Thank you all for your feedback. Would reducing a 8" to 6 reduce performance a lot, even if i will add just 3 Feet of Duct vertically? ( no elbows ) @ Lillo, if i had to get a custom made one, what type of this rigid material would be best, and do any of you know where i could get this material?

But, if the reduction occurs only at the end, it's like the Venturi principle. Not all that serious. It IS serious to reduce a duct over its entire length. It does degrade performance to squeeze a duct down to a reduced size for a length. But, at a singularity, not a length of duct, it is less than serious. Keep on researching. Look into "supply plumbing pipe sizing rules", as an analogy. Combine that with "Venturi principle". Within a few weeks you will have learned a lot.

Remember that resistance varies by the 4th power of the radius and surface area by the square. When you go from 8" to 6" that's a 25% reduction in radius. So Davidro1 is right. If you have any length of the duct that's 6", you'll be increasing resistance by 64 times (4 to the 4th power). Even an adapter will force the flow through a surface area 1/4 the size intended. There are many factors that affect true flow rates, including the total length of the duct, radius, and location of the blower. But there's a reason why high flow vents require an 8" duct. Why do you want to pay the money for a 1200 CFM hood and only get 600? It's worth the money to install an 8" duct.

You guys are right. I have never changed a Range-Hood myself, and i have learned a lot with your help. I will be installing an 8", to keep my Range hood's power and Performance. My manual does say to use a Rigid Duct. Would you guys consider a Galvanized 30 gauge duct "Rigid" enough for a 900 CFM? like this one: http://www.ventingdirect.com/vent-a-hood-vp502-8-diameter-modern-themed-stainless-steel-round-duct-for-vent-a-hood-range-hoods/p1039086?y=18&term=vp%252D502&x=37

or would a 6063 Aluminum Pipe (14 Gauge) Be my best option?

Certainly there are lots of variables here but the one factor in question is why do you need to vent at that high a CFM in the first place? In most modern construction where the house is tight, a bigger exhaust hood is not necessarily better. In some regions the code states that if you vent more than 400 CFM you need to provide make-up air so you don't suck air in from places you don't want to. Even if you have a 6 burner gas stove you will not have them all on full throttle at once unless you are in a commercial kitchen. Exhausting 1000 CFM from a well constructed house is lots of air out and you will quickly create a negative pressure inside the envelope. Its best to exhaust the right amount of cooking gases. 100 CFM for every 12" of cooking surface is enough by most standards from what I read. I agree there are other variables. Just my 2 cents for what its worth. Of course an 8" exhaust vent is lower back pressure than a 6" on your hood fan.

I guess it is time to review a few realities of cooking ventilation -- described here without much justification. Appropriate references and rationales can be found in some of my previous comments to Appliance forum threads. In no particular order:

A ventilation blower, normally rated at its zero static pressure flow rate, can only operate at the CFM on its fan curve that applies to the house negative pressure it has managed to pull the house down to (including pressure losses between the hood and outside through the ducting). In other words, a 1000 CFM blower that is trying to ventilate a fairly well sealed house may only pull 200 CFM through the cracks (or back-draft any combustion appliances present).

The necessity for make-up air (MUA) is to (a) allow the cooking ventilation to achieve a good part (2/3 say) of its rated flow rate, and (b) eliminate pressure differences high enough (house interior more negative than -0.03 inches of water column) to back-draft combustion heaters and dryers. (See the many MUA threads on this forum.)

To first order, the number of burners that are operating does not determine the flow rate required at the hood to achieve good containment of the captured cooking plume. Every part of the hood aperture needs enough velocity that the rising cooking effluent does not "reflect" back out of the hood. We don't have means to block off the part of the hood baffles not being impinged by the plume, so the entire area contributes to the required flow rate. Schlieren photographs showing "reflection" for commercial hoods are available on the Internet, and in particular in Greenheck's CKV Design Guide. The desirable air velocity is around 90 ft/min, equivalent to 90 CFM/sq. foot of hood aperture, measured inside the lower lip of the hood.

