Attic Fan

[NickCoons]

I'm wanting to build a solar attic fan (instead of buying a kit) to keep the price down as much as possible, and am looking for suggestions on where to look for parts.  My attic has about 8,000 cubic feet, so I'd also like to know how I can figure out how many CFM the fan will need to be to exchange the air and give me a good temperature drop.

My first thought for a fan was to go to a junk yard and look for an old electric radiator fan that's 12" or 14" or so, then find a solar panel on eBay that will handle it.  Am I going to need any other pieces to this, or can I just connect the fan to the solar panel?  I plan to put a switch in-line that will be mounted on the side of the house so I can turn it off for the winter months.  I would imagine a regular household switch would work even though I'll be using DC instead of AC.

Open to suggestions... thanks!

[luckeydog]

you may consider having a battery for storage and using a temperature switch to turn on and off. I find myself needing the fan more in the evening when the heat dose build up.

with a battery you can use the panel to charge it on cooler mornings and have some reserve power to take you into the night when you are trying to sleep.

I am not sure if you want to complicate things with a battery , charge controller, temp switch but it is just some suggestions.

and yes you can run the fan motor directly off the solar panel if your solar panel is large enough.

the only problem I can see with this is that in the morning hrs. your fan will kick on and blow out all the cool air from the night cooler temperatures. and suck in heat that was not there before. this is assuming you forget to shut off the switch from the night before this is where a thermostat would work nicely.

Luckeydog

.

[NickCoons]

Thanks for the ideas.  I'm going to put a thermometer up in the attic to see how the inside temperature and outside temperature correlate throughout the day to see if using a thermostat (or perhaps maybe simpler, a timer) would be helpful.

[Nando]

Interesting verrrry interesting

My suggestion is for you to buy the fan with the case and the thermostat to turn it on when the temperature is around 95 F in the attic.

The fan needs to be quite and not vibrating, it is a nuisance.

I have 2 fans in my house that has 2400 square feet that represents around 8000 cubic feet of attic space, the fans have a total of 2600 cubic feet capacity so about every 3 minutes the air is exchanged and still the air in the attic may go abuot 30 F higher than the outside, this due to the insolation of 850 watts/m^2 of roof surface, in my area.

The breeding holes in the brick walls and the overhangs must be kept clean and open for proper operation.

Nando

[NickCoons]

My suggestion is for you to buy the fan with the case and the thermostat to turn it on when the temperature is around 95 F in the attic.

I live just outside of Phoenix, so the thermostat would probably be useless.  The fan would turn itself on as soon as the sun rose, even in December :-).

The fan needs to be quite and not vibrating, it is a nuisance.

Is this result something I can get from an electric radiator fan off of a junk yard car, or will I need to buy an attic fan kit for this?

I have 2 fans in my house that has 2400 square feet that represents around 8000 cubic feet of attic space, the fans have a total of 2600 cubic feet capacity so about every 3 minutes the air is exchanged and still the air in the attic may go abuot 30 F higher than the outside, this due to the insolation of 850 watts/m^2 of roof surface, in my area.

Any way to calculate the CFM of a fan if it's otherwise unknown?

The breeding holes in the brick walls and the overhangs must be kept clean and open for proper operation.

I have gables on the east and west ends of my house, each side with a 2.5-foot x 3-foot screened vent.  I had planned on putting the fan on one of these to blow the air out.

[Bruce S]

NickCoons;

   If you do go with a car fan, and want to use standard house switches get the better ones and pay attention to the amp rating.

I have a similar setup to replace our burnt out kitchen exhaust fan. I went with lowering the voltage to control the fan speed the 12Vdc ones once turning , will work on voltgaes down to ~4Vdc and keep turning, PWMing it will be better, but it's what I had on hand. Also, since these do take some juice to get them going, I use a SLA battery that a small ICP solar battery minder keeps topped off.

I am using one out of a 1990's 280Z and it can really push the air:-)

I did change out the blades and made use of the box fan blades sized to the opening.

One thing you might want to think about trying; use a comparator circuit and go with two fans. Let the circuit decide which area is cooler and pull the cooler air in and at the same time pushing the hotter air out with the other fan.

You could then go with smaller sized fans but the ability to "move" more air.

Hope this helps!

Bruce S

[tecker]

 If you have adequate ventilation at the upper most point of you r roof and not getting and the temp climbs . Then then add some positive pressure with cooler air from a Garage or soffet or covered patio. A fan built into the ceiling of a Garage or Laundry room is nice.

[spinningmagnets]

I bought a 2' attic fan (110 VAC, $65) and installed it to blow out at one end of the attic. The stores estimate in our area was about $5/month of electricity for the 6 hot months of the year. Without changing the A/C thermostat, it cut our A/C bill about 1/3 (saving $30/mo). It had a built-in thermostat, and I wired it to a wall switch so I could turn it off during the six cold months (attic would still get hot...YEA SOLAR!). Since there are attic vents around the entire roof edge, I noticed once when I was in the attic that air was coming in from the sides, so only half the attic was getting a significant cooling effect. I was going to add a second fan, but moved.

Later I visited a friend who had a $260 "whole house" fan with impressive performance. It is a 3-1/2' fan mounted in the ceiling near the center of the house. A switch would have to alternate the A/C or fan, as the fan pulls air from inside the house up into the attic. The larger diameter turned slower and quieter, and it had a beefier bearing, too. A white screen halfway hid it, and gravity drop louvers above it closed when it was off. I have also heard good things from the "chef-hat" looking turbine attic vents. If a couple are mounted about three feet below the roof peak they are not visible from the street, and should work well.

If it wasn't for zoning and municipal codes I'd like to put a small windmill/VAWT on the roof, and on the same vertical shaft as a DC whole house fan hooked to a PV panel. when the suns out, its faster, but in the evening, it still circulates a little. The fan could be slightly elevated and ducted, and whenever it gets hot enough for the A/C to come on, a flapper can be actuated that will redirect its flow so its from the outside air to the middle of the attic, and separate from the inside of the house.

Because of the ceiling insulation, I was surprised the attic fan helped so much. Please post your results, Cutting energy use with cheap available junk is my favorite subject. -Ron