted by lichen
    I don't care about comfort
    (for the purpose of this post anyway),

    I just want to know whether I'll save enough in electricity costs over 10 years to make up the substantial difference in cost.

    I use 300 therms of gas per heating season.
    Turn t-stat down to 52'F and
    you'll save more than enough in gas
    to make up the substantial cost difference.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  6. #6
    Join Date
    Mar 2004
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    1,042
    The other really important question here is if you have air conditioning!

  7. #7
    Join Date
    Oct 2005
    Posts
    5
    thanks chapmanf, after reading the study i can see why it's so difficult to rate blowers in any meaningful way.

    no air conditioning. in a couple years the furnace may be part of a dual fuel heat pump setup, we'll see. i'd definitely get a variable speed air handler if i go that route. but for now, the fan is only on when the furnace is.

    dan your reply made me laugh, but it really makes no sense. the *more* i use my furnace or fan, the more likely i am to realize significant savings with variable speed. it's like saying 'drive less so you can buy a better car with the money you save on gas!'

  8. #8
    Join Date
    Mar 2004
    Posts
    1,042
    You don't use an air handler with dual fuel. The furnace blower is the blower, and the heat pump's coil goes on the furnace's exhaust side. So if you want variable speed when you add a heat pump down the road, you will neither need a new furnace then or you will have to get the VS blower now.

  9. #9
    Join Date
    Apr 2004
    Posts
    7,680
    Originally posted by chapmanf

    Now part of what could be going into docholiday's comment could be that if you're underducted, with high static, there's a good chance you're not getting correct airflow with the PSC motor. Put in an ECM and that motor is going to sense the problem and ramp up to give you correct flow--and then it might be drawing more power than the old one even though it is more efficient--but that's drawing more power to move more air, to give you the correct flow you didn't have before. That's worth something anyway.
    Yes, thank you for stating it correctly.

  10. #10
    Join Date
    Oct 2005
    Posts
    88
    Somehow last night one of the most important points slipped my mind completely. It's one of those things that's stunningly obvious once pointed out, but easy to miss at first.

    All the inefficiency in an electric motor makes heat. Whether it's IR heating in the windings or bearing friction, every bit of energy that goes into a motor and doesn't come out as motion comes out as heat.

    Because the blower motor is in the furnace's airflow, all motor inefficiency contributes to the heat output of the furnace. Holding everything else equal, if you change out a blower motor for a more efficient one that uses 100 kilowatthours less over some period, you will burn 3.412 therms more gas over that same period to make the same amount of heat. (That's a 'perfect' furnace; at 80% it would be 3.412/.8 = 4.265 therms.) This is a balance that's explicitly mentioned in the front of the GAMA directory of efficiency ratings.

    Because the price of gas is usually well below the price of electricity, this still tends to be a good trade; you just can't look only at the electricity savings without taking out the cost of the equivalent amount of gas.

  11. #11
    Join Date
    Feb 2005
    Location
    Dothan, Al
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    3,453

    lichen

    Lets put it this way....
    No matter what you save, yur only talking a blower motor
    If that variable speed motor has to be replaced with ten years you will cancel all of yur savings on the repair bill.
    I'd much rather go with a multispeed blower.

    JMHO,
    Richard

  12. #12
    Join Date
    Dec 2002
    Location
    Rochester, MN
    Posts
    5,304

    Re: lichen

    Originally posted by bornriding
    Lets put it this way....
    No matter what you save, yur only talking a blower motor
    If that variable speed motor has to be replaced with ten years you will cancel all of yur savings on the repair bill.
    I'd much rather go with a multispeed blower.

    JMHO,
    Richard
    What have you replaced? The whole motor or just the modular?


    I'd never got back to a multispeed blower.. I am willing to pay for comfort/quiet.

  13. #13
    Join Date
    Feb 2005
    Location
    Dothan, Al
    Posts
    3,453

    Re: Re: lichen

    Originally posted by mayguy
    "What have you replaced? The whole motor or just the modular?"


    Have not either, have been told my price for motor from my dealer - I know what I would have to charge to replace it.


    "I'd never got back to a multispeed blower.. I am willing to pay for comfort/quiet." [/B]

    I glad you are willing to pay - I ,however, believe in the KISS principle. Also I believe that you are paying a LOT more for a LITTLE more comfort - my 10 Seer, single-speed system keeps me quite comfortable - Thats why I can't understand why I should expect better?? My home stayed between 45 & 55% humidity this summer while maintaining 73-74 degrees 'f', my utility bills are high but not outragious, and they are high when I don't even use my Hvac system. What would be better ??? How much better is yur system?? Plus, how old is yur system - wait till you have to replace board / motor / or both. Then tell me its still worth it.

    This was said Respectfully,
    Richard

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