I currently have an exhaust hood that produces a lot of back draft due to limitations of the ducting and a 4" diameter exhaust.

The hood has a 7" circular output. It uses a reducer immediately from the output to 4" and then up through a cabinet and then 90 degrees out to the side of the wall - approximately a 5 foot run.

I notice a lot of back draft coming out from the hood fans.


Would there be less back draft and better performance if all the ducting was 7" until it goes out to the wall?

Would it help even if it was 7" through the cabinet before (or after) the 90 degree turn, or will it make no difference in performance where the reduction occurs?

I was thinking since the reduction causes turbulence and negative pressure/work, that moving the reducer away from the fan should increase performance, however I was also thinking that the placement of the reducer in the system should have no effect on the net flow rate.

I played with some fluid mechanic simulations and my current thinking is that having a short run of 7" duct in the cabinet and then a reducer to 4" before the 90 degree elbow, then the 5 foot run of 4" to the wall would be best. Moving the reducer just a foot or so away from the fans gives some space for the air to begin accelerating into the reducer, now moving faster the air can take the 90 degree turn at a higher flow rate. As opposed to having the 90 degree elbow at 7" than the reducer after. This would also be easier to implement than 7" all the way to the wall.

Attached is a SketchUp pic of the current situation.

enter image description here

  • 1
    $\begingroup$ First I would change the 90 elbow to a swept elbow. $\endgroup$
    – Solar Mike
    Jan 17 at 17:39
  • $\begingroup$ this is like a narrow nozzle on a garden hose ... the size of the hose makes a big difference in water flow for the same nozzle $\endgroup$
    – jsotola
    Jan 17 at 17:40
  • $\begingroup$ a range is an appliance that combines oven and stove. I think you mean an exhaust hood. $\endgroup$
    – Tiger Guy
    Jan 17 at 20:54
  • $\begingroup$ is there a chimney on the outside? that might help too $\endgroup$
    – Pete W
    Jan 17 at 23:22
  • $\begingroup$ @SolarMike Thanks for the feedback. That's a good point. I did some initial searching and it seems hard to find that in 7" steel hvac ducting $\endgroup$
    – cyboman
    Jan 19 at 0:51

2 Answers 2


Would there be less back draft and better performance if all the ducting was 7" until it goes out to the wall?


Would it help even if it was 7" through the cabinet before (or after) the 90 degree turn

any section that was 7 inches would help the same amount.

Moving the reducer just a foot or so away from the fans gives some space for the air to begin accelerating into the reducer

You are way over-thinking this. It's an exhaust vent, not a Formu1a 1 diffuser.

A 7 inch pipe passes about three times as much air as a 4 inch one with an equivalent velocity, and it's velocity that leads to pressure drops. That said, this is not a guarantee that this will solve your problem, since it's not obvious that you have one.

  • $\begingroup$ Haha yes, I do think I'm over engineering this problem. Sometimes it's fun sorry if it's annoying! Your feedback helps and makes sense, I was thinking that the reduction will provide the same net back pressure at the source. no matter where in the system it occurs. After reading your post I'm thinking that the shorter the run of 4", the less work the fan has to do to "push" the air through. Such that having the reduction at the end, with the least amount of 4" duct, would yield the best performance. Does that sound right? $\endgroup$
    – cyboman
    Jan 19 at 0:50
  • $\begingroup$ @cyboman, it will be the same at the beginning or end $\endgroup$
    – Tiger Guy
    Jan 19 at 14:24
  • $\begingroup$ Can you clarify? This seems contradictory to you stating having 7" right to the wall would increase performance. $\endgroup$
    – cyboman
    Jan 19 at 18:13
  • $\begingroup$ @cyboman, the total head loss is the total of losses from all pipes, this is what determines how much air leaves the fan, 4" at the beginning or end makes no difference. $\endgroup$
    – Tiger Guy
    Jan 19 at 19:46
  • 1
    $\begingroup$ Right, to be clear, the least resistance and head loss is created by maximizing duct size throughout the system. In this scenario that's 7" right to the wall with the reducer to 4" at the end. $\endgroup$
    – cyboman
    Jan 21 at 2:02

The fan belongs somewhere in, or downstream of, the smallest section. Commercial range hoods have the fans on the roof. Then you have no backdrafting. If your fan is bigger than 7" and located in the hood upstream of the vent, you are bound to have problems unless you build a very precise cowl for the fan.

Does it matter where a reduction happens in a fluid system to the amount of back pressure at the source?

What matters is where the fan is. Put it far enough down the duct that there isn't any possibility of backflow returning up the duct to the work area.

  • $\begingroup$ Thank you for the great comment this helps! Thing is as you've alluded to, with the residential range hood, the fan placement is fixed at the intake of the system. I think the cowl route is likely outside my ability and budget. Given that, would you recommend running 7" duct for length of the system, and then reducing down to 4" at the wall? It seems to me this would provide the least resistance. $\endgroup$
    – cyboman
    Jan 20 at 0:15
  • $\begingroup$ Get a cheap bilge blower and stick it on the pipe outside. amazon.com/Attwood-1749-4-Blower-Resistant-4-Inch/dp/B003EX02DA/… $\endgroup$
    – Phil Sweet
    Jan 20 at 0:18
  • $\begingroup$ Would this work in tandem with the range hood fans? In any case, I'm not sure that's going to be a practical install and very resistant to harsh winters here. I wonder if an inline fan for grow rooms / green houses could be used in tandem with the range hood to increase performance. It could be right before the 4" wall exhaust to "boost" the air flow. They seem pretty affordable. $\endgroup$
    – cyboman
    Jan 20 at 0:32
  • $\begingroup$ Given the placement of the fan is fixed, would you recommend 7" duct all the way to the wall? Or given the reduction to 4" at the exhaust the difference is negligible? $\endgroup$
    – cyboman
    Jan 20 at 0:35
  • $\begingroup$ Need an explosion-proof fan. Bilge fans are explosion proof. Don't want to go kaboom because of the wrong motor type. Grow room fans aren't explosion proof. $\endgroup$
    – Phil Sweet
    Jan 20 at 13:55

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