Author Topic: Energy efficient box fans  (Read 10830 times)

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Mary B

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Energy efficient box fans
« on: March 24, 2015, 03:17:10 PM »
Are there any high efficiency box fans out there on the market? I use one in winter to move cold air from the other end of the house to the living room where the corn stove sits. It runs 14-16 hours a day so the power use adds up fast. On low the one I have is using about 80 watts(varies a bit with how clean the blades are, I have cats who are shedding... fur builds up in a day!). I need to come up with a higher efficiency way to move air.

joestue

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Re: Energy efficient box fans
« Reply #1 on: March 24, 2015, 04:35:16 PM »
the larger commercial box fans with metal blades have a 6 pole induction motor, the 20$ fans use a 4 pole.. the 6 pole motor drives larger fan blades at a higher rpm.

i have been considering connecting my 1/2 hp 8 pole induction motor to a 20 inch box fan blade.. i think it might spin it faster than the 4 pole units.

you might try and measure the rpm and the torque needed to spin the existing fan, then go from there.

electrondady1

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Re: Energy efficient box fans
« Reply #2 on: March 24, 2015, 09:28:30 PM »
would a duct on the fan move the air to were it was needed more directly  perhaps a smaller fan could then do the job

Mary B

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Re: Energy efficient box fans
« Reply #3 on: March 25, 2015, 04:15:24 PM »
Ducting won't work, cold air is at floor level and you get the best results trying to move the cold instead of trying to push warm air into a room and have that push cold out.

Thought about a different motor, maybe 1/4HP ECM motor and controller... the motor in that fan doesn't have much life left in it anyway after 8 years of use.

electrondady1

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Re: Energy efficient box fans
« Reply #4 on: March 25, 2015, 05:39:10 PM »
pull the cold air out of the room at floor level via the duct.
 let it empty next to the stove

Bruce S

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Re: Energy efficient box fans
« Reply #5 on: March 26, 2015, 09:09:15 AM »
There was an old post on here were GHURD Norm or someone had converted a 110Vac box fan to 12Vdc and how much less power it consumed.
Might do a search in the older threads; I'm doing some searches, but "work" keeps getting in the way  ::).
Cheers
Bruce S
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OperaHouse

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Re: Energy efficient box fans
« Reply #6 on: March 26, 2015, 11:13:48 AM »
Fractional HP AC motors are notorious for being inefficient and I assume by going with a 1/8 hp DC motor would save about 30%.  At camp I made a wooden box with four computer fans in it for my wife to cool off.  It would be interesting to see how a 9X9 fan arrangement of computer fans compares with your fan.

kc7noa

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Re: Energy efficient box fans
« Reply #7 on: March 26, 2015, 12:31:21 PM »
Iv heard that the best cooling ... is also in the placement of your fan ....

An attic fan to pull in cooler air from outside (through open windows) -- out through the roof -- has dual benefits ...

TDC

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Re: Energy efficient box fans
« Reply #8 on: March 26, 2015, 06:20:16 PM »
I think a fan is an inefficient way to move heat, especially at a low temperature differential. Ducting would be more efficient, it moves more air from point A to point B without most of the energy going into mixing the air.   

30 years ago I dealt with the same situation.  I installed a copper coil in my wood stove just below the flue exit,  it connected to an old water heater tank at the other end of the house using a couple of garden hoses.   I machined a centrifugal pump and used a small fractional hp ac motor, probably 1/10 hp or less.  It worked ok, but the pump was poor and the coil in the stove may have been undersized.  Today I would use a Bosch 12v intercooler pump, I measured about .8 amps at 12v, about 1A at 14v. They also have a brushless pump used in the Chevrolet Volt battery and Corvettes. 

Mary B

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Re: Energy efficient box fans
« Reply #9 on: March 26, 2015, 06:36:22 PM »
No basement under the stove area to bring a duct to...

pull the cold air out of the room at floor level via the duct.
 let it empty next to the stove

TDC

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Re: Energy efficient box fans
« Reply #10 on: March 26, 2015, 06:40:13 PM »
Iv heard that the best cooling ... is also in the placement of your fan ....

