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Thread: Partially closed vents - why a bad idea?

  1. #1
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    Partially closed vents - why a bad idea?

    Hello Pros,
    Firstly, thanks to all the pros and other forum members for always taking time to reply to questions here.

    I was curious about the general advise here that discourages closing of supply vents as a room temperature management strategy.
    I understand closing a non-trivial percentage of vents/registers will be detrimental to the system. However, is this strategy still bad in the following case ?

    Assume 2 rooms - R1 and R2 connected to the same main duct. Cool air travels first to R1 and then to R2. However, say due to improper duct work/sizing they get different air-flow through their supply registers ..V1 and V2 (V1 >> V2). Further assume the return vent is in the common area (and not individually situated in R1 and R2) and the room R1 is not used at all.

    In the above case, why partially closing V1 to allow more air flow through V2 is a bad idea ?
    Isn't this essentially "balancing" air distribution without using an actual damper.

    To stretch this further, why should V1 be not fully closed, if there's a third room (R3) connected to the same duct and similar to R2 is not getting enough air ?

    I understand the "real fix" will be a better duct work. Also, I'm not suggesting that every other vent in the house should be closed like this.

    Just trying to understand the reasoning why closing a small number of vents to manage air distribution (as a quick-fix) is a bad idea.

    Thanks again !

  2. #2
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    To be perfectly honest I didn’t take the time to completely read that long of a post but I did try and get the gist of it.

    Basically closing vents is perfectly fine. However, if you create too much of a restriction then you’ll lower the total CFM across the evaporator too much, and that is bad. Does that answer your question or am I missing something?
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  3. #3
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    Without test equipment your guessing.
    Better to get someone on site with the equipment. Check the TESP and see if there’s wiggle room. If not then you’ll need duct work.

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  5. #4
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    Quote Originally Posted by Froodo View Post
    Hello Pros,
    Firstly, thanks to all the pros and other forum members for always taking time to reply to questions here.

    I was curious about the general advise here that discourages closing of supply vents as a room temperature management strategy.
    I understand closing a non-trivial percentage of vents/registers will be detrimental to the system. However, is this strategy still bad in the following case ?

    Assume 2 rooms - R1 and R2 connected to the same main duct. Cool air travels first to R1 and then to R2. However, say due to improper duct work/sizing they get different air-flow through their supply registers ..V1 and V2 (V1 >> V2). Further assume the return vent is in the common area (and not individually situated in R1 and R2) and the room R1 is not used at all.

    In the above case, why partially closing V1 to allow more air flow through V2 is a bad idea ?
    Isn't this essentially "balancing" air distribution without using an actual damper.

    To stretch this further, why should V1 be not fully closed, if there's a third room (R3) connected to the same duct and similar to R2 is not getting enough air ?

    I understand the "real fix" will be a better duct work. Also, I'm not suggesting that every other vent in the house should be closed like this.

    Just trying to understand the reasoning why closing a small number of vents to manage air distribution (as a quick-fix) is a bad idea.

    Thanks again !
    closing off vents doesn't necessarily make more air go through other vents. Typically it just put more back pressure on the blower and the blower cant over come it so you essentially just create a restriction. Most HVAC systems are already restricted when installed due to lack of knowledge and poor practices.

    However some scenarios it might work. And some blower motor have the ability to overcome more resistance than others.

    So lots of variables but in general its a bad idea....

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  7. #5
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    Thread Starter
    Quote Originally Posted by jacob-k View Post
    closing off vents doesn't necessarily make more air go through other vents. Typically it just put more back pressure on the blower and the blower cant over come it so you essentially just create a restriction. Most HVAC systems are already restricted when installed due to lack of knowledge and poor practices.

    However some scenarios it might work. And some blower motor have the ability to overcome more resistance than others.

    So lots of variables but in general its a bad idea....
    Quite interesting!
    I then wonder, how installing a balancing damper ( a more preferred approach, I guess) is different? Should it not similarly create more restriction and not necessarily allow to go through other vents?

  8. #6
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    Thread Starter
    Quote Originally Posted by JayMan7 View Post
    To be perfectly honest I didn’t take the time to completely read that long of a post but I did try and get the gist of it.

    Basically closing vents is perfectly fine. However, if you create too much of a restriction then you’ll lower the total CFM across the evaporator too much, and that is bad. Does that answer your question or am I missing something?
    Yes it does. Thanks for replying.

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  10. #7
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    Quote Originally Posted by Froodo View Post
    Quite interesting!
    I then wonder, how installing a balancing damper ( a more preferred approach, I guess) is different? Should it not similarly create more restriction and not necessarily allow to go through other vents?
    It may or may not be....

    Really you want to know what the static pressure of your system is prior to doing any of this....

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