Even with MUA, the fan curve will likely show that for typical pressure losses in the system (outside to inside to outside), a blower's zero static pressure rating should be around 1.5X the flow rate determined from the 90 ft/min requirement.

Hoods need to overlap the cooking appliance sufficiently to capture the rising, expanding, cooking plume. Expansion can typically be pictured as conical, arising from every point on the hot pan surface, such that significant effluent is within a zone 10 degrees from vertical. This varies with pan temperature and whether gas burner effluent is also present. Nominally, a hood entrance aperture overlap of 3 inches on each exposed side of the hood relative to the cooktop is taken as a practical/affordable design point.

Rigid duct means galvanized steel sheet metal duct, and not accordion flexible aluminum duct. Stainless steel can also be used. The goal is to resist a grease fire internal to the duct.

Looks like this thread ended in 12/2016 but something that was not mentioned applies to my current case. New construction/Hood to outlet distance is 22 feet with only one 90 elbow at wall above stove/Requires In-Line Blower of 800 CFM or more according to Broan technical service help. This application requires an 8" duct at minimum and the only question I am working on now is where is the best place to mount the blower. Beginning, middle or end of the duct run.

Ah! That too has been addressed before a number of times, but even I wouldn't want to search for the particularly relevant threads. Observations, (see also messages on measurements from forum member clinresga) indicate that with a long duct (longer than a silencer, at least) lowest sound level at the cook is obtained by using a distant in-line or roof blower and an intermediate silencer (muffler).

Such silencers for different diameter ducts may be purchased from Fantech, where some useful interfacing duct parts can also be obtained. Fantech is also one source for in-line blowers, but most kitchen ventilation systems adopt roof blowers. A roof blower will incorporate the roof cap and external damper to supplement the damper that should be just above the hood. (In very snowy areas where a high mounted up-blast blower is needed, more commercially oriented sources such as Greenheck may be considered. Also a roof pedestal to adapt the up-blast blower to the roof slope will be required.)

What happens noise-wise is that there are four main sources of noise.

• baffle hiss due to air turbulence at the baffles

• hood internal transition and duct turbulence

• rumble from motor unbalance, if any

• fan blade tip turbulence

Of these sources, those on the roof side of a silencer that are higher in frequency than the silencer acoustic cut-off can be significantly reduced by the silencer. I have a Wolf Pro Island hood with a nominal (hanging in free air) 1500 CFM rated roof-mounted blower (supplied by Wolf but sourced from Broan/NuTone/Best). Between the hood and blower is an in-line Fantech silencer. Some system characteristics are:

• Blade tip turbulence noise and duct turbulence noise are not noticeable

• Some motor rumble interacting with my roof and the ducting can be heard, but most is suppressed by wrapping my 10-inch duct with automotive sound deadening material. A stiffer roof would help. Tearing the assembly apart and having the blower disk professionally balanced would probably also help.

• Primary noise is baffle hiss.

• One can talk normally to another under or across the hood. The system isn't "quiet" but it also isn't obtrusive. (Quiet is defined as a properly installed mid-level Miele dishwasher.)

• Estimated actual flow rate with low pressure loss MUA is around 900 CFM and this flow rate for this largest Wolf hood (specific flow rate 90 CFM/sq.ft of aperture) fully captures and contains the effluent from a Cooktec 3500W induction wok and 36-inch induction cooktop operating with any combination of hobs. (Full disclosure: I haven't tried searing on five hobs while wokking, but I don't doubt that it could be captured and contained.)

So, I would recommend a down-slope (centrifugal) roof blower if there are no conflicts with the weather, roof design, and attic path, and an in-line silencer placed as near to the hood as practical. (My silencer's hood side end is roughly 1/3 of the way to the roof.) This type of roof blower can also be used on a wall.

