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  1. #1
    Join Date
    Apr 2008
    Location
    Toronto, Canada
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    166

    Furnace: DC Motor Cost Savings

    Hi. does anyone have real cost savings data between a dc motor furnace (ecm) and AC variable speed? i am trying to decide if i should spend the extra money on an ecm motor furnace.

    oddly the "unit amps" listed in the rheem RGFD specs (ecm) says 8.7 and the RGRK specs (ac variable speed) "motor full load amps" says 6.8. too bad the two dont have the same specs so i can compare.

    my main issue is with potential repair costs: the ecm furnace parts costs are crazy high compared to the AC furnace. but will the cost savings in energy make up for it? thats what i am looking for

    brett

  2. #2
    Join Date
    Jul 2002
    Location
    Zelienople, Pa
    Posts
    2,965
    You don't go with a variable speed for "cost savings" only.
    You will get your money back in comfort 10 fold.
    Increased dehumidification in the summer means you can set the t-stat a few degrees warmer and still feel comfortable.
    More even temperatures throughout your home for comfort.
    Couple your system with a high efficiency air cleaner and you can leave the fan run 24/7 for a fraction of the cost of a standard blower.

    Look at the big picture and get an extended warranty for peace of mind if you're worried about service bills.
    How tall are you Private???!!!!

  3. #3
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,789
    A VS motor can save you enough in electric to cover the difference in replacement cost.
    Weather it will or not depends on many things. System static pressure. The higher the static, the more electric it uses.
    Weather or not you run your fan 24/7. If you do, it will save you money.
    Length of heating and cooling seasons. The moe you use the blower the more it can save you. Mild heating and cooling season, you won't save much.

    As above. VS blowers are comfort enchancements to your system design.
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  4. #4
    Join Date
    Feb 2005
    Location
    cincinnati ohio
    Posts
    2,024

    vs cost

    heres a link you can try . Set everything the same but use the vs option on one and use the standard motor on the other . Do both coolin hours and heating hours to find the savings . Let us know how yours comes out . http://www.hvacopcost.com/
    My avatar is a picture of a Goodman Silencer .....These were commonly used in Goodman country ....Photos by hvac tech ( PaysonHVAC )

  5. #5
    Join Date
    Apr 2008
    Location
    Toronto, Canada
    Posts
    166
    hello thanks for your answers! my question wasnt with respect to single speed vs variable speed. i was asking the difference between an AC motor and DC in terms of power draw.

    what is the difference, typically, between the power draw in amps or watts between a modern furnace that uses an AC motor vs a DC motor, running at the same speed, say, the highest.

    this appears to me to be the only real cost savings between the two types of furnaces. since the parts costs for a DC furnace are drastically higher, i want to determine the net potential cost of each system assuming a break down requiring a parts replacement.

    this will then help me to determine if i want to get a DC furnace or not. i understand that stages, modulation and variable speed are about comfort more than about savings.

    the two furnaces in question are the rheem 2-stage variable speed AC and the rheem modulating.

    again, i GET that modulation/stages provide comfort. what i am trying to work out is operating + maintenance costs.


    brett

  6. #6
    Join Date
    Jul 2006
    Location
    NW burbs of Detroit
    Posts
    6,058
    There is a definite cost saving in the VS motor.

    But like the the other guy said, that all depends on how it is applied.

    I can tell you that from a training seminar an engineer did all the math

    (Which I will admit is above my head) and with the motor running 24/7 it cost about as much as a 100 watt bulb.

    Using an AC multi speed motor left to run 24/7 would cost as much to run as a 20cu ft side by side refrigerator freezer.



    I am over simplifying but that's a good basic analogy

  7. #7
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,789
    What your refering to as a single speed, is a standard PSC motor.
    What your calling a DC motor, is a variable speed motor.

    If order to have savings with a VS/(DC) blower, your duct static can't be too high, or it will use more electric.
    If your duct work is sized right, the VS motor will use about half as much electric as the single speed.


    Rheem doesn't have a variable speed AC motor.
    I think your confusing a multispeed, with variable speed.
    A Multispeed motor is a standard PSC motor.
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  8. #8
    Join Date
    Jul 2006
    Location
    NW burbs of Detroit
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    6,058
    Thanks for the heads up Bennthere. I forgot to mention that was on continuous fan operation which is the lowest rpm on the VS motor.

  9. #9
    Join Date
    Nov 2000
    Location
    Eastern PA
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    68,981
    Quote Originally Posted by plexus View Post
    hello thanks for your answers! my question wasnt with respect to single speed vs variable speed. i was asking the difference between an AC motor and DC in terms of power draw.

    what is the difference, typically, between the power draw in amps or watts between a modern furnace that uses an AC motor vs a DC motor, running at the same speed, say, the highest.

    this appears to me to be the only real cost savings between the two types of furnaces. since the parts costs for a DC furnace are drastically higher, i want to determine the net potential cost of each system assuming a break down requiring a parts replacement.

    this will then help me to determine if i want to get a DC furnace or not. i understand that stages, modulation and variable speed are about comfort more than about savings.

    the two furnaces in question are the rheem 2-stage variable speed AC and the rheem modulating.

    again, i GET that modulation/stages provide comfort. what i am trying to work out is operating + maintenance costs.


    brett
    You are asking a question in a manner that is difficult to answer. Figure out what it is you really want to know and ask that question without the additional parameters.

