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Old 06-09-2012, 03:15 PM   #1
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Attic Gable Fan - How to Reduce Current?


Newbie here.

I installed a gable attic fan last year that seems to work just fine.

During the annual visit by our AC contractor, the technician mentioned that the 3-Amp draw by the fan was excessive, leading to two potential issues:

1. Premature burnout of the fan motor
2. Cancellation of the supposed benefit of better attic ventilation by the cost of the electricity to power the fan.

He told my wife (I wasn't there), that adding a capacitor in the circuit would limit the current used by the fan. My concern with that is with some other things I've read about the cap potentially causing motor hum due to affecting magnetic field collapse in the motor. Also, I've not been able to find any articles that address this.

We recently moved to Georgia from Washington (state), where we didn't even have AC, so most of that stuff is pretty new to me.

So the questions to the experts here are 1) is adding a capacitor to reduce run current a good idea? and 2) if so, how is this done?

The fan motor is 1/5 HP single phase, 1050 RPM, 120V, 3.3 A. The thermostat is set for it to kick on at 95 degrees, which it hits in there pretty much every day starting in April or so.

The attic is still hot, but more tolerable than I remember it being last year when I installed the fan.

Thoughts?

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Old 06-09-2012, 03:28 PM   #2
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Attic Gable Fan - How to Reduce Current?


Quote:
Originally Posted by DONTZAPME View Post
Newbie here.

I installed a gable attic fan last year that seems to work just fine.

During the annual visit by our AC contractor, the technician mentioned that the 3-Amp draw by the fan was excessive, leading to two potential issues:

1. Premature burnout of the fan motor
2. Cancellation of the supposed benefit of better attic ventilation by the cost of the electricity to power the fan.

He told my wife (I wasn't there), that adding a capacitor in the circuit would limit the current used by the fan. My concern with that is with some other things I've read about the cap potentially causing motor hum due to affecting magnetic field collapse in the motor. Also, I've not been able to find any articles that address this.

We recently moved to Georgia from Washington (state), where we didn't even have AC, so most of that stuff is pretty new to me.

So the questions to the experts here are 1) is adding a capacitor to reduce run current a good idea? and 2) if so, how is this done?

The fan motor is 1/5 HP single phase, 1050 RPM, 120V, 3.3 A. The thermostat is set for it to kick on at 95 degrees, which it hits in there pretty much every day starting in April or so.

The attic is still hot, but more tolerable than I remember it being last year when I installed the fan.

Thoughts?
I don't get it? The fan has a nameplate of 3.3 amps but someone thinks that 3 amps is excessive? I'm confused?!?!?

Personally, you installed the fan, but does your attic already have a ridge vent? if so, I would just omit the fan altogether, attics are going to be hot no matter what...

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Last edited by stickboy1375; 06-09-2012 at 03:31 PM.
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Old 06-09-2012, 03:31 PM   #3
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Attic Gable Fan - How to Reduce Current?


Quote:
Originally Posted by DONTZAPME View Post
the technician mentioned that the 3-Amp draw by the fan was excessive

He told my wife (I wasn't there), that adding a capacitor in the circuit would limit the current used by the fan.

The fan motor is 1/5 HP single phase, 1050 RPM, 120V, 3.3 A. The thermostat is set for it to kick on at 95 degrees, which it hits in there pretty much every day starting in April or so.
Offhand it sounds like a crock but a series capacitor properly chosen will reduce fan current. I used to use salt water rheostats to reduce fan speed.

Is he
well meaning but ignorant
not well meaning and not ignorant
not well meaning & ignorant
?

If you know your roof dims the recommended Air Changes per Hour can be easily worked out.

Last edited by Yoyizit; 06-09-2012 at 03:35 PM.
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Old 06-09-2012, 03:37 PM   #4
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Attic Gable Fan - How to Reduce Current?


A 3A motor will cost about 25 cents/day to run it 10 hours a day. You decide if this is too much $$$$$

As for a capacitor that might make a power factor correction but that benefits the power utility not you. You only pay for real power (kW) so power factor correction is of little benefit...
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Old 06-09-2012, 03:42 PM   #5
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Attic Gable Fan - How to Reduce Current?


Unless you actually use the attic for something I would not bother using the fan and just make sure there is enough insulation up there.
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Old 06-09-2012, 05:57 PM   #6
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I was reading this post and don't understand how a 1/5hp fan can draw 3.3 amps. where is all the electricity going? I always thought HP and amps go together. Maybe I'm just stupid can some one take the time to explain what I'm missing? Thanks .
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Old 06-09-2012, 06:01 PM   #7
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Attic Gable Fan - How to Reduce Current?


Quote:
Originally Posted by Yoyizit View Post
Offhand it sounds like a crock but a series capacitor properly chosen will reduce fan current. I used to use salt water rheostats to reduce fan speed.

Is he
well meaning but ignorant
not well meaning and not ignorant
not well meaning & ignorant
?

If you know your roof dims the recommended Air Changes per Hour can be easily worked out.
Ditto.....

3A for a 3.3A motor sounds like your getting a pretty good deal....

Personally...I would not use the fan.....you would be better off having more vents.

I installed one....and removed it recently. The problem is that at certain times the winds blows such that the fan it fighting the wind....

At any given time, the wind is going to be hitting your house at a certain angle....as it blows over the roof it tends to push air in on one side and suck it out the other....unless that fan is at the top of the roof....or your wind always blows from the same direction and you have the fan in the right position....the fan will spend a good part of the time fighting the wind.

That is what mine was doing. I gotbetter luck cooling my attic by installing one of those rotating vents....
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Old 06-09-2012, 06:08 PM   #8
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Attic Gable Fan - How to Reduce Current?


A ridge vent would vent the whole roof not just a circle, Cost 0 to run, will never brake down in the middle of summer like most do, will vent no matter which way the winds blowing, looks far better then any other vent, will never rust.
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Old 06-09-2012, 09:34 PM   #9
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Attic Gable Fan - How to Reduce Current?


The problem here is power factor. 1/5 HP is 150 watts. At 80% efficiency, the motor should draw about 1.6A. If it's drawing 3A, then the power factor is only about 0.5 which is excessively low. A capacitor in parallel with the motor would correct for this and reduce the current draw to something closer to ideal. However, since your electric meter only charges you for REAL power, not APPARENT power, this should have no noticeable effect on your electric bill. I wouldn't mess with it.
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Old 06-09-2012, 10:35 PM   #10
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Attic Gable Fan - How to Reduce Current?


Several keep saying do away with the fan and add more vents, I live in Fla and put in 4 off ridge vents and continuous ventilated soffit all the way around the house and my attic was unbearable, I put in an attic fan and it keeps it about 35 degrees cooler now, I personally don't buy into all the venting, I believe if you had a continuous wind blowing full time maybe, but in my opinion it's all about air movement in the attic and that fan is moving air, I wouldn't change a thing.
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Old 06-10-2012, 10:12 AM   #11
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Attic Gable Fan - How to Reduce Current?


Roughly, it seems to be normal for motors in the 1/6 hp range to lose about half the volts x amps inputted to them, for whatever reasons.
Along these lines, small motors that put out 10W lose about 90% of the VA in, and 5 hp motors may only lose 10%. Motors as large as 1 hp don't do all that well.

These three breakpoints can fine tuned by looking in the Grainger catalog; it lists volts, amps and rated hp.
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Old 06-10-2012, 10:16 AM   #12
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Attic Gable Fan - How to Reduce Current?


If you install proper vents and ridge vents, convection will take care of the rest, the attic will always be hot, regardless, unless of course you condition the space.

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