A) Moving bends away from the blower won't reduce the pressure loss of the bend, but may avoid interactions with the blade turbulence close to the blower.

B) If your real flow rate is 550 - 600 CFM, as it may be, use those values to choose a duct diameter such that the velocity of air in the duct is between 1000 and 2000 ft/min, with the lower end (1000) better for cold ducts and minimal noise. Too large a duct leads to too low a velocity resulting in grease condensation. Avoid rectangular ducting unless it is large enough to have a pressure loss per foot equivalent to the desired round ducting or it is the only shape that will fit. This is not just sectional area dependent. Use an on-line calculator if necessary.

C) I am unaware of any information suggesting damper location is a noise variable. Normally one has a damper at the outlet and one just above the hood.

Real noise abatement requires enough duct length to install a silencer, and use of a remote blower to be beyond the silencer.

Thank for you the detailed response!

(A) So I get from this that perhaps for different reasons it nonetheless does make some sense to put distance between the hood to the first turn. We’re trying to keep the ducting inside the cabinet above, so that means about one foot straight up before the bend.

(B) So I will stick with the round opening. That makes sense about grease condensation with a too-low velocity. My range hood maxes out at 850 CFM. I see online that for 600-900 CFMS, 7” is the minimum duct size recommended with actual recommended being 8-10 inch. However, do I take from your comment about “real flow rate” that I may actually experience less than that? And I suppose I won’t be maxing out the hood all the time. The next level down of 400-600 CFM, which seems to warrant a minimum duct size of 6 inches with 8 inches recommended.

It seems like an 8-inch duct is the sweet spot between those recommendations for me, right?

(C) The hood will be mounted on the kitchen wall which is the exterior wall of the house. So really, I’m talking about a maximum of 2-3 feet of ducting and only because I’m going out the top for a foot and then turning 90-degrees and then going through the wall and out. If I went out the back of the range hood (which I won’t because I can use only the smaller rectangular opening) I would have 1 foot…foot-and-a-half of ducting. So silencer and remote blower seem overkill.

Thanks again. Let me know if you have any reactions to the above. Otherwise, I’m going up the top 7” round hole with 8” ducting to the top of the cabinet, which is about a foot above the hood, and then going 90-degrees and out the wall. I’ll have the damper at the outlet and install an in-line damper right above the hood.

Thanks again! (and if anyone else has any other comments, I would appreciate them!)

Wow, thank you! That is very helpful. The diagram is very useful. I will see if we can cut an 8 (or 9)-inch hole or failing that, use a transition. I'll report back on what we chose and how it goes. Thank you very much.

I'm not sure 9-inch duct is available as a standard item, although it might be easily fabricated. Available dampers and duct caps, however, should drive the size selection of the duct that connects them together.

Got it. I'll think we'll go with 8" and see how my duct guy thinks we can transition. Thanks!

kazu1234 What hood did you end up getting and how do you like it? We aren't happy with our hood. It's way too loud. So I'm back to researching options. We're considering getting a remote blower (mega mega bucks unfortunately) or just buying a new hood. I think buying a new hood would end up being cheaper.

Hey Rachel

We went with a Zephyr 30" 850 CFM Typhoon Under Cabinet Hood. This hood is made to be installed directly below a cabinet. Here's a pic. https://zephyronline.com/product/typhoon-under-cabinet-hood/

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If the hood you're thinking about replacing is a typical free-standing range hood (say like this https://structuretech1.com/wp-content/uploads/2020/01/Range-hood.jpg), there's no reason to go with this one. We went with this one because we replaced an under cabinet, non-vented hood, and it was a simpler swap out versus getting rid of the cabinets above in order to put in a classic vented hood

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As to how we like it, we're still figuring it out. At the high end of power, it's pretty noisy. But we no longer get any smells lingering in our kitchen and going out the rest of the first floor. So that's great.

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I'm not sure how it's doing on collecting grease, however. We've cleaned it a couple of times and I've never found any grease in the grease collection areas. The grating above the fans does get greasy, so some greasy air is being sucked in (and as I said, we don't have any smells at all). That said, I'm cleaning grease off of the top of the range too, which puzzles me since shouldn't all that grease being going into the hood?

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This is our first vented range hood, however, so I'm still trying to figure out what is and what is not because we're using an under cabinet model versus a normal range hood.

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Good luck!

We have realized that we are getting oil droplets collecting on the grate protecting the fan, directly above the range.

As discussed above, we went out the 7” hole at the top of the fan and transitioned to 8” ducting to hopefully help with noise while preserving CFM. I assume that the oil collecting like this means we are suffering from what commenter kaseki above warned might result – “too large a duct leading to too low a velocity resulting in grease condensation.”

As I reported in the comment directly above though, since installing the fan, we no longer gets cooking smells in the kitchen. Also, when we go outside to the vent, we can smell the cooking smells there. So some ventilation is happening.

