range hood duct, asking guidance on

I’ve just stumbled on this (great) site looking for answers to my situation.

Though not an inspector, just a regular homeowner, am hoping/thinking inspectors might be the most reliable source for my question.

Here’s the situation:
– I want to replace my 200 CFM range hood with a 600 CFM model.

– The old range hood uses 6" duct, the new one 8".

– The 6" vent hole was put right at the bottom and to the left of the cabinet holding up the range hood. The original installation had a sheet metal box in the cabinet, with a hole in the bottom of the box over the range hood, and a hole in the side of the box going to the outside.

– I took the sheet metal box out, replaced it with corrugated flexible tubing. The flex duct goes up out of the range hood, circles back down to the vent, then makes a 90 degree turn to go outside. Between the weak range hood and all those turns, steam doesn’t get drawn up at all.

That’s the situation. Now am pondering the solution.

  1. One option is to enlarge the 6" hole to 8" and put another sheet metal box there.

  2. Another option is to plug the 6" hole and make another 8" hole higher up inside the cabinet. Lowe’s/HD sells adjustable ducting. It’s about a foot long, has four sections that can rotate. (You’ve seen it. I’m sure.)

The problem for me is that it’s a brick wall on the outside of the house, so I’d have to open a new 8" hole in a brick wall. No longer DIY at this point. That makes me like option 1. But even though option 1 is cheapest and easiest, it seems a terrible idea. I picture 600 CFM of air coming up out of the range hood and running smack into the top of the box.

Have you inspectors ever seen a box like this used with a high volume range hood?

Might it work if I could find a metal shop to round the corners of the box so the air slides through the hood better?

What would you do if you had this issue in your house?

Hope I haven’t given you a headache. Wish I could post a few photos, would help a lot. Looking forward to hearing your thoughts.

I am having a hard time following what you have done with the ducts. What you want is round pipe coming from the range hood to the exterior with a minimum amount of changes of direction. Use straight lengths of pipe and elbows, avoid using flex. A good sheet metal worker - HVAC tech can probably do this for you very quickly for not too much cost. The four piece thing that Lowes sells is called an adjustable elbow if I read your description right. If the range hood has a rectangular exhaust opening you will need a fitting called a square to round, a sheet metal shop can make one for you. I would try to use 6 inch pipe with your 600 cfm range hood and see how that works for you before chopping up your bricks.
See this link from a range hood manufacturer

Quick thought: metal shop can fabricate a new box, you will have to give exact dimensions and they weld up.

Cutting hole in brick is not a diy job, but call around to core cutting companies in your area, might be a reasonable choice.

Do you need 600cfm fan? You may be able to duct through smaller pipe will be less venting, and more noise. Check with manufacturer.