Duct Reducer for Bathroom Venting Fan?

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  #1  
Old 10-30-07, 05:44 AM
D
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Question Duct Reducer for Bathroom Venting Fan?

I just installed a 110 CFM bathroom venting fan and am ready to install the ductwork to the outside.

The fan instructions say to "connect 4-inch round ductwork to damper / duct connector ... to a roof cap or wall cap," which is easy enough.

However, I noticed last night at Lowes that the fan manufacturer (Nutone) also sells a venting-fan-ductwork kit that contains a 4" to 3" reducer.

So, I have two questions:

1. What is the function of the reducer?
2. Do I need it for my installation?

BTW, for my configuration, the run from the fan to the wall is only about 4 feet.

Thanks.
 
  #2  
Old 10-30-07, 06:17 AM
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Reducer

The reducer allows you to connect a 3 inch pipe to the 4 inch outlet port on the fan. What size pipe do you want to use?
 
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Old 10-30-07, 06:49 AM
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You would use the reducer if you had an existing installation with a three inch vent and did not want to change the vent. If you are installing a new vent, make it a four inch one.
 
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Old 10-30-07, 10:58 AM
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You want the least air resistance as possible. That means you want a 4" pipe rather than a 3", you want to use a smooth, solid metal or PVC pipe (not corrugated flexible pipe), you want as few bends as possible, you want the pipe to be as short as possible, and you want it to go mostly horizontally and as little vertically as possible.
 
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Old 10-30-07, 04:42 PM
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What Jonh said. Since you are increasing the size of the fan, you should increase the duct to allow the improved air flow. The reducer is in the event that replacing the old 3" is a very hard(read impossible) thing to do.
 
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Old 11-02-07, 12:21 PM
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No reducer for you

Thanks everyone for the feedback.

I was planning on using 4" PVC.

Interestly, the manufacturer pooh-pooh'd flex-duct, yet they sell a DIY-kit that contains exactly that!

Anyway, Originally I was going to go straight up, which is a run to the roof of about 6 feet, with one "L" at the junction between the pipe and fan-damper. This is a vertical run.

 / \
/   \
     \
      \
      |\
      | \R
      |  \O
      |   \O
      |    \F
(fan)_|     \
             \


The horizontal run would have not have an "L", but the roof vent would be just above the gutter line on the roof.


 / \
/   \
     \
      \
       \
        \R
         \O
          \O
           \F
(fan)_______\
             \


An alternative would be to introduce two L's to bring it out horizontally but further away from the gutter line. This would also be about a 6 feet (total run).

         ___\
         |   \
         |    \
(fan)____|     \
                \


Another alternative is to run it out the other side of the house to a wall vent, which would keep it completely horizontal with one "L" to turn it to 90 degrees. However, this would introduce a run of about 10-11 feet total.

I'm not sure what is more difficult/costly (or susceptible to problems/leakage/etc) - a roof vent; or a wall vent over vinyl siding. I'm probably going to have someone do that part anyway since I'm "height challenged."
 
 

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