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Thread: Downside of a More Powerful Attic Fan?

  1. #1

    Default Downside of a More Powerful Attic Fan?

    Our attic fan worked only 2 or 3 years before making noises. And its thermostat failed to work correctly now. It would usually be on unless we manually switch it off - luckily we added a switch when first installing it.

    So, we want to upgrade it to a slightly better and hopefully more durable model (from a 1000 CFM model to the 1250 CFM model). However, the better and more durable model would also mean a more powerful fan motor designed for a bigger attic, and that would require more soffit vents which we don't have.

    What would happen if we install the more powerful attic fan anyway (without adding more intake vents)? We don't have air conditioning in our house, so I don't think we would lose any cooled air from inside the house (getting sucked up into the attic through leaks by the more powerful attic fan).

    Is there any other downside to using the overpowered attic fan?

  2. #2
    In the Trades Bob NH's Avatar
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    A more powerful fan will use more kWh. It could also be noisier, but the new model might be quieter.

    If it is a variable speed or belt-driven fan you can adjust the speed to will reduce the CFM.

    Is the air coming in the soffit vents or going out? Fans are often set up to pull in through the soffit vents and exhaust out the gables or roof.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    A lot of times, with an attic vent, people pull air from the house and exhaust it through the attic to the outside.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  4. #4

    Default Bob

    Quote Originally Posted by Bob NH View Post
    A more powerful fan will use more kWh. It could also be noisier, but the new model might be quieter.

    If it is a variable speed or belt-driven fan you can adjust the speed to will reduce the CFM.

    Is the air coming in the soffit vents or going out? Fans are often set up to pull in through the soffit vents and exhaust out the gables or roof.
    Hi Bob, thanks for your reply.

    No, the model I am looking at isn't a variable speed (or belt-driven?) fan. The CFM can't be reduced. Will it create some kind of a vacuum or suction in the attic and do unexpected things to our house?

    Yes, the soffit vents will all be used as intake vents (even then there are still not enough intake vents when a more powerful attic fan is used). The fan will pull in air through all the soffit vents and exhaust out the roof. Since the better and more powerful model requires more soffit vents than our smaller attic has (the more powerful fan is designed for a bigger attic), it creates a mismatch. When there is intake restriction because of insufficient soffit vents, will it overwork or heat up the fan motor?

  5. #5

    Default jadnashua

    Quote Originally Posted by jadnashua View Post
    A lot of times, with an attic vent, people pull air from the house and exhaust it through the attic to the outside.
    Hi jadnashua,

    Thank you for your help and suggestion.

    According to your suggestion, I would then be doubling the function of the more powerful attic fan and using it as a whole house fan too (besides using it as an attic fan)?

    The attic fan will be mounted on the roof. Would I then need to add some type of vents on our ceiling (between the interior of the house and the attic) to allow air flow from the house through the attic, the roof, to the outside? What type of vents would I need to put on the ceiling?

    Should I make sure that some windows will be left open when the fan is operating? What else should I look out for when using the attic fan partially as a whole house fan?

  6. #6
    In the Trades Bob NH's Avatar
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    Restricting the flow of the fan actually reduces the power because it reduces mass of air being driven by the fan. It is like the effect that you will notice if your shop vacuum gets plugged; the blower runs faster.

    If your fan is the type with the typical fan blade then it won't develop enough pressure to affect the house.

    If you have the "water column" pressure data for the fan you could get some idea of the effect. One inch of "water column" is about 5 pounds per square foot. That is less than the design load for any ceiling or roof. In many cases it will not say "water column" but will just give the number.

  7. #7
    Retired Defense Industry Engineer jadnashua's Avatar
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    TO use it as a whole house fan, yes, you'd need a vent into the house and leave a window or two open. If you have an access to the attic, you could leave that open and see if it helped. People often leave them running all night to bring in the cooler night air. You might not want to do that during the hot part of the day and restrict it to just venting the attic.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  8. #8

    Default Bob NH and jadnashua

    Hi Bob and jadnashua,

    Thank you both for your continuous help.

    Bob, I tried but couldn't find the pressure data for the attic fan. Can I mention Company, Brand or product names (and their links for info) in this forum? If yes, I can just say them and that should make it easier for you to help me more.

    jadnashua, what is the logic behind "you might not want to do that (using it as a whole house fan) during the hot part of the day and restrict it to just venting the attic"? During the hot part of the day, you would want to focus on getting rid of the hottest air in the attic first and foremost, and not to worry about the warm/not-so-hot air inside the house (the outside air would probably be also warm then and won't help much anyway)?

    From the discussion here, it seems that putting in a more powerful (overpowered) attic fan won't hurt anyway, if we just watch the few things mentioned above.

    But why does the manufacturer say on the product box or info sheet things like: "Remember: Always have a balanced ventilation system. In no case should the amount of exhaust ventilation exceed the amount of intake ventilation."? Do they say that not to avoid failure or disaster, but just in order to achieve optimal performance?

    And what do they mean by "the amount of exhaust ventilation" and "the amount of intake ventilation"? What do they mean by "AMOUNT"? Do they mean the venting AREA (OPENINGS)? But part of this system is now POWERED. So, by AMOUNT, do they mean CFM (flow rate)? When there are not enough soffit vents to serve as intake for the more powerful exhaust fan, would air just rush through the soffit vents FASTER to make up for the less area than required? Sure, there may be some pressure differentials created here and there, but they should NOT be enough to create any failures or premature failures, right?

    Thanks again for your continuous great help!

  9. #9
    Retired Defense Industry Engineer jadnashua's Avatar
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    If you ran the fan all night and brought cool air into the house, you may want to then close off the house. You could still ventilate the attic, but pulling in hotter outside air probably would end up heating the house that was pleasantly cool from the night air.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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