Roof Mounted Attic Fan on a Dimmer Switch?

Discussion in 'Electrical Forum discussion & Blog' started by bjferri, Jul 19, 2009.

  1. bjferri

    bjferri DoD Army

    Messages:
    150
    Location:
    Maryland
    I have an Nutone attic exhaust fan I would like to slow down a bit. Can I put a dimmer switch of some sort on it? Any ideas? They sell similar items to control ceiling fan speed - is that an option?
    Last edited: Jul 19, 2009
  2. Speedy Petey

    Speedy Petey Licensed Electrical Contractor

    Messages:
    996
    Location:
    NY State, USA
    NO, not a dimmer. You need a motor speed control, but it MUST be rated for the motor it is controlling.

    Verify the amperage of the motor and go to a real electrical supply house and get what you need.
  3. bjferri

    bjferri DoD Army

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    Location:
    Maryland
    I have also have a Humidistat and Thermostat connected. Do I need to verify amps of those items too? Where would I make the connection for the motor speed control? I'm trying to find the fan amps as I write this...

    Thanks.
  4. bjferri

    bjferri DoD Army

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    Location:
    Maryland
    Found it! 4.3 amps.
  5. Speedy Petey

    Speedy Petey Licensed Electrical Contractor

    Messages:
    996
    Location:
    NY State, USA
    Put the speed control last in line before the motor. You do not want to vary the voltage into the two stats.
  6. bjferri

    bjferri DoD Army

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    Gotcha! Thanks again!
  7. Thatguy

    Thatguy Homeowner

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    Putting it is series with an incand. lamp probably won't damage it. Less wattage= more speed reduction.
  8. bjferri

    bjferri DoD Army

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    Location:
    Maryland
    Can you clarify this? I really don't understand...

    Thanks...
  9. Thatguy

    Thatguy Homeowner

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    Location:
    MD
    It's a series circuit: a 120v source in series with the lamp in series with the fan.

    If the fan draws 100w@120v and the lamp draws 100w@120v, then when you put them in series each will get half voltage and 1/4th the power, so the fan will move more slowly and the lamp will hardly glow at all.

    The trouble is an incand. lamp has a resistance that depends on the current through it, so you should try different size incand. bulbs until you find one that gives you a satisfactory fan speed.

    With 7-1/2w the lamp will probably glow full brightness and the fan won't move and with 100w the opposite will be true, depending on the fan size.
  10. bjferri

    bjferri DoD Army

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    Location:
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    I think I understand, but does this mean I will always have a light burning in the attic?
  11. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
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    Location:
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    Yes, until the bulb burns out, and then the fan stops. Depending on the motor controller, some act like that and generate a fair amount of heat. Some chop the output.

    You might want to check the motor itself...some have multiple windings, allowing more than one speed. They save money by not wiring them up. All you may need is a 2-3 position switch, properly wired to control speed. If there are more than two wires to the fan motor, look closely at the data plate. You might be lucky. Also, there may be a set of jumpers...those may allow an alternate speed.
  12. bjferri

    bjferri DoD Army

    Messages:
    150
    Location:
    Maryland
    I'm not exactly sure what to look for but am I safe to buy a Motor Rated Fan Speed Control to replace the existing on/off switch? So long as the amps are okay?

    I was also told I may be able to rewind the motor but if it's easier replacing a shutoff switch I'd rather do that. Or but a smaller motor and replace... This is not my forte...

    Thanks again...
  13. ActionDave

    ActionDave Electrician

    Messages:
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    Location:
    Colorado
    For the cost of rewinding the motor you could buy three fans that run at different speeds.

    Is this a whole house attic fan? The kind that sound like an air plane on the runway? On the fan motor is a Horse Power rating. Find it and post it and you'll get some better advice.
  14. bjferri

    bjferri DoD Army

    Messages:
    150
    Location:
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    It's not measured by HP - it's an attic roof fan measured as CFM. I think my only option is to replace motor and blade assembly. I was told I could replace the on/off switch with a motor rated speed control switch but I'm not sure what it'll do to the humidistat/thermostat.
  15. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,139
    Location:
    New England
    This is probably how those sensors work...they act like a switch. When they sense the proper conditions, they either close or open the switch. So, if that is the case, it's not much different than adding an on/off switch. The motor controller would either have power applied, or not as determined by the switches in series before it.

    the motor probably has a data plate on it...if you post that, someone may be able to tell you what your options are.
  16. ActionDave

    ActionDave Electrician

    Messages:
    351
    Location:
    Colorado
    All motors have a rating. Are you moving air out of your attic only or out of your house?

    The motor rated speed control won't hurt your stats if you put it last in line in the circut but it has to be the right kind of control- not a dimmer, not a ceiling fan control, not a 100W bulb- one able to handle the size motor you have.
  17. Thatguy

    Thatguy Homeowner

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    Location:
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    4.3A leaves out the use of a 100w lamp unless you want very slow speed.:(
  18. ActionDave

    ActionDave Electrician

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    351
    Location:
    Colorado
    I just saw this. Do what Speedy says and you are all set.
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