Window exhaust fan - looking for lower RPM's

Discussion in 'General Electronics Chat' started by Avalanche Breakdown, May 5, 2007.

  1. Avalanche Breakdown

    Thread Starter Member

    Mar 22, 2007
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    I'm about to replace the present window fan in my bedroom/recording studio. I smoke, but you'd never know it stepping into the room, as I'm positioned right smack in front of the fan. All smoke goes out the window, saving things from smelling like stale smoke, and gunking up the equipment.

    The least unpleasant fan going (considering I have to listen to music in here) is the Bionaire BW2300. I can say this with utmost confidence, having tried MANY different fans over the last 8 years. I'm about to buy my second one during this time, but, even the Bionaire is more distracting than I'd like. The goal is to see if there's a safe, and very neat and clean way to slow the RPM's down for less noise. I don't need the full CFM of this fan, as it's fairly powerful even on low, so a side benefit would be less energy flying out the window.

    A couple years ago, I asked a local electrician about this, and remember leaving with the impression that the motors might actually need to be replaced with lower RPM models of the same size (if they could be gotten, and for a reasonable price). Reason being is that it's not a simple matter of using a rheostat between the wall plug and the fan, especially when the control buttons are electronic, as in the Bionaire. But electronic controls or not, it's the motors themselves that are physically wired for multi-speed operation, and not just different resistances on a control switch, or some such notion I had at the time.

    Anyway, I'm still on a quest to have the quietest window fan ever. Buying a new BW2300, then replacing the two motors is definitely too expensive. Since burning down the house is not an option, is there a way to SAFELY mod this fan?
     
  2. Avalanche Breakdown

    Thread Starter Member

    Mar 22, 2007
    20
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    Additionally...

    I've already considered building a homemade job out of four or five 120mm computer fans mounted on plywood, but there are too many problems going down that route.

    Edit: Small pic, but this is the fan I'm talking about.

    BW2300
     
  3. wireaddict

    Senior Member

    Nov 1, 2006
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    That info didn't show the wattage or current the fan requires. There are fan speed controls which I think will handle up to 600W. Since you have a recording studio I'd use the electrically quiet type; I connected two of them to ceiling fans on the same circuit with the smoke alarms & never had any trouble with them falsing & they never bothered an AM radio in the house either.
     
  4. Avalanche Breakdown

    Thread Starter Member

    Mar 22, 2007
    20
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    Thanks, wireaddict... I'll check into that, as I believe most window fans range about 50 - 75 watts. If speed controls run up to 600 watts or so, sounds like it should run fairly cool and easy on a relatively light load.

    Are such controls just simple rheostats? Would they affect the operation of electronic control buttons on the fan? The Bionaire also uses an IR remote, so with all the extra complexity, perhaps the fan may be sensitive to undervoltage?
     
  5. awright

    Well-Known Member

    Jul 5, 2006
    84
    7
    I hear you say that you want a quiet window fan and are concerned about the technology of slowing it down to reduce noise and excessive energy loss in exhausted heated air.

    If your physical layout will allow it, you are FAR better off using a remotely located tubeaxial fan at the far end of an acoustically absorbent duct. There is a limit to how quiet a fan of a given capacity can be made if you are sitting there looking at the fan blades a few feet away. It is definitely the case that slowing down the fan is a very effective noise reduction method, since fan noise is proportional to the fifth power of speed. but for a given airflow which is, after all, the purpose of installing a fan of any type, you are far better off with a remote fan and absorbent duct.

    Look at the Grainger part #5C516, 17, or 18 (depending on air flow requirements) duct-mounted fans and part #5E282, 83, 84 insulated flexible ducts for an idea of what I am talking about. I have the 4" model in my attic as a bathroom exhaust fan at the end of about 10 feet of flexible, insulated duct and I have to listen carefully to tell if it is running or not. These fans can be operated at reduced voltage when full flow is not required.

    To be effective in minimizing fan noise, the fan should actually be located remotely. It is relatively ineffective to, say, mount the fan in the window and then run the flexible duct around the room you hope to be quiet because the duct is not a very effective noise BARRIER with the result that the fan noise can leak into the room through the duct wall. It COULD be effective if the flexible duct in the room is boxed in with gypsum board.

    The duct should be about 8 or 10 duct diameters long and should not be in a straight line from the fan to the room (i.e., it should have bends).

    If you use a speed control, either use variable transformer-type dimmers or tapped transformer or inductor speed reducers, since the common dimmers/motor controllers that employ SCRs or Triacs to chop the current generate electromagnetic radiation that can raise havoc with audio systems. I was once called down to Hollywood from San Francisco to trouble shoot a noise problem in a recording studio and the problem turned out to be the lighting dimmers. You can get small variable autotransformer dimmers (at a premium price) that fit into a standard wall electrical box from electrical supply houses. They produce no electrical noise.

    awright
    Acoustical Consultant
     
  6. mozikluv

    AAC Fanatic!

    Jan 22, 2004
    1,437
    1
    hi awright,

    "To be effective in minimizing fan noise, the fan should actually be located remotely. It is relatively ineffective to, say, mount the fan in the window and then run the flexible duct around the room you hope to be quiet because the duct is not a very effective noise BARRIER with the result that the fan noise can leak into the room through the duct wall. It COULD be effective if the flexible duct in the room is boxed in with gypsum board.

    The duct should be about 8 or 10 duct diameters long and should not be in a straight line from the fan to the room (i.e., it should have bends)."

    i like your suggestion on minimiziing fan noise. i have the same problem here in my bedroom. everytime i smoke i have to turn on the fan and it gives out annoying noise. i'll try your suggestion. thanks

    moz
     
  7. Avalanche Breakdown

    Thread Starter Member

    Mar 22, 2007
    20
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    Also agreed that would be a great solution, noisewise. Unfortunately, I can't do it here.

    Just a FYI for those who might want to try that kind of setup... I did look into this possibility a couple years ago as well, and found general consensus that the Panasonic fans are among the quietest. There was another brand, but can't remember the name at the moment.

    With limitations on what I can do/build in this room, I'm gonna' take another look into fan speed controls (previously did that too, but forget why I decided against it, and didn't know about the electrically quiet ones). Hopefully they will allow the electronic controls on the Bionaire to work properly.
     
  8. TwoPage

    New Member

    Apr 10, 2009
    1
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    I think i have the solution for you. A Hertz regulator, I use them to slow down fans in SkyScrapers. Put it in line with the power feed at the panel for the fans. this will give the fans the same amount of power just at a lower hertz and then you have 60 speeds.... And the fan is not forcing more making noise just getting a slower heartbeat.... And yes they do make smaller ones than the ones needed for a skyscraper.
     
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