Do not fail to look up the silencer physical size for the duct size you intend to run to ensure it can be fitted in the available space.

kas

@kaseki, if you were to do it again would you do the same config? What would you change?

How would you compare your system to a similar size VAH with regards to noise (interior & exterior), effluent capture, and cleaning?

(a) Changes: The only thing I would change, assuming staying with residential grade stuff, is that I would balance the blower fan disk before installing it on the roof. (Not that it is inaccessible, but it isn't clear without some disassembly whether easy disassembly is feasible. Also the continuing depression has driven a lot of balancing shops out of business, making it uncertain how easily or quickly the blower could be put back in operation. Acquiring a new disk and balancing it might be an alternative allowing a simple swap.) Or, I could have put up an up-blast commercial blower that, due to their indirect design, might (!) be better balanced, but I haven't tried it so who knows.

(There is a related part to this question. In hindsight, I would evaluate to a greater depth of analysis electric heating of the MUA instead of hydronic heating through a heat exchanger. The reason is that the installation cost may be comparable (energy cost would be more) but the pressure loss would be less and hence a smaller MUA blower might work. Note that I haven't finished this part of the project due to other projects being stacked on.)

(b) VaH Competiveness: I am not sure that there is a VaH that is as large as this hood.

I have an instinctive aversion to the word magic if not associated with fantasy.

I am confident that removing squirrel cage blowers and cleaning them and all the sheet metal in the throw zone is more work than cleaning baffles and wiping down the hood interior. I do not and never have owned a VaH so some views of mine are necessarily speculative unless other reported experiential evidence supports them.

Given equal hood size, the capture should be equivalent. Given adequate air velocity at the hood aperture in each case, the containment should be equivalent. This may mean different CFM rated blowers, however, due to different pressure losses and different fan curves for the respective blowers.

There is no doubt (and certainly earlier clinresga measurements support this) that a VaH system will be noisier than a remote blower baffle system with an intermediate Fantech silencer. New VaH models may be quieter than older versions and be at least competitive, noise-wise, with in-hood blower systems with baffles.

(c) External noise: In rare cases, such as narrow spaces between properties or inconvenient hood duct exit points, exterior noise may have to be suppressed. This could be done using an axial in-line blower (Fantech again) and a downstream silencer between the blower and the roof or wall cap. This implies two silencers, the other for obtaining noise reduction in the kitchen also. If two silencers won't fit but the exterior noise removal is critical and other duct terminus options are unavailable, then one might as well use a blower in the hood and one silencer. Even a hood blower and no silencer may be quieted by a sufficiently long duct. Given sufficient funds to expend on this, an exterior commercial blower run at a low blade tip speed could be another possible noise pollution solution.

Thank you. Great info.

I assume you've not needed to clean the silencer, exterior blower, or anything else other than the baffles? Do you expect to have to clean these at some point or on any regular basis? One bit made about VAH is that it does a better job of removing contaminants from the air stream so what is sent to the duct and eventually to outside is much cleaner air than what typically exits a baffle (and then flows to the silencer & external blower).

BTW, I think I'd much prefer to clean the baffles every few months and an external blower every 3 or so years than clean the VAH Magic Lung system once per year.

I haven't seen a need to clean beyond the hood, at least so far. Next time I'm on the roof I'll try to remember to look at the blower exit port for any signs of grease. One might reasonably have concern about the silencer as a grease trap; keeping the air speed up when cooking greasy stuff may be sufficient to minimize particle collection. Variations in post-hood particle deposits probably follow variations in temperature of the surfaces; running the hood before generating effluent can help warm the ducting in cold weather.

The principle that baffles use to remove much of the larger diameter end of the grease particle spectrum is the same as the VaH blower assemblies use: centrifugal impingement. If VaH has published a graph of collection efficiency vs. particle size, as commercial baffle suppliers do, I'd like to know about it. What we won't likely ever get is a plot of actual measurement results of the baffles used in otherwise reputable residential hood fabricators' hoods. For similar baffle shapes, however, extrapolating from the commercial data should be a reasonable rough approximation. Results will be air speed dependent.

