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Thread: DC blower motor on furnace

  1. #1
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    DC blower motor on furnace

    I recently had a heating contractor over to give estimates on a new forced air gas furnace. He wanted to upgrade me to a higher end model that has a DC blower motor, rather than the usual AC blower. He said that because it is DC, it will use way less electricity, I think he said 1/8th or 1/10th of what a comparable AC blower would use. So you could therefore leave it running continuously, circulating air in the house, without driving electricity bills way up.

    Somehow I have trouble believing this.

    Any thoughts?

    Thanks

  2. #2
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    It uses less electricity by varying the speed of the blower motor with respect to how quickly the space is meeting setpoint. The electricity consumed by the motor is negligible anyhow, no matter what kind of blower motor you use, when compared with the heating fuel cost. If you want to Google this new motor, Google "GE ECM motor". Also Google "Zebra Instruments", which is the scan tool to service this motor.

  3. #3
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    Thanks for that info. He did say it was variable speed, but even if it reduced speed to half, still seems like it would not provide the savings he predicted.

    Also, regarding a standard AC blower, if we assume 600 watts running 24/7, that looks like 432 kwh/month. At .20 per kwh, that is $86/month.

  4. #4
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    I think he may be overstating the figures a little. By using variable speed, AC or DC, you can match the motor speed to the air flow requirements. If a low air volume is needed, the motor speed can be reduced, and the energy it draws is reduced. The energy reduction is by the cube of the speed, so at 1/2 speed, the energy required is 1/2 x 1/2 x 1/2, or 1/8th. This is all very real and can be proven by looking up the "affinity laws" for more details.

    BUT... (and this is a big butt),

    When running at full speed, there is NO energy savings, in fact the Variable Speed Drive drive will actually consume MORE energy than just running the motor. So the claim he makes, while true for a portion of the operating time, is not to be confused with the overall cost of running a HVAC system.

    The real savings come from circulating the air continuously, which evens out the overall temperature and maintains a better comfort level for the occupants. In the old system, you would heat up the place and while the blower was running, the air felt warm. But when the t-stat was satisfied, the blower shuts down and the air stratifies, leaving your feet feeling colder even though the air at the t-stat level is warm. That makes you either suffer with cold feet, or, like most people, turn up the t-stat. What a continuous circulating system does is let you feel better with a lower setting, and the heating can cycle on and off less often.

    Now if you ran a blower motor full speed 24/7, the electricity would cost you a lot. But by using low speed from a VSD blower for circulation and only turning it up when the burner is on, you save energy comparatively to running full speed. The overall savings compared to a non-continuous system is what you really need to investigate, and that's a little more nebulous.

    The AC vs DC issue is just a technology preference. You can do it either way, but typically a DC motor was more trouble because of wear parts like brushes, or was more expensive because it needs to use permanent magnets (PMDC). The GE ECM motor is a PMDC motor with a built-in VSD, produced n high enough volume to make it competitive to standard motors (or rather, less uncompetitive). It is also a bottom-up redesign to achieve very high efficiency compared to a similar 1 phase AC PSC motor, the kind you would need if you wanted to use a VFD on a 1 phase motor. So that, coupled with variable speed capability, makes a nice package for energy efficient HVAC systems.

    What they don't show you on their literature however is how it would compare to a 3 phase motor, which is what a lot of their competitors use. I have heard that if you use a 240V 1 phase supply to a VFD, then use a 240V 3 phase motor, it achieves even higher efficiency than the ECM. But if you have to have the voltage doubler on the line side of the VFD to accept 120V input, they are about the same. So GE has essentially conceded this product to be valuable only to the North American marketplace. Still, there are a lot of HVAC systems here...

  5. #5
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    I have the DC blower motor on my two stage zoned gas system. I like it and would buy again but I quit running the fan on continous because of the cost. Nothing is cheaper than off.
    Tom
    TBLO

  6. #6
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    Quote Originally Posted by ptonsparky View Post
    Nothing is cheaper than off.
    It might cost you more in fuel than the added electoral cost of running the fan.

  7. #7
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    We are probably comparing apples to oranges. I have a 100 yr old house that needs stratification of the air to get the dang thing to shut down at all on some winter nights.
    Tom
    TBLO

  8. #8
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    I actually install the VFD method Jraef has discribed..it is just what jraef has said and it is a very dependable system and if you want continuous air flow that is the method of choice..
    Life is temporary, heaven is forever. live life like it is your only chance to make a difference..

    to do nothing is the surest way to achieve nothing..

  9. #9
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    OK thanks jraef.

    Those affinity laws are interesting.

    Not sure how I'd feel having the blower running continuously, especially if it is circulationing just lukewarm air. Probably be good in the summer tho.

    And it seems like a bit of stratification could be ok on a cold nite if the bedrooms are all upstairs.

    I guess you would also have to factor in the earlier replacement cost of blower from running it 24/7.

  10. #10
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    we use it in locations where a more even temp is desired and the cost of individual zoning is to cost prohibited..then you speed it up as the fire box increases with temperature and slow it down when it is just circulating..normally done by the furnace controller..
    Life is temporary, heaven is forever. live life like it is your only chance to make a difference..

    to do nothing is the surest way to achieve nothing..

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