But we get the oil condensation in the picture and there is no oil collecting in our grease traps, so there seems to be some air ducting velocity problems.

Could this be solved by replacing the 8” ducting with 7” ducting? Does 1” difference in ducting diameter really change the performance of the fan by this much? Could there be a different problem?

Thanks for any thoughts!

When moisture and grease particulate laden air flows through a duct, there is turbulence and these two components will be brought in contact with the duct wall. Slow moving air will allow condensation; fast moving air will allow impingement collection. In general, both likely happen at different ratios depending on temperature and fan setting. I wouldn't change a duct diameter just for this reason.

What you have for a hood is unconventional. Commercial hoods and "pro" residential hoods use baffles that cause two 180-degree turns in the air flow. The larger grease particulates are deliberately impinged on the baffle surfaces, and given some baffle angle will drain toward a collection point. Even without angle, the grease is trapped where it can't easily drip down onto the cooktop.

I don't know the details of this fan scheme, so I won't comment on what it might be doing with grease. When you initially wrote that you were having the hood installed, I didn't bother with commenting on its design -- sorry. Whether grease dripping from the fan grid was intended or not I cannot say, but it likely is consistent with the physics of the hood design.

Again let me pontificate for this forum that the farther a residential hood deviates from long-time optimized commercial hood design, some portion of performance and efficiency will be lost.

Running the fans for a minute before generating any greasy effluent might help with condensation, but I think the fan blades here are causing grease collection around the grill as well as wherever it is supposed to collect.

I have a related question. Hope you don't mind. Our hood duct wall cap has a flap. I don't know if it's a damper? It's at the end of the duct. Anyway, the flap flaps when it's windy. Which wakes us up. Bedroom's above the kitchen. My contractor put a weather strip (or something like it) inside the duct to reduce the noise. The flap hits the strip, instead of the metal now. It's slightly quieter, but still annoying. He didn't have anymore ideas. He said it's a perfectly good cap. Should we buy a new cap? We have a Bluestar hood, will any brand cap fit? I think he bought a generic brand cap from Home Depot. Any suggestions appreciated!

One solution, often used on dryer vents, is a magnetic lock that hold the damper closed when not in use, but when the blower starts its pressure breaks the magnetic bond, the damper opens, and the pressure loss (back pressure) is very small once opened.

For kitchen ventilation hoods, there should be a damper at the exterior exit, and one just above the hood. This should minimize flapping. Those with external blowers will likely have a damper integrated with the motor/fan assembly that cannot open from exterior wind turbulence, but only when the motor is running.

In your case, you could buy an in-line damper and have it installed just inside wall at the exit cap. This is now an accessibility question.

kaseki Thank you! So the cap with a flap at the exterior exit isn't called a damper? And do you think negative pressure is causing the flapping? In our case, the motor is inside the hood itself.

There is a cap, and there is a damper. Some caps come with dampers. Depending on what is available, you may have to put your own damper inside the wall if the cap doesn't have one.

Negative pressure? Well, transient pressure differentials can cause flapping. If the wind causes a low pressure area outside the cap relative to inside, and there isn't another damper inside, then the outer damper can flap (in the breeze).

Start by adding a damper above the hood. Make sure after it is installed that no sheet metal screws limit its up travel or block it from closing. This damper would be closed by gravity.

Thanks for your thoughts, kaseki! It's a relief that you don't think the inch difference in venting could be making that much difference. That at least means I don't have to redo the venting. I take your point about new designs. Yea, we did try to go a cheaper route by using a design that allowed us to simply swap in a vented under-cabinet design for our existing under cabinet rather than go through the more expensive option of putting in a normal hood. That may be a future project.

For now, I have reached out to Zephyr to see if they have any thoughts. I'll keep the board updated on how this goes!

Thanks again kaseki!

Here is how grease separation works on Zephyr under cabinet:

https://youtu.be/GBFiqNWJoO0

Had ours for ten years and no grease drips on grills. Run at higher speed. Low speed (250 CFM) vented up through duct with a 90 plus the exhaust cap may drop fpm below 500 which allows grease to condense.

8” at 250 CFM is 715 FPM.

7” at 250 CFM is 936 FPM.

Add in losses of the 90 elbow in the duct and the vent cap you may be below 500 FPM. Run your hood on medium or high speed and see if the problem goes away.

So after a back-and-forth with Zephyr customer service, the rep said that the same thing happens on the under-cabinet hood the rep owns: " I have one of these units installed in my home as well, The grease trays only capture a minimum amount of grease when the self-clean procedure is done I recommend doing a surface wipe of the fan covers and letting the fan run longer after cooking more of the grease/steam residue will be expelled from the kitchen vent area. I usually let the fan run an extra 10-20 minutes depending on what I've cooked. "

So I think that means that commentator kaseki was right a number of comments above when he cautions about deviating from established hood design. I will keep that in mind if we ever replace this hood.

Thanks kaseki (and everyone else who contributed their thoughts)!