Ducting for under cabinet range hood, how to offset to get by stud?

Submitted by ninian1927 t3_11azplu in DIY

Hi, I'm looking at replacing my 30 inch over the range microwave with a range hood. The fan is useless and there is no external venting at present time. I plan to go into the cabinet directly above and then out the wall, the cabinet and range hood will be on the exterior wall.

My problem is there is a stud that I will likely hit if I take the direct approach, so I need to offset to the left a bit and then go out to the wall. These would all be very small ducts in terms of length, but 8 inch diameter. The manufacturer says to use 2 45 degree ducts instead of 90s as 90s will cause more turbulence and noise. I can't seem to wrap my head around how I could go from the top of the range hood and out the wall using two 45 ducts? Am I missing something obvious here... I clearly am!

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Only-Replacement-198 t1_j9vfqp3 wrote

Hard to know without a picture. An 8" diameter seems like overkill.

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ninian1927 OP t1_j9vnrv1 wrote

You're probably right, I figured if I was going to do it may as well future proof myself. The range hood I'm looking at has a 600cfm or 900cfm option. I figured I may as well go with the highest.

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knowitallz t1_j9w02q9 wrote

Could you remove the stud and go straight an build a square box to replace said stud?

You want to go straight show as possible. Otherwise yeah a lot of noise (turbulence) so 45 then 45 again

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ninian1927 OP t1_j9w5rqf wrote

It's a load bearing exterior wall so I'm not sure, beyond my comfort level really. I guess my question is, would only using two 45s still allow me to offset a few inches to the left and then out? That's the part confusing me, the duct cutout in the hood is directly in middle of it ( on top), so I need to start there with duct, go left a few inches, and then connect to the exterior wall vent duct which will be likely at top (inside) of cabinet

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knowitallz t1_j9w6qc1 wrote

Then go left with two 45s and go out. 45 is just a less of a quick turn. Less noise by a lot. I have a straight shot with 8 inch pipe straight up and run a 900 cfm fan that has 4 settings. I cant hardly hear it at full blast

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ghostridur t1_j9w5khd wrote

I would use 2 adjustable 90s to make a short angle and a offset you would be less restricted than 2 straight 45s.

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ninian1927 OP t1_j9w6jhg wrote

Would that still let me line up with the wall duct? I guess I need to go to the hardware store and get some ducts so it makes better sense in my head. I'm guessing I'd need to cut them down. I keep thinking if I angle any less than 90 It's impossible to line up with the wall duct which will be sticking out starting... But I'm not smart!

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ghostridur t1_j9w6udw wrote

Yeah you can spin a adjustable 90 into some really weird offsets and contorted angles. Next time you are at the store just play with one a bit and see what you can make it to by spinning the individual sections separately.

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caucasian88 t1_j9vkwvj wrote

Just so you know it's code to install a UL listed microwave with an integral fan over the stove. So you could in theory just install a new one.

How big is your stove to warrant an 8" exhaust? What's the CFM? It sounds like you're oversizing.

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ninian1927 OP t1_j9vnm23 wrote

I have a gas range, it's 44,000BTUs. Perhaps you're right. The range maxes out at 900cfm but I imagine only having it on level 1/2 most of the time. I could go down to the 600cfm option and use 6 inch. I may or may not still hit the stud but would have to do some cutting to see.

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caucasian88 t1_j9vr5ro wrote

100 cfm per 10,000 btu. You're super oversized.

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ninian1927 OP t1_j9w56ds wrote

From different research I'm not sure the CFM advertised is true CFM so they say to overshoot what you need.. but you're right I don't need 900cfm but it's 4 speed so I'll likely run it much lower.

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