Good point about air speed. Dust collection systems must always run at the same speed—for effective separation and to prevent drop-out in horizontal ducts. Some people will run them at lower speeds for tools that produce low volumes of dust and then wonder why the post cyclone filters get clogged faster.

I'd love to see CKBD or similar do some tests of various exhaust systems both effectiveness and noise. I suspect that a VAH cage is massively less effective @ 200 cfm. Any idea how a baffle would compare?

"than clean the VAH Magic Lung system once per year."

Opaone - I wasn't sure if you mean that you believe the VAH hood would only need to be cleaned once per year. It would need to be cleaned as often as any baffle filter hood. That is, depending on what you are cooking - hamburgers, pork chops, bacon, vs. vegetarian dishes - the grease build-up inside the VAH hood housing is the same as would be on baffles, and needing the same frequency of cleaning.

opaone "I suspect that a VAH cage is massively less effective @ 200 cfm. Any idea how a baffle would compare?"

As I recently noted in another thread, both means of grease particle impingement collection depend on air velocity, so my present view is that it will be best when slow cooking greasy foods, for example bacon, that the blower be kept at a higher speed than otherwise needed to ensure no reflection at the baffles (or whatever counterpart applies to VaH configurations, such as the metal piece between two blowers.

Apologies if this thread reads like an irrational conversation. My July 15th submission was intended to answer a noise question that now seems to have disappeared.

Can anyone explain in detail the actual harm of using a smaller-than-rated duct size for a 500+CFM fan? Everyone is saying "you shouldn't do it", but don't really explain why for those folks who have situations where they have to do it. I'm installing a range hood over an island below a vaulted roof ceiling. The total run would be less than 5' with one 90 and one 45 turn. What issues would be encountered by using a 4" duct with a 500CFM fan?

A 500cfm hood over an island is already undersized to begin with. Island hoods generally need to be larger and therefore also have higher CFM, because they are not as protected from cross ventilation as a wall hood would be.

If you then reduce the duct diameter to something ridiculously tiny such as 4", you reduce air flow even more. You would be better off removing the hood altogether. It doesn't really do its job in this configuration; might as well leave it out and enjoy the sight lines. That'll distract from the cooking smells and the grease on the cabinets.

Or you could of course follow the advice given in a all the excellent archived threads in the Houzz forums. Figure out the appropriate size for your island hood (probably something on the order of 40"+ wide and 27"+ deep, give or take) and then compute the CFM so that you get an airflow rate of 90 CFM per sqft of aperture. Then apply a correction factor of at least 1.5 to adjust for inevitable real-life performance losses (the nominal CFM specified by the manufacturer is measured without any duct at all). You'll probably end up with around 1000+ CFM nominal flow reate. But that's assuming you install it according to the manufacturers recommendations.

For a 1000 CFM hood, they will most likely tell you that you need about 8" diameter ducts or better. If you instead reduce your duct diameter by a factor of four, your performance will go down by about a factor of 16. That would mean your fancy new 1000 CFM hood only gives you 60 CFM. And we are right back to getting rid of the useless but expensive noise maker that you installed and enjoying the sightlines, instead.

A lot of things are negotiable, even if not necessarily advisable. But duct diameter really isn't one of those things. Make the duct to small and there literally is no point in having a hood.

Not to mention a ridiculous (~100 ft/s) air velocity in the duct if 500 CFM were actually forced (at a few inches of water column back pressure) through a 4-inch duct using some aerospace blower, but for any likely (tolerable) hood blower the flow rate would be negligible. Start with M's suggestions and we can refine them if needed.

Note, duct air velocity for cooking equipment should be in the range of 1000 - 2000 ft/min, with lower likely to be quieter, so for an actual 500 CFM, an 8-inch duct will be in the zone, but a 10-inch duct will also work.

