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View Full Version : attic fans CFM AC vs solar DC



James Baker SD
06-21-2015, 11:23 PM
Looking at putting a fan on my roof to help vent the attic. A typical AC powered fan takes about 200W to achieve 1200 CFM. When looking at solar powered fans, they claim around 1200CFM for 20 to 25W solar panel. Something doesn't look right to me. What am I missing? Super efficiency of DC motors? Lying admen who write the copy?

Phil Thien
06-21-2015, 11:33 PM
It is the admen thing.

They don't move anywhere near as much CFM as your typical AC fan.

Jim Matthews
06-22-2015, 8:04 AM
What works in roof ventilation is a ridge vent and clear soffit intakes.

Anything else is just another hole where water will get in.

If you have loose fill insulation, make certain the soffits are clear.

http://www.finehomebuilding.com/how-to/articles/a-crash-course-in-roof-venting.aspx

James Baker SD
06-22-2015, 12:07 PM
What works in roof ventilation is a ridge vent and clear soffit intakes.

Anything else is just another hole where water will get in.

If you have loose fill insulation, make certain the soffits are clear.

http://www.finehomebuilding.com/how-to/articles/a-crash-course-in-roof-venting.aspx

I have a ridge vent and it seems to do nothing.

Yesterday I checked it out. The plywood nailed to the rafters only has about a 1 inch gap at the hip ridge. That inch is my only attic exhaust along the rectangular portion of the roof. It is clearly covered by a heavy material, presumably the underlayment to keep the rain out. No light gets through it at all and it is stiff enough to not give any when I push on it. Not sure if this is how a ridge or hip vent is to be built.

Then I went up on the roof to see what the ridge vent looked like from above. Under the vent covering shingles is a wire mesh about ” high. That would be the potential exit point for the hot trapped air. It is at least 140 degrees inside the attic now. Air temp outside is 87. The wire mesh felt cool to the touch, certainly cooler than the shingles still in evening sunlight. Shingles were warm, but I would not call them hot, no problem holding my hand against them. I would expect that the metal mesh was at least be warm if the hot attic air were venting out through it. I touched it a couple of feet from each end and in the middle and same cool sensation each time. Also, no noticeable air flow out through the mesh, but slight breeze blowing made it hard to be sure. Certainly no rush of hot air exiting the attic

I have soffits vents all around the house, but they are virtually invisible from inside the attic due to the design of the roof. They are in a rather large overhang that is actually lower than the interior ceilings. The sloping roof comes to about 8" above the ceiling at the locations of the outside walls, then continues below the level of the wall. Trying to crawl there, my head hits the underside of the roof with my belly on the ceiling before I can get close enough to really see the soffits from inside. Layered insulation appears to stop at the walls, but hard to be sure what is in the overhang with the soffit vents.

Anthony Whitesell
06-22-2015, 1:16 PM
I have a ridge vent and it seems to do nothing.

Yesterday I checked it out. The plywood nailed to the rafters only has about a 1 inch gap at the hip ridge. That inch is my only attic exhaust along the rectangular portion of the roof. It is clearly covered by a heavy material, presumably the underlayment to keep the rain out. No light gets through it at all and it is stiff enough to not give any when I push on it. Not sure if this is how a ridge or hip vent is to be built.

Then I went up on the roof to see what the ridge vent looked like from above. Under the vent covering shingles is a wire mesh about ” high. That would be the potential exit point for the hot trapped air. It is at least 140 degrees inside the attic now. Air temp outside is 87. The wire mesh felt cool to the touch, certainly cooler than the shingles still in evening sunlight. Shingles were warm, but I would not call them hot, no problem holding my hand against them. I would expect that the metal mesh was at least be warm if the hot attic air were venting out through it. I touched it a couple of feet from each end and in the middle and same cool sensation each time. Also, no noticeable air flow out through the mesh, but slight breeze blowing made it hard to be sure. Certainly no rush of hot air exiting the attic

I have soffits vents all around the house, but they are virtually invisible from inside the attic due to the design of the roof. They are in a rather large overhang that is actually lower than the interior ceilings. The sloping roof comes to about 8" above the ceiling at the locations of the outside walls, then continues below the level of the wall. Trying to crawl there, my head hits the underside of the roof with my belly on the ceiling before I can get close enough to really see the soffits from inside. Layered insulation appears to stop at the walls, but hard to be sure what is in the overhang with the soffit vents.

Sounds like the installer placed the roofing felt over the ridge opening.

Once upon an time I read that you should not have both ridge/soffit and gable vents. I had several people talk about condensation in their attics this winter. We had a ton of snow up here. I explained what I knew about the soffit/ridge vent airflow, then quickly thereafter saw and article about "blocked" ridge vents. At least one person I know, shoveled the roof off just near the ridge vent, and the condensation and air dam issues went away.

I purchased the house with gable vents. The attic was always warm. I installed a ridge vent and felt an immediate difference. I still have both. I don't have any plans to close off the gable vents.

To the OP. Are the AC and solar fans the same size? Do they list the RPMs?

Jason Roehl
06-23-2015, 7:15 AM
Looking at putting a fan on my roof to help vent the attic. A typical AC powered fan takes about 200W to achieve 1200 CFM. When looking at solar powered fans, they claim around 1200CFM for 20 to 25W solar panel. Something doesn't look right to me. What am I missing? Super efficiency of DC motors? Lying admen who write the copy?

Considering 25W is about 1/30 HP, I highly doubt that you can move 1200 CFM with that. Even a 120 CFM claim would be suspect to me.

Michael Weber
06-23-2015, 5:17 PM
Well crud. I was about to get a solar fan but now I'm highly suspect. Anyone have factual data?

Jerome Stanek
06-23-2015, 6:47 PM
remember that you are talking dc not ac motors.

Mike Henderson
06-23-2015, 6:57 PM
remember that you are talking dc not ac motors.
Doesn't really matter - power is power, whether it's AC or DC. You need the same power to move the same air, no matter how the power is supplied.

Mike

Greg R Bradley
06-23-2015, 8:23 PM
I have done a lot of research on using solar to vent the attic as it makes far more sense than using solar to generate AC. I was planning on using two 200watt solar panels.

James Baker SD
06-23-2015, 9:52 PM
I have done a lot of research on using solar to vent the attic as it makes far more sense than using solar to generate AC. I was planning on using two 200watt solar panels.
Greg, are you planning to use regular solar panels dedicated to run AC fans? The fans I was asking about have small panels built into them, typically in the 20W, 25W or 30W range that power that single fans, usually an 18V DC motor.

None-the-less I got my original question answered to my satisfaction and ordered a pair of AC fans today, 204W each that claim 1200 CFM each. I plan to wire them to have one suck air into a north facing gable and one to exhaust at the other end of the house from the south facing roof. I simply do not have enough room to place adequate area of passive intake vents.

I had hoped the solar fans would work and not cost me money on my utility bill and be quieter since the roof one will be above our bedroom, but I just could not believe they would move enough air to do the job and I only have so much energy (myself) to expend on installing fans.