An attic fan to pull in cooler air from outside (through open windows) -- out through the roof -- has dual benefits ...
I doubt Mary wants to open windows in the winter. ;)

Mary B

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Re: Energy efficient box fans
« Reply #11 on: March 26, 2015, 06:46:23 PM »
No way to duct to this room... no basement under it and no crawl space. Floor joists are resting on gravel from a couple test holes I drilled to see if I could make a hatch to get under this floor and insulate better. So stuck with fan pushing cold air. 

I actually ran across that fan conversion to DC doing a search for energy efficient box fan in google! was one of the top 10

thirteen

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Re: Energy efficient box fans
« Reply #12 on: March 26, 2015, 08:39:12 PM »
Side thought. Make a false beam (or 2) and put it on the ceiling and run it from the back to the front or to other rooms as needed.
I made one for my water lines years ago and in two houses I've made fake beams that run down the ceiling giving it a beam look. I  even made one that fits in the corner of the wall and ceiling down a hallway. This was to help hide it plus the new ducks are out of the way and can improve the looks or rooms with a little thought. They can be any shape from a triangle to a square to a rectangles. They bolt onto the rafters or upper floor joist or wall studs.  13 
MntMnROY 13

Mary B

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Re: Energy efficient box fans
« Reply #13 on: March 27, 2015, 04:32:34 PM »
Then I have to push cold UP and that is a direction it does not want to go! Plus I tried it with poor results. My floor plan is very open, I can see the far bathroom wall 40 feet away from where I am in the living room. 5 foot wide arch into the kitchen then the bathroom is off the kitchen. I place a fan in that doorway and temps in the bathroom will be close to what they are in the living room next to the corn stove. Considering most of my water piping is in that area keeping it warm is a must! Kitchen runs about 5-7 degrees cooler which is okay, when I cook it warms fast enough. Cold air from upstairs comes down the stairway into the kitchen also and the fan action pushes it into the living room and helps setup a convection current where warm goes up through the ceiling grate that is 4 feet in front of the corn stove. I have a booster fan in there on a thermostat for really cold weather but it rarely runs. With the bathroom being 30 feet away(to its doorway) it is the hardest room to get heat too.

TDC

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Re: Energy efficient box fans
« Reply #14 on: March 27, 2015, 08:01:56 PM »
Just a couple thoughts.....    Computer fans could be series connected for 24v or 48v, etc. They don't have to be arranged in a panel, maybe a couple here, a couple there, etc.  Just keep the circulation loop going.   I just tested a couple 80mm fans, both pulled significantly less than rated current.  At 13v, one was .12 amps, the other was .17. 


Mary B

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Re: Energy efficient box fans
« Reply #15 on: March 28, 2015, 04:47:44 PM »
I had 2 120 volt computer style fans in a frame for awhile... noise level was horrible! Maybe if I can find very low noise fans... I run on low all the time so not moving a huge amount of air just enough to keep a circulation going. Setup for 24 volts with a small A/C supply.. would give me an option for no grid power events that last more than 12 hours. Use the old box fan frame and hang the fans on a frame inside the grids to keep the cats from sticking  a nose into one...

TDC

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Re: Energy efficient box fans
« Reply #16 on: March 28, 2015, 09:08:18 PM »
Yeah, I got to thinking about the noise. Dropping to 12 or 11 volts made a big difference in the noise of those 80mm fans.  Large, slow fans are probably more efficient also.  I have several old PC's to collect fans from,  I'll try running the array at various voltages and compare to a box fan for noise and flow.  Another idea, 12 volt automotive radiator fan at reduced voltage.  I'll check with a friend that works for an industrial dc motor mfg, he might have something. Any idea of the rpm your box fan is turning? DC fans would also take the load off your inverter. Hey, put the inverters and controllers in the cold room... with the fans pointed in the right direction. ;)

Mary B

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Re: Energy efficient box fans
« Reply #17 on: March 29, 2015, 04:41:14 PM »
inverters reside in the living room which is also the coldest floor in the house! No basement under this end. Bathroom end is cold when I have the corn stove running so that is why the fan. I need to see how many CFM I am moving now and look at small 24 volt computer style fans.