For an idea of pressure loss, use the calculator here:

https://www.engineeringtoolbox.com/duct-friction-pressure-loss-d_444.html

but note that bends are worth more feet of duct than their real length

https://www.energyvanguard.com/blog/duct-design-3-total-effective-length

Adding baffle pressure loss (a few tenths of an inch at 90 CFM/sq. ft. maybe), one can add the baffle pressure loss to the duct loss and then see where one is on the blower's fan curve (flow vs. pressure loss). This yields the flow one will get with all the windows open and no screens.

This brings us to makeup air, which you will need to provide by some passive or active system, and its pressure loss to be added to the values previously described for a corrected point on the fan curve. Please review MUA related threads here.

My contractor has installed a 4 inch flexible metal duct that runs from the above range cabinet down through the wall to exit below the floor (house is an over-hang). I want strong ventilation but am having trouble finding any hoods with a 4 inch duct. He says he can reduce a 6 inch ducted hood down to the 4 inch in the space of the above range cabinet where the 4 inch ducting begins. Is this acceptable? Do I need to ask him to rip through the wall and ceiling to reinstall a larger (and rigid?) length of ducting?

Flexible ducting is a fire risk as if accumulates grease. It's not allowed by code, but not all building inspectors know about that.

Running ducts up and down is also a problem. It can result in grease pooling at the low spots. My building inspector won't pass anything that doesn't continuously go up.

6" is tight and won't work well. Most hoods need 8" or more. 4" is pretty pointless. Might as well not have a hood. You can look online for tables that allow you to compute flow through ducts. Even going down a small amount affects flow rates a lot

Wow! Your contractor may wish to review generic information on HVAC principles, not to mention relevant building codes.

a) Cooking ventilation ducts have to be steel. Flexible ducts are usually aluminum.

b) Not far above this comment are some links related to duct pressure loss calculators. Play with them to see what pressure loss values result from various duct lengths, bends, and flow rates. You will not likely have a hood blower that can work much beyond 2 inches of water column pressure (including the hood baffles pressure loss and make up air pressure loss.

c) The only devices that might be designed for 4-inch ducts are over-the-range microwave ovens with an exceedingly wimpy (and useless) fan intended to move a bit of the cooking effluent outside.

d) Clearly (I hope) a few inches of length of 4-inch duct would not cause a large pressure loss, but longer lengths will. Even six inch diameter ducting is too small for "strong ventilation" unless very short in length with no bends.

e) The order of selection here is backwards. Start with the cooking area, determine the hood capture area, determine the CFM required given the capture area, then plan the ducting such that the velocity in the duct is in the 1000 - 2000 ft/min range. You will have to do some arithmetic.

f) Pick a hood with the correct overlap.

g) Estimate the pressure loss of the baffle and ducting and choose a blower that will yield the desired CFM when subject to the estimated pressure loss.

All of the parameters and concepts related to the above advice are addressed in threads on this forum.

@ltaichman What your contractor is proposing is quite irresponsible and dangerous. As mentioned previously:

- NEVER use flex duct of any kind. Both for safety and performance.

- NEVER have effluent airflow go down. Up or horizontal only. This is both for safety and performance. The effluent will be hot and hot air naturally rises and with considerable force (think about hot air balloons). You can quickly reduce your performance by 80% or more trying to force hot air to go down.

As @kaseki mentioned, size the duct appropriately. Undersize duct will greatly affect your performance (think a 400 cfm hood only operating at 50 cfm) and could be dangerous if pressure builds up and pushes effluent out of the duct.

More: https://www.gardenweb.com/discussions/5161173/hood-faq

Thanks for confirming my concerns, everyone!

Ok, we're changing the plans. We will now use rectangular steel ducting and go horizontal over the tops of the cabinets about 6 feet. Will 3" x 10" be sufficient for 530 cfm hood with an 8" opening?

3x10 is really tiny. If you look it up, at best it's equivalent to a 6" round duct.

