Limited-use, daytime-only system

Hi, everyone. I'm new to solar energy and new to the board.

I'm considering installing solar powered attic fans in my NW Florida home, not only for savings but also for operability after hurricanes knock out power for several days. I'll need up to 4 fans. Comparable AC fans are rated 2.8 amps and would cost at least $10 per month to operate. I've looked at the solar kits sold by several dealers, but they come with a 20-watt panel which would leave the fans rather underpowered, at best, as I figure it.

I should need <3 amps per fan at 12 volts (about 34W each). As a daytime only system, could I purchase one PV panel with enough wattage to supply all the fans and hook them directly to it, omitting the need for batteries, etc? Would you approach the problem another way?

Thanks for your inputs. This is a great board.


  • BB.BB. Super Moderators, Administrators Posts: 30,953 admin
    Re: Limited-use, daytime-only system

    If you look around here, I think that this has been discussed before... I believe that one person even though that using old electric automotive radiator fans from a junk yard was a great way to, cheaply and reliably, move the air.

    Yes, you can do this without batteries. You would have to check the ratings of fans vs the solar panels to make sure that you don't over voltage the fan motors (especially if they are electronically commutated--to avoid the whole issue of brush wear and replacement on DC motors).

    Of course, if you have windy afternoons on hot days, you could put some regular wind-turbines on the roof. Or even just open (with screening) more entry and exit points for the air to move out on its own.

    One thing to look at is the CFM and back pressure ratings of your fans... You talk about 3 amp AC and 2-3 amp DC fans. Remember, (sorry this is a bulletin board, so I will make the statement--that you probably already know well) is that the "power" for the AC fans is 10x that of the DC fans you are talking about here. 3amps*120VAC=360watts, vs 3amps*12VDC=36watts. Just so that you are not surprised that the DC fans don't move but a fraction of the air that an equivalent AC fan would.

    My weather is not too hot here--so generally I don't worry about (or have experience with) fans in my home. For those occasional bouts of hot weather we get, I somethings think a whole-house fan would be nice--instead of a window A/C unit.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2System2 Posts: 6,290 admin
    Re: Limited-use, daytime-only system

    Thanks, BB. Yes, I had seen the other thread about it. I originally put my questions there but got no replies. But, no, I hadn't considered the relative "power" of the fans. Now that you've pointed it out, it makes perfect sense: same amperage but 12V vs 115V. I guess what fooled me was that the CFM claims of both fans were nearly identical.
  • BB.BB. Super Moderators, Administrators Posts: 30,953 admin
    Re: Limited-use, daytime-only system

    I don't know which fans exactly you are looking at... But, check the charts (back pressure vs volume). My guess is that the AC fans are much better at providing volume against back pressure.

    Low voltage DC fans are generally pretty pathetic when there is back pressure involved. Also watch the scales on the charts--frequently they change scale factors (or even measurement units) across product lines.

    Also, in researching another DC Fan question here--I was looking for voltage ranges for generic DC brushless fans and found that the 12 VDC range with a low voltage of 4-8 VDC and a high range of 13.2-13.8 VDC might be an issue with solar panels--especially if they are oversized (in voltage). Many "12 vdc" rated panels are actually running 17 vdc or higher and I wonder if that is just too much for the typical unregulated 12 vdc fan...

    However, I checked the 48 VDC rating for Delta Fans, and saw it was something like 32-80 VDC... Get the right panel(s) and that would be a nice wide range to run with an unregulated panel.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • RoderickRoderick Solar Expert Posts: 253 ✭✭
    Re: Limited-use, daytime-only system

    I had a solar attic fan put in about 7 years ago. We noticed an immediate cooling to the house, to the tune of 10 degrees F. Going 90 to 80 is a big deal. The fan broke after about 2-3 years. Its integrated panel was a flexible, amorphous silicon pad about one foot square. What happened was that the panel stopped producing electricity; the fan motor was fine.

    Now, the good news. We didn't even notice that the fan had stopped turning. Just having an opening near the ridge of the roof was enough to cool the house by convection alone. I was going to connect a good-quality rigid panel to the fan, but I'm now thinking there's no need.

    My neighbor put in those turret-type vents on their roof (they look like a big Jiffy-Pop), maybe 3 years before we put in the solar attic fan. Those turrets are still turning every time a breeze blows. I would go for those if you have any wind at all. Remember, even if they're not turning, they're still openings in your roof through which convective heat can rise and escape.

