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Attic (Solar) fan efficiency claims?

LorenAmelang Member Posts: 4
edited May 2016 in THE MAIN WALL
The 2012 building code sets a minimum of 2.8 CFM/W for Residential Ventilating Fans - the same number that qualifies larger fans for EnergyStar (small fans can be even worse). Non-solar, AC powered through-the-roof attic vent fans seem to range from 2.8 to 6 CFM/W for traditional motors, up to 6 to 16 CFM/W for the newest models.

But if you look at solar powered fans:
5	 365	73
15	1007	67
20	1280	64
25	1450	58
30	1550	52
36	1628	45
60	1995	33
The mechanical designs don't look that different from my ancient AC powered fan. Its motor looks to be 18% efficient (425W for 1/10 Hp). I don't have a CFM spec for my fan, but the best similar designs do around 1500 CFM, so I suspect it does under 3.5 CFM/W. The latest fractional Hp ECM motors look to be 65% to 85% efficient, so a potential gain of under 5X with a new motor - maybe reaching 17 CFM/W.

Similar sized solar fans commonly claim three times that efficiency! What is the trick that nobody is calling them on?

Here's one AC powered gable (flat vertical surface) fan that matches the solar specs:
Energy Saving Brushless DC Motor
Power Consumption 30 Watts
Motor Speed 1,100 RPM
Air Flow @0.1" SP 1,560 CFM California Energy Commission Tested
CFM/WATT 52.0 - The CA Energy Commission Highest Ranked Efficiency Product!
Dimensions The housing is 14-3/8" ID, approximately 14-5/8" OD

Its mechanical design looks utterly conventional, the only difference from mine is that it is not facing a through-the-roof domed enclosure. I suspect gable fans are tested in open-air free space... For their models with roof flashings, the CFM/W drops - 35, 31, 27, 22, 19, and then they are down in the range I could reach with a new motor.

But the solar roof fans don't have the open air advantage. They face roof domes just like mine. How do they claim such high CFM/W numbers?

Of course CFM/W is not the ultimate goal. I suspect the need for cooling lags way behind the peak of solar input. In inland California with good insulation, the A/C seems to get hit hardest as the sun is fading away, and it continues to run well into the evening. I guess one could tilt the fan solar panel way to the West... But it seems a grid powered fan with sensors inside the attic and outdoors, and a differential controller with a bit of intelligence, could do a far better job.

My other top concern is sound level. The solar fans I've seen were far quieter than my ancient AC fan. But I suspect that's because they rarely run at full speed and specified CFM. Or maybe never do...

Does anyone know the details of how fan ratings are derived?


  • Rich_49
    Rich_49 Member Posts: 2,766
    What exactly is it that you want this fan to do , what will it's function be ?
    You didn't get what you didn't pay for and it will never be what you thought it would .
    Langans Plumbing & Heating LLC
    Serving most of New Jersey, Eastern Pa .
    Consultation, Design & Installation anywhere
    Rich McGrath 732-581-3833
  • LorenAmelang
    LorenAmelang Member Posts: 4
    I guess that's part of my question. The existing fan was there when we bought the house, in its hole through the roof, but it has become too noisy to leave it running. We're convinced it helps cool the attic, but that may be just because the intake "frieze block vent" area is about one-eighth of the current recommendation. (Which helps explain why the fan is noisy and worn out.)

    I plan to open up lots more passive intake vent area, but the passive exhaust vent area is also a bit low, so perhaps a fan might still be helpful. "Everybody" is telling us to get a solar fan, "they are wonderful, and quiet". But as I wrote above, I'm very suspicious of their claims - and I'm not anxious to tear up the roof to replace the whole fan housing assembly.

    So I'm trying to understand where the magic is. Can I just replace my existing motor with the best available ECM motor, and get the high CFM and low noise through my existing roof assembly? Buy that efficient gable fan and point it through the existing roof assembly?

    Can a pure DC motor be that much more efficient and quieter than an AC-sourced ECM motor? I can't imagine the magic is getting power from a tiny dedicated solar panel.

    Maybe once the new vent area is opened up, I just remove the old fan and leave the duct as a passive exhaust vent?
  • Rich_49
    Rich_49 Member Posts: 2,766
    There is no magic . The fan does not cool the attic , it brings air you paid to condition from the house into the attic causing you to spend more money on cooling . I guess if that does not matter to you , then yeah , it cools the attic .
    Spend your money sealing the ceiling and lowering the temp in that attic instead .



    You didn't get what you didn't pay for and it will never be what you thought it would .
    Langans Plumbing & Heating LLC
    Serving most of New Jersey, Eastern Pa .
    Consultation, Design & Installation anywhere
    Rich McGrath 732-581-3833
  • ChrisJ
    ChrisJ Member Posts: 15,442
    In my opinion,
    Just guessing here, they're lying.

    Just like shop vac manufacturers claim their 1/2 - 1 HP vacuum is magically 6 HP.

    There are some fancy fans out there, I use really expensive 120mm (5"?) fans in my computers that are much quieter than cheap ones and still move a lot of air, but they're nothing magical.

    I don't know how I feel about fans in an attic, if I had air conditioning equipment up there I may consider using fans but otherwise I think soffit vents, gable vents and a ridge vent are plenty.

    My own house has absolutely no ventilation in the attic as of yet. There's no equipment up there either though and fact is it's done fine for 150 years so I'm hesitant to mess with it.
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment