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  1. #1
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    Question duct size question - can i exhaust a 6" duct out of a 4" hole?

    in the plans for my basement remodel, the architect has called for two medium/high volume fans. one is to meet code to technically be a 'whole house fan' - which i can't see that i will ever really use. the other is a shop-fan - to exhaust any odors from chemicals etc from my workshop - which i will probably use quite a bit.

    both fans are panasonic 150 cfm fans (http://tinyurl.com/5gtqtr
    ) with 6" duct. in both locations i have a straight shot - maybe 3 or 4 feet out of the rim joist (no bends necessary). i'd rather not cut a 6" hole in the rim joist - so my question is - if i do a direct 6" duct run - can i use a 6-4" reducer at the end and the go out of a 4" vent?

    or is such a reduction going to create too much noise/pressure from the restriction.

    thanks for any pointers,
    -brett

  2. #2
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    Default Re: duct size question - can i exhaust a 6" duct out of a 4" hole?

    or is such a reduction going to create too much noise/pressure from the restriction.
    I'm thinking it would .... as well it would reduce the CFM and put strain on the of the exhaust fan shortening it's life.

    Think of a funnel with it's wider mouth and the smaller spout .... it's the size of the spout that controls how much flow will occur.

    Or ... blow through a straw and realize how much back pressure there is when reducing the opening on the other end.

    The size of the duct has a finite capacity for CFM....

    4 inch - approx. 100 CFM
    5 inch - approx. 200 CFM
    6 inch - approx. 300 CFM

    The fan you are using is spec'ed at 150 CFM with a 6 inch duct.
    With the description of a 3 or 4 foot run with no elbows this would be the ideal case where the fan would be running at or near capacity.

    Now reducing the exhaust opening down to 4 inch which only has the capacity to pass 100 CFM would create back pressure and effectively reduce the exhausting down to at least 100 CFM ( probably less ). This back pressure would also put a strain on the fan motor since it's trying to move more air than is flowing .... so the motor will over work and wear out quicker.

    I'm thinking of one of two other options ....

    Down sizing the fan to the next model of 110 CFM using the 4 inch duct and with the short run without elbows would allow flow at or near the rated capacity. This would be around the same output as the method you propose and with less strain on the fan motor.

    Or ...

    Using the proposed fan but with reduced to a 5 inch duct would allow for the CFM to be nearer to the capacity needed. This would still result in a back pressure on the fan but certainly more beneficial than reducing down to the 4 inch.

    However .... if it's mandatory to need the 150 CFm you might try contacting the manufacturer to see what size of ducting would work as a compromise.

    Side note ....
    You mentioned this exhaust fan would be located in the basement.
    Since the exhaust fan will be removing air you may have to consider a source for " make up " air to replace that which is being removed. This should be considered if there are heating and /or hot water heaters that use combustion.

    The exhaust fans can create negative pressures affecting the exhausting of those combustion devices .... affecting the drafts for venting.
    The negative pressure that can exist can prevent those combustion devices from operating if too much air is taken away for their firing or you could have back drafting in chimneys


    Just a thought.
    "" an ounce of perception -- a pound of obscure "
    - Rush

  3. #3
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    Default Re: duct size question - can i exhaust a 6" duct out of a 4" hole?

    hhmm.. I suspect you're right. Perhaps I can convert to a rectangular duct and cut a 4x7 hole in the RIM joist vs. a 6" round. I guess it's time to talk to my structural engineer again - thanks for the tips.

    Regarding the negative pressure issue - i'm planning on leaving plenty of badly installed gaps between the doors and windows ... just kidding... we're installing some passive inlet vents - eg. http://tinyurl.com/69m87g to meet code.

    Also - we only have one appliance in the basement - the indirect gas boiler - which has it's own inlet vent.

    Thanks for all great detail - you know a lot about this stuff!

  4. #4
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    Default Re: duct size question - can i exhaust a 6" duct out of a 4" hole?

    fyi - for others reading this thread. our structural engineer approved cutting a 5" circular vent centered in the rim joist - so i just need to reduce to 5" at the end which should be fine CFM-wise.

  5. #5
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    Thumbs up Re: duct size question - can i exhaust a 6" duct out of a 4" hole?

    our structural engineer approved cutting a 5" circular vent centered in the rim joist
    Thanks for the update ... good stuff.

    My hats off to you working with an engineer throughout your projects.

    Regarding the negative pressure issue - i'm planning on leaving plenty of badly installed gaps between the doors and windows ... just kidding... we're installing some passive inlet vents - eg. http://tinyurl.com/69m87g to meet code.
    ....LOL .... humour is always welcomed

    I see you were steps ahead of me ( not hard to do ) thinking of make up air.
    "" an ounce of perception -- a pound of obscure "
    - Rush

  6. #6
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    Default Re: duct size question - can i exhaust a 6" duct out of a 4" hole?

    Quote Originally Posted by canuk View Post
    My hats off to you working with an engineer throughout your projects.
    thanks. he's actually a lot cheaper than the architect

    i'd rather spend an extra few hundred bucks to make sure i don't do something to make my house fall down

  7. #7
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    Default Re: duct size question - can i exhaust a 6" duct out of a 4" hole?

    Quote Originally Posted by brettmarl View Post
    thanks. he's actually a lot cheaper than the architect

    i'd rather spend an extra few hundred bucks to make sure i don't do something to make my house fall down
    Architects ... they .... ummm ... nope ... better not go there.

    Few hundred bucks could save thousands ..... it's money well spent
    "" an ounce of perception -- a pound of obscure "
    - Rush

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