    A final thought - when you place your vents, consider whether you will be blocking or shading space that you might later want to use for solar hot water or solar electric.
  • System2System2 Posts: 6,290 admin
    Re: Limited-use, daytime-only system

    Thanks for all the advice. Roderick, it's funny you bring up that point. I did some calculations and found that the free area ratio between soffit and ridge like 80-1. My roof has ridge vents, but a hip roof has so little ridge and so much perimeter that the pressure just builds from the ridge down until until it eventually pushes the hot air out the soffits. (I checked this with a smoke bomb.) So, more opening at the top of the roof is definitely needed.

    It's amazing, though, how generic the information is about attic ventilation and how much of it seems driven by what someone is selling. I've computed attic area, volume, and researched characteristics of the spaces under complex roofs. But at the end of the day, there's just very little info other than the standard 300-1 area-to-vent ratio (or 150-1, depending on who you ask).

    I may just save the solar plans for our next house, as moving to a new job is likely. But I still appreciate everyone's inputs and the information on this board. Thanks, everyone.

    Helicopters don't fly; they beat the air into submission!
  • BB.BB. Super Moderators, Administrators Posts: 30,953 admin
    Re: Limited-use, daytime-only system

    Here seems to be a good place to start for numbers:
    Natural ventilation caused by a chimney effect or by wind movement is the most economical ventilation method. The quantity of ventilation air depends on opening size, temperature rise and wind movement. Tests have shown that effective natural ventilation can be achieved by providing inlet and outlet open vent areas of approximately 1 square inch per square foot of attic area, when roof slopes of 3/12 to 5/12 are used. This vent area should be the net open area rather than gross vent size. Some prefabricated vent materials may have only 60 percent net open area. Thus; if a vent area of 10.4 square feet is required, the gross vent area would be 10.4 / 0.6 = 17.3 square feet. [National standards appear to require only 1/2 this amount of open vent area. The open vent area calculated by these formulas is to be divided roughly equally between inlet and outlet.]

    Suppose a 1,500-square-foot home requires the above soffit vent area. If the total soffit length of the home is 100 feet, the vent width for a continuous soffit vent would be (17.3 x 12) / 100 = 2.1 inches. Ventilators should be purchased on the basis of net ventilation opening.

    Roof overhangs of 12 inches or more provide ample space for inlet (soffit) vents. Outlet vents should have the same net area as inlet vents. This is easily provided by continuous ridge vents or individual roof vents for gable-roofed homes. The roof ridge length may not be long enough on a hip roof to use a continuous ridge ventilator. In this case, several individual roof vents, well spaced near the ridge, should be used.

    Combine with this Powered Vent calculation:
    Powered attic ventilators should provide at least 10
    air changes/hr. Multiply the total square footage of the
    attic by 0.7 to get the minimum CFM necessary. For
    dark roofs, add 15% to this number. For particularly
    steep roofs, a slightly higher rating is recommended.
    Only soffit vents should be used as fresh-air intakes
    for powered attic ventilators. Gable vents should not
    be used, as rain and snow can be drawn into the attic.
    To calculate the total minimum vent intake area, divide
    the CFM of the powered attic ventilator by 300
    and multiply the result by 144. This will give you the
    net free area, in square inches, that should be installed.
    A minimum of one square foot of inlet area for
    every 300 CFM of HVI Certified fan capacity is required
    for proper fan operation.

    Say 1,000 sq foot attic-- 1" per sq foot = 1,000 net free sq" = 6.94 sqft for natural ventilation (7 sqft inlet and 7 sqft outlet)

    1,000 sq foot attic fan -- 0.7 * 1,000 sq foot = 700 cfm
    700 CFM / 300 cfm per sq foot of inlet = 2.33 sqft (inlet net free area)

    (2.33/6.94) * 300 LFM (linear f/m) => implies natural airflow is ~100 LFM or 100 CFM per 1 sqft Net Free opening

    Seems about right to me (I would not expect more than 100 Linear Feet per Minute air flow via natural circulation). Using a powered ventilator can reduce the vent sized by a factor of 3. If you can have lots of natural air flow vents (per above recommendations), then it would appear that this is a good place to start... Remember that screened/protected vent net free opening may only be 0.6 of raw opening.

    Also, I would believe that a solar powered DC vent fan would need larger inlet vents (reduced backpressure) as they probably cannot draw 1/8" of water and draw 300 LFM (300 CFM/sqFt) through a vent opening.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
Sign In or Register to comment.