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Thread: Booster fan off while drying... Is this Bad?

  1. #1
    DIY Junior Member o_mega05's Avatar
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    Default Booster fan off while drying... Is this Bad?

    Hi All,

    Is it bad to turn off the booster fan/inline exhaust fan while using the dryer? The reason being, I live in a townhome, and the furnace sits right next to the neighbors wall. Whenever we run the dryer, they hear and feel loud rattling and vibration from the booster fan.

    Am I causing any harm by turning off the booster fan while the dryer's running? While trying to research the problem, it seems like the booster fan is more of an enhancement to increase better air flow, thus drying clothes faster and efficiently.

    I appreciate any feedback on this matter. Thanks in advance.

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    Plumber jimbo's Avatar
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    What has the furnace got to do with this? If the booster fan is for the dryer exhaust, then drying performance will be serriously affected if you do not run it.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    FWIW, if the booster fan makes a lot of noise and vibrates, it may be time to clean it! By its nature, even with the filter in a dryer, some lint gets into the ductwork. It can cake onto the fan blades, and when it does, several bad things can happen: vibrations, excess wear, and it could create a potential fire hazard. A booster fan is normally only required if the duct run exceeds the manufacturer's specifications. Depending on the type, not running it could add even more restriction and create more problems.
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Junior Member o_mega05's Avatar
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    Thanks for the replies.

    @jimbo, sorry for the newbie knowledge. My booster fan ductwork is placed right next to the furnace, so it appears like it's connected. I'm a new homeowner so this is all foreign to me.

    @jadnashua, I was planning on taking apart the fan myself and cleaning it since every appliance service/repair shop in my area don't seem to deal with booster fans. A "fire"was my major concern so I will make sure to address this ASAP. Thanks again for this helpful insight.

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    In the trades Dana's Avatar
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    Quote Originally Posted by jimbo View Post
    What has the furnace got to do with this? If the booster fan is for the dryer exhaust, then drying performance will be serriously affected if you do not run it.
    Even dryers without booster fans will often back draft atmospheric drafted furnaces & hot water heaters in small or fairly tight houses. With booster fans it could be even worse, and it should be tested before making any further decisions. Ideally this would have all been tested as part of the commissioning of the system(s), but "should haves" don't always conform to real-world practices. Before re-commissioning the booster it's good to first verify that there's no spillage on any atmospheric combustion equipment in the house when all kitchen/bath fans + the dryer is running.

    If the dryer vent piping is longer than code for operation without a booster, it could indeed present a fire hazard (higher with a gas-fired dryer than with an electric, but still non-zero for the electric.) If there's no way to run it (with or without booster fan) without a backdrafting/spillage risk, it may be time to spring for an unvented heat-pump clothes dryer. Even though they're a bit slower to dry than conventional models, they're more efficient on total energy use than old-school dryers, and they don't have booster fan noise or backdrafting issues to contend with.

    If the spillage issues are OK and it really needs the booster, using 8-12" sections of flex-duct between the fan and the hard-piped sections of vent will reduce the mechanical vibe/noise issues.

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    DIY Junior Member o_mega05's Avatar
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    @Dana, awesome info, much appreciated. I live in a tri-level townhome so the booster fan was necessary as it vents out from the second to the third floor. The noise was an issue with the neighbors upon moving in a few years ago and elevated to vibration just recently. Upon reading more and more about booster fans, the first thing I took from it, was to clean it on a regular basis.

    Can you explain spillage and backdrafting a bit more? Thanks.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Any burner in your house, unless it is in a closed combustion situation (where it draws its own combustion air from outside and exhausts it in a totally closed vent) relies on the house being neither pressurized nor in a vacuum situation. A dryer does not use air from outside, but blows a LOT of air out - this must come from within the house. Depending on how well sealed the house is, it could find the easiest path to pull in air is through things like the flue from your furnace or gas water heater. Those sources could easily have exhaust gasses from their burners in it...now, you're pulling in exhaust fumes. If your house is all electric, with no burners, then while this is a HVAC issue, it's not particularly a health/safety issue. Essentially, when you exhaust air, you are pulling in air from somewhere - this could be leaks in the house, an open window/door, or worst case from the chimney of other devices.

    When the air through the dryer is restricted - the flow slows, the temperature inside and in the flue will rise. The thermostat may or may not be able to compensate properly with reduced air flow. Other than the potential backdrafting, this is the other big potential problem with any dryer.
    Last edited by jadnashua; 03-18-2013 at 10:22 AM.
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    In the trades Dana's Avatar
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    What Jim said- exhaust fans and dryers de-pressurize the house. If the lowest-impedance path for the air to enter the house to equalize the pressure is an un-powered water heater or furnace flue, it can suck exhaust products from those burners into the house, which is a serious health hazard.

    If the exhaust venting on the furnace & water heater are driven by blowers on the unit (always the case for condensing burners, not so much for 80-83% efficiency burners) you're fine, even without ducted-in combustion air. But if it's relying on the buoyancy of the hotter exhaust for getting up the stack, you're potentially susceptible. All atmospheric-drafted burners have an opening to dilute the exhuast product with room air to lower the dew point and reduce condensation in the flue, but that also provides a path for air to move down, the flue into the room if the house is sufficiently de-pressurized by dryers & exhaust fans.

    A draft-hood over the exhaust port on an atmospherically drafted hot water heater looks like this (color & exact shape will vary):



    On hot air furnaces the dilution-draft openings are usually incorporated into the cabinet sheet metal. Schematically it looks like this:



    But details & appearance will vary by make and model.

    Almost all units with 85%+ efficiency will have draft-inducer blowers, even if they're not condensing 90%+ units, so the name-plate efficiency numbers may give us a clue, if you can't figure it out in other ways.

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