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  1. #1
    Join Date
    Feb 2005
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    Dothan, Al
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    Can anyone tell me where I can get info on why's of using a variable speed blower to obtain higher eff.
    I can understand varying the blower speed if you vary the compressor speed also. In other words, varying the blower speed as the load on the house decreases and your system output decreases.
    What i can't seem to grasp is changing the blower speed without changing the unit output load.

    My big problem is that when you lower speed, the system runs longer. I can see where comfort could be better, but what about energy eff. if the system runs longer, even at a lower amp draw.

  2. #2
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
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    33,399
    ECMs use much less juice than their PSC counterparts. Anything that uses less juice in a system raises the SEER and HSPF. Those ratings figure in what wattage a blower motor uses, usually estimating but if you see a rating with a matching furnace or air handler, the exact wattage was used.

  3. #3
    Join Date
    Apr 2004
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    7,680
    Your required elctricty input is less with the same cooling output. Thats efficiency. The other features of VS such as dehumidifying properties are all comfort related.

    You cannot confuse comfort and efficiency, they are not the same but the better system considers both.

  4. #4
    Join Date
    Dec 2002
    Location
    Houston,Tx.
    Posts
    15,506
    Originally posted by docholiday
    You cannot confuse comfort and efficiency
    Is that on the same lines as you cannot confuse needs with wants? Lol!
    __________________________________________________ ____________________________
    “Reality is merely an illusion, albeit a very persistent one.” –Albert Einstein

    “Experience is a hard teacher because she gives the test first, the lesson afterwards.”-Vernon Law

  5. #5
    Join Date
    Nov 2003
    Posts
    257
    The purpose of the lower speeds is to improve comfort. Most systems with variable speed blowers aren't run at the lower speeds for more than a few minutes unless there is a need for extra dehumidification. The lower speeds at startup help to get the indoor coil to temperature sooner than with a standard PSC blower. This helps promote better humidity removal in cooling and less cold-blow in heating. An unfortunate side effect of increasing the efficiency in cooling mode has been a decrease in humidity removal capability, the use of ECM motors has allowed manufacturers to combat this problem.

  6. #6
    Running longer (and lower) in the summer means more dehumidification (especially when you're not at the limits of the system). More dehumidification means more comfort, or the same comfort at a higher temp. Same comfort at a higher temp means less expense to be comfortable (and they don't take that part of it into consideration, they assume same temps when calculating efficiency). Also better mixing (more even temps), less noise, smoother start/stop, etc - most of which (other then more even temps) don't affect efficiency.

  7. #7
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
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    11,086
    What i can't seem to grasp is changing the blower speed without changing the unit output load.
    Perhaps re-thinking your term "unit output load" might clarify the matter for you. If you are thinking in terms of what emerges from the supply registers in relation to blower speed, this isn't where you want to look. The true load is from the return air passing over the evaporator coil. If you slow the blower speed without reducing compressor capacity, the coil will run at a colder temperature and will result in colder air emerging from the supply registers.

    However, let's say the heat load and humidity levels are constant in the space being conditioned, and the outdoor ambient conditions are constant. First we fire up the system on high blower speed. After the system stabilizes, it will reduce both the sensible and latent heat of the conditioned space. The ratio of sensible to latent heat reduction will favor sensible heat due to maximum airflow over coil at rated BTU of coil and condenser.

    Now, same system, same conditions, but reduced blower speed. The system will now reduce both sensible and latent heat as before, but the ratio tips toward latent heat removal over sensible.

    Overall, the system will run at shorter intervals under high speed conditions because the sensible heat reduction is faster, leading to faster thermostat satisfaction. Longer intervals will be the case with reduced blower speeds due to slower sensible heat reduction to satisfy the thermostat.

