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I currently have a range that was hacked in terribly by a previous contractor with flex pipe. I want to replace it with rigid duct to make it quieter and increase performance too hopefully.

The range has a 7" circular output. Right now it uses a reducer immediately from the output of the range from 6" (with a 1" gap covered with foil tape) to 4" flex and then up through a cabinet and then 90 degrees out to the side of the wall - approximately a 5 foot run. It properly should have a 7" to 4" reducer so there is no gap. I actually only see a 7" to 6" at the local store so I'd probably have to put that with the existing 6" to 4" to eliminate the gap.

I notice a lot of back draft coming out from the range fans. It does suck air but there is also back flow blowing out from the range. This is from the large reduction I'm assuming.

To make matters worse the rigid run out to the wall needs to jog over to the wall opening a good foot or so. Now it's flex tubing so that's not a problem, however I imagine if I go rigid duct, I'll need a pair of 90 deg turns potentially to make it over to the wall opening.

Questions:

  1. Would there be less back draft and better performance if all the ducting was 7" until it goes out to the wall and have the reduction to 4" there?
  2. Would it help even if it was 7" through the cabinet before the 90 degree turn, or will it make no difference in performance where the reduction occurs?
  3. I'm wondering if using a small amount of flex at the end would be OK?
  4. A small amount of flex is better than all flex?

Attached is a SketchUp pic of the current situation.

range duct

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  • Each elbow increases resistance, so straighter is better(within reason). 45s are better than 90s. 7 to 4 inch might be a problem also. It will depend on what the hood requires. 7 inch seems odd size if you only need 4 inch. A mix of flex/rigid should not matter much. Flex will make more gentle curves.
    – crip659
    Commented Jan 15 at 20:17
  • @crip659 well the hood requires/wants 7" the entire way, but b/c of the clearance to the roof the contractor would not put in larger than a 4" hole in the wall. That's why I'm wondering if if makes any difference where the reduction occurs. Immediately out of the range in the cabinet? Or right before the wall exhaust, so that the bulk of the ducting with be 7" instead of 4"?
    – cyboman
    Commented Jan 15 at 20:40
  • Elbows plus the reduction makes the fans work much harder. I think the reduction at the end is better(but not sure). The reason for the four inch limit might be important. A limit because that is the biggest that can safety be made in a support member is good, because that was the biggest hole saw they had, not so good.
    – crip659
    Commented Jan 15 at 21:00
  • @crip649 Thx for your feedback. The reduction at the end seems better to me also, however then I think of the analogy of water through a hose and it seems to suggest it doesn't really matter where the reduction occurs, the net reduction in flow will be the same. With the reduction at the end, the fan might start off with less back pressure as it fills the whole 7" duct, but when it hits that end reducer it's gonna be the same back pressure I think.
    – cyboman
    Commented Jan 15 at 21:42
  • "Is this worth it?" can only be determined by you. Nobody else can determine whether the expense will be "worth" it you vs the time & money expenditure.
    – FreeMan
    Commented Jan 15 at 23:57

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