For the engineers here.

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  • vaking
    Veteran Member
    • Apr 2005
    • 1428
    • Montclair, NJ, USA.
    • Ryobi BT3100-1

    For the engineers here.

    I am looking at my attic insulation and ventilation and came across things I have hard time understanding.
    I have a fan in the roof in the attic to exhaust hot air to prevent attic from overheating. Pretty standard feature, 14" diameter fan operated by thermostat - I am sure many people here have something similar. Current fashion recommends using solar powered fan in such situation and without thermostat. The topic of fan in the attic lately became quite controversial. If you google "attic ventilation" you will find plenty of articles saying that attic fan is useless and unnecessary and should not exist at all - regardless what powers it. Some places (like State of Georgia) outlawed use of attic fans except solar powered. I looked at technical characteristics and got puzzled. Most common traditional attic fans are designed to operate on 115Volts, using 3.5 - 4 Amps, so it consumes 400 - 450 Watts. Such fans are rated for about 1000 - 1600 CFM. But then you look at solar powered fans and those claim to have 500 - 2500 CFM even though they are connected to solar panels rated for 20 - 60 watts and equipped with motors rated at 10 - 40 Watts. Leaving aside loaded discussions about advantages of solar power or fan in the attic - I am just wondering - how is it possible that a fan with 10 watt motor have similar air-moving capacity as another fan with similar dimensions but with 350 watt motor?
    Alex V
  • JimD
    Veteran Member
    • Feb 2003
    • 4187
    • Lexington, SC.

    #2
    Alex,

    The answer is the one I think you are suspecting. 60 watts is so much less than 350 watts that there is no question the AC powered fan moves a lot more air. Electric motors are all quite efficient so there is no chance the solar unit is significantly more efficient. It is probably less efficient since it operates at low voltage. The fan is more of an opportunity for optimization but even so, you can't gain enough to make the solar unit similar. Another thing working against the solar unit is the fact that the sun doesn't shine at peak levels all the time. It is undoubtedly rated at optimal conditions which rarely exist. So whatever it can do, it won't do it often. Most of the time it will produce much less effect.

    I had the builder add a thermostatically controlled fan to a house we had built when moving to SC in 1999. It had good soffit vents but, due to the roof profile, not much ridge venting. So we added the fan. I switched if off in the winter. When possible, I think the best thing to do is to have adequate ridge and soffit vents and not use mechanical ventilation. But when you need more, a powered ventilator is valid option. I would be leery of solar powered units, however. Inexpensive solar cells tend to not be very efficient and I wonder how long they will last.

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    • capncarl
      Veteran Member
      • Jan 2007
      • 3564
      • Leesburg Georgia USA
      • SawStop CTS

      #3
      If you wonder why the 110v vent fans were outlawed just take a look at them. The retail price was about $79, open motor,cheap thermo-witch and metal only slightly thicker than beer can metal, it was a wonder every house they were installed in didn't burn down! My house went through 6-8 that ran 24-7 from April till October in 170 degrees. All eventually burned up or melted down, literally. Finally went to ridge vents.
      capncarl

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      • cwsmith
        Veteran Member
        • Dec 2005
        • 2737
        • NY Southern Tier, USA.
        • BT3100-1

        #4
        Perhaps not an obvious reason, or even something that may have any significance whatsoever. Perhaps there's been little if any data recorded of such events, but I do know of two occasions where vent fans were the target of lightning strikes. Both of those were in the small village that we used to live. Just those two events in a small population area would lead me to think of them as a possible concern.

        Vent fans are generally installed at the high point in a house, and the electrical ground would offer a potential path for attraction.

        Bigger concern would be that the few vent fans that I've seen, as 'capncarl' described, were pretty cheaply made. There are exceptions of course, but I do think overheating, poor installation, etc. would provide enough cause that would lead to concerns.

        Merry Christmas to everyone,

        CWS
        Think it Through Before You Do!

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