    Ideally, for best efficiency, the system would be able to operate a blower at higher speeds for higher sensible/lower latent loads and operate at lower speeds for higher latent/lower sensible loads. The reduced blower speed (with ECM) and reduced outdoor ambient (resulting in lower compressor head pressure) will offset the longer run times to accomplish dehumidification and space temperature reduction.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  8. #8
    Join Date
    Feb 2005
    Location
    Dothan, Al
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    Thanks for response:

    Shophound,
    Thanks for very good explanation. Further questions please as I have never had to deal with ecm motors.
    What factor determines the speed of the motor - what factors involved?
    Also are the ecm motors variable over the entire range ( from 0 to full speed ) or are they more like a two or three speed motor?
    And yes, I was concerned about the coil freezing if the blower speed got too low, if the refrigeration wasn't reduced.

    [Edited by bornriding on 02-11-2005 at 05:27 PM]

  9. #9
    Join Date
    Apr 2004
    Posts
    7,680
    before you get overly excited about 2 stage cooling dehumidfying better you need to look at the latent work being done during that time frame. The latant capacity during the low stage is generally not as rewarding as you may think. Just because its running longer doesnt truely mean its going to dehumidify better.

    Maybe someone who has data can share thier findings.

  10. #10
    Join Date
    Apr 2004
    Posts
    7,680
    The ECM's are designed for a specific unit, the software determines the flow rates. Generally in a furnace you would have several cfm settings.

    Fan on -(about 1/2 nominal cfm if selected)
    Heat - faster than fan only (proper for furnace design)
    Cool - Fastest select
    10% increase
    10% decrease
    15 percent decrease (dehumidify).

    ECM's can be programmed to run at any speed the programmer wants but generally he has protection built in depending on the unit. They can also be programmed to be a single speed motor. Best thing is to read the configurations for each peice of equipment and be sure not to use a module not configured for the unit in question.

  11. #11
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,086

    Re: Thanks for response:

    Originally posted by bornriding
    Shophound,
    Thanks for very good explanation. Further questions please as I have never had to deal with ecm motors.
    What factor determines the speed of the motor - what factors involved?
    Also are the ecm motors variable over the entire range ( from 0 to full speed ) or are they more like a two or three speed motor?
    And yes, I was concerned about the coil freezing if the blower speed got too low, if the refrigeration wasn't reduced.

    [Edited by bornriding on 02-11-2005 at 05:27 PM]
    On residential applications I am still on the learning curve in regard to variable speed blowers, but my understanding is that static pressure most often determines ECM motor speed. The electronic equipment monitors the torque and speed of the motor, which is affected by the static pressure of the equipment and ductwork.
    On the commercial equipment I'm familiar with, the blower motor is an AC motor regulated by a frequency drive. Depending on application, it is often regulated by a static pressure sensor in VAV applications.

    Variable speed motors are likely variable through the entire range, but for residential HVAC applications, the full range isn't needed.

    The coil would freeze under reduced airflow conditions if the capacity was not reduced either by superheat regulation (TXV) or staging the compressor, or both. If the equipment is sized correctly for the house, long run times, which normally lead to a freeze-over of the coil under reduced airflow conditions, would not be a significant factor. A TXV would regulate superheat to prevent liquid floodback to the compressor.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  12. #12
    Join Date
    Feb 2005
    Location
    Dothan, Al
    Posts
    3,453

    VS drives

    Shop,
    I, like you, are familiar with the commercial / industrial concepts of blower speed control ( freq. drive)
    but as you said, this control is based on "load conditions" when the system has various zones.
    I just wasn't sure what was being accomplished ( other than the manuf. getting a better seer rating ) by variable blower speeds on a residential, one stage cooling system.

    And Doc,

    When you say 'not to use a module not configured for the
    unit', I'm not sure what the 'module'is. I assume ( that word again ) that the module is the electronic board that controls the motor, and that the board and the motor go together. So do not put just any 'board' with any ECM. Or do you mean the entire evaporator unit ??
    Is not the 'module' a frequency drive system, and what makes the ECM motor special??

  13. #13
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,086
    Born,

    Check out this link for ECM motor info:

    http://www.geindustrial.com/cwc/prod...ecm&lang=en_US
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

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