Mary B

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Re: Energy efficient box fans
« Reply #18 on: March 29, 2015, 04:48:17 PM »
Looking at specs online on low the typical box fan is moving about 1,000 CFM of air. So now the search is one for a low power 24 volt DC motor to graft into it. That many muffin fans would sound like a jet engine!

MattM

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Re: Energy efficient box fans
« Reply #19 on: March 30, 2015, 04:39:53 PM »
For applications going against the thermodynamic flow you're better off with high speed fans.

Mary B

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Re: Energy efficient box fans
« Reply #20 on: March 31, 2015, 04:43:51 PM »
This is actually going with the flow, normal convection gets some heat to the bathroom but not comfort level to shower. I am boosting the cold convection that flows towards the corn stove.

Bruce S

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Re: Energy efficient box fans
« Reply #21 on: April 01, 2015, 07:30:27 AM »
Mary B;
I just finished re-purposing 9 80mm 12Vdc 2.50a computer fans out of some Dell 520's.
I have wire tied them together in a 3x3 configuration for testing as an attic fan replacement.
I'm also reusing a old P/S rebuilt long ago for a 120mm fan setup, that I can adjust down from 13Vdc to 5Vdc.
The fans do not want to start at anything lower than 12Vdc, but once running they do pretty good and fairly quiet at 9Vdc. I did this in the IT department as just a "TEST" bed so wasn't able to get true noise levels or current draw, but the power supply is only good for about 2a.
I'm sold on the amount of air these push!! I'm looking to build this up to 16 fans total (opening of the gable area).

Hope this helps,

Bruce S
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MattM

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Re: Energy efficient box fans
« Reply #22 on: April 01, 2015, 11:30:14 AM »
The high blade count fans will use more electricity, be less efficient, and wear out faster.  But the trade off is they are quieter.

Bruce S

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Re: Energy efficient box fans
« Reply #23 on: April 01, 2015, 11:52:16 AM »
The high blade count fans will use more electricity, be less efficient, and wear out faster.  But the trade off is they are quieter.
Do you have any documented proof of this?

I have 80mm and 120mm fans that have 4,5 & 6 blades from 5+ years ago and they still work.
At 12Vdc and even full out 2.5a that's only 30watts I've replaced 120Vac 0.9a in server rooms 4 times already.

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joestue

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Re: Energy efficient box fans
« Reply #24 on: April 01, 2015, 07:24:19 PM »
Bruce, that statement is too general anyways, covers too many different trade offs.

The other statement he made doesn't really fit either, unless you intend that the momentum of the air, accelerated to a high velocity is what keeps it flowing.. shedding much of the cfm into vortexes along the way.

Small fan blades spin at high rpm anyways to keep the same tip speed ratio.
My cursory observation is my 8 pole induction motor drives a 20 inch box fan blade at a higher rpm than does the stock 4 pole motor, I'm not really surprised.
Probably looking at less than 30% efficiency for the stock box fan motor.
You could double this efficiency by belt driving the fan from a 1/8th hp 3450 rpm induction motor.. or better yet use a dc motor, or a series wound motor run from rectified ac or dc.

Surplus center has a 1/15th hp 6 pole "3 speed" fan motor for pretty cheap btw.. but it will probably suck up more than 80 watts simply because its a more powerful motor.  That company also has a 225 rpm ceiling fan motor for pretty cheap, but 225 rpm is a bit slow for a 20 inch box fan.

A 20 inch box fan at 850 rpm corresponds to 5400 rpm for an 80mm computer fan btw.. some of them do run that fast but would probably be a 12v .2 amp fan.. most are  half that current which would correspond to 4K rpm.

Mary B

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Re: Energy efficient box fans
« Reply #25 on: April 02, 2015, 04:06:57 PM »
I was looking at DC motors over on surplus center. I don't need multi speed much... a simple voltage regulator to control fan speed would work fine. I need to pull the fan apart and see what size the shaft is before I order something. I would prefer to not have to engineer that if I can help it.