    The fact is that a variable speed blower is going to save energy usage, usually about a fifth of the energy used by an AC motor. As mentioned, there are other factors for comfort and energy savings that are not directly associated with the direct energy usage.

    Your inquiry is "is it worth it to pay the additional cost for a variable speed blower" and the answer has been given; yes it is.
    Government is a disease...
    ...masquerading as its own cure…
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  10. #10
    Join Date
    Apr 2008
    Location
    Toronto, Canada
    Posts
    166
    thanks for your replies. i will try and be more specific. i can choose to go with a 2-stage PSC motor rheem furnace or their modulating ECM+ model. the additional upfront cost is a bit limiting but doable if i can justify it. the issue for me is the cost of parts. i will be getting a 10 year p/l warranty regardless but if the furnace lasts longer, i am concerned about the difference between replacing a $1000 blower vs a $400 blower in the simpler PSC motor model.

    my house is small, 1200 sq ft, 2 story and the exiting ductwork will not be changed and is c.1975 when the existing furnace was put in.

    should i just not worry about it and spend the extra money and get the modulating ECM furnace or should i stay away on the Keep It Simple Stupid principle?

    the existing furnace is fine for me in terms of comfort so anything more is great. i am sure a mod/vs furnace is potentially amazing in term of comfort but for me right now costs are the biggest issue.

    also resale value - if i sell the house will a potential buyer say "oh great they have a good but simple furnace installed that is more matched to the house" or "too bad they didnt put in a top of the line" or "fantastic! they have a modulating furnace installed" or "oh no, they have one of those variable speed furnaces installed thats going to cost an arm and a leg to maintain"...

    what are your professional experiences with the rheem modulating furnace vs the 2-stage? i guess that will tell me more accurately than anything how reliable these things are in the long run.

    brett
    brett

  11. #11
    Join Date
    Jul 2006
    Location
    NW burbs of Detroit
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    Reem makes a good dependable mod furnace.
    But the more bells and whistles....well, you know what I mean.

    My opinion is this: If Modulating furnaces are THAT good, why aren't more mfrs making them?
    The only other one who makes a mod is York, and the technology they use is slightly diff from Reem.



    The fact is that the price of gas is going to do nothing else but INCREASE.

    So on the face of it I would say that a ROI calculated today might not look favorable in today's dollars, but with the price of gas going up all the time
    the payback will probably grow more to your favor each year.

    But is that mod furnace going to eat you alive in repairs?

    Personally I would not take the chance since there are only two mfgs using that technology.

    ECM motors on 90% furnaces have been available since the late 80's and early 90's from Heil and others.

    Everyone sold was recalled by Heil and the other mfrs had their problems too..

    Here we are in 2008 and now they are as common as cars on the road.

    I would suggest a 2stg vs 95% furnace. They are priced with in range of most folks income, and even if you have to get a loan to afford one I would recommend it, as the reliability of these type furnace are thru the roof.

    The Ecm is always going to be the most expensive part, but gas prices will be so high they will make the investment worthwhile.

  12. #12
    Join Date
    Jan 2004
    Location
    PA
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    68,789
    The Rheem mod, has a lifetime replace the furnce if the heat exchanger fails warranty.
    Not sure if it can be transfered to a new owner.

    ECM motors seldom fail. Its usually the module when their is a failure. The module is a lot less then the motor and module.(can't get the motor without the module.)

    The mod is a comfort furnace. So don't buy it for savings.

    Most home buyers would like a top of the line furnace already in the house their buying.
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  13. #13
    Join Date
    Jul 2006
    Location
    NW burbs of Detroit
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    6,058
    Quote Originally Posted by beenthere View Post
    The Rheem mod, has a lifetime replace the fence if the heat exchanger fails warranty.
    Not sure if it can be transferred to a new owner.

    ECM motors seldom fail. Its usually the module when their is a failure. The module is a lot less then the motor and module.(can't get the motor without the module.)

    The mod is a comfort furnace. So don't buy it for savings.

    Most home buyers would like a top of the line furnace already in the house their buying.
    Only thing I see wrong with that is what history has taught us.

    Luxaire made a product called a "HEAT PIPE" furnace. Life time warranty.

    They all went defective and customers had to settle for a lower quality replacement furnace.

    Lennox Pulse. Sweet piece of technology: Heat Exchanger were bad in a high percentage. Government action forced replacement (I think it was a forced government recall, not 100% sure. But a huge recall happened) and again the customers got a replacement that was no where good as the original.

    Lifetime warranties on heat exchangers are great and companies do honor them.

    But the 90%years I put in 20 years ago are no where near as good as whats available to day.

    I have had some heat exchangers go bad on me and in most cases the customer wants the newer technology.

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