In the IRC, Table M1506.2, the formula given for finding the equivalent circular duct diameter for a given non-circular duct is (4 x sectional area)/perimeter. Hence, for a 3 x 10, one can calculate 4.6 inches. A 6 x 10 would yield 7.5 inches equivalent. That would be a more reasonable duct.

(This formula is fairly conservative, because for a square 8 x 8 duct the equivalent circular duct is calculated to be 8 inches. We can presume that a 8 x 8 duct will have somewhat better flow than an 8-inch circular duct, but it isn't a lot better flow due to edge effects in the corners.)

Thanks for looking that up. I was a little surprised by the numbers that I got from that table. Your calculation sounds a lot more plausible. And yes, I would assume that in particularly tight configurations (such as only 3" width), the turbulence from the sharp edges can easily make a rectangular duct work worse then a round duct. I just can't picture anything close to laminar flow with those sharp corners.

Hi. New to this group. wondered if i could bum some advice. i'm wanting to replace the ridiculous range hood that came with my house 13 years ago. i'm looking at a zephyr typhoon or hurricane what are 850 and 695cfm respectively or a Cosmo qb75 which claims 900cfm. if you have any suggestions (or another similar size) on which one is best, i'd appreciate it. 30" under cabinet. Now, the real reason i'm on here is because my setup is a 3.25x10 rectangle duct straight up through the cabinet about 4-5' and then transitions to 6" round. i was planning to remove the 6" and go up to 8" for the majority of the run, but removing the 3.25x 10 will be a much bigger undertaking as it's all framed in with sheetrock. is it worth it to improve that short run of rectangle? also my roof cap is 6" so i was planning to reduce 8" to 6"roof cap right at the end, wasn't sure that would reduce performance, otherwise, gotta open the roof...

My guess is that any time you drop down to 6", even if it is for only a short stretch, you'll seriously compromise the efficiency of the system. But from reading on this forum over the years, I gather that the details are a lot more complicated. A lot depends on the specifics of your hood and on its blower.

From personal experience, I can say that we have an old Vent-A-Hood hood (relabeled Viking) that came with the house. It's a fairly nice hood. It served us well for many years. After the remodel, we moved it into the in-law kitchen. It vents straight out. But the outside vent is poorly accessible so we didn't enlarge it. I think it's only 6" unlike the huge opening that we have in our main kitchen. After the move, the formerly good hood became barely functional. I have been told this is partially a result of the type of blower that Vent-A-Hood uses and which is more susceptible to back pressure. No way of verifying whether that's true. The in-law kitchen doesn't get enough use for us to fix the problem. But for the main kitchen, I absolutely wouldn't be happy with such poor performance.

The gist is, I recommend either upgrading all your ducts to a bigger diameter, or if that's not something you want to do right away, you could just install the new hood and see how it works. If you are not happy with the performance even when all windows are open, then you know the duct needs to be upgraded. If the problem only happens when the windows are closed, then you know that make-up-air needs to be provided. It's possible that both upgrades are required in addition, of course, to getting a decent-size hood.

good advice,thanks. i think i found someone to replace my roof vent so i guess i'll upgrade it to 8". do you suppose i should go to 10" termination just in case i upgrade hood again someday? although i don't suspect we will because going from a 30" range to 36 or bigger would cause us to have to rip out a lot of cabinets. it really surprises me a few inches of restriction would cause that much hindrence. Water would just increase pressure in that small section to compensate, maybe air is different? I had not considered doing make up air (figured window might work) so do you think that is acceptable? this range hood claims 850cfm and the manual does not even mention makeup air (zephyr typhoon) also it asks for 7" duct and i'm going 8. one think i couldn't figure out was that it says 3.25x10 is ok for stubbing out of the unit, but then it also wants 7" round as minimum. 32.5sq inch vs 38sq inch, so rectangle is less than 7" but they don't seem to worry about that difference? and 6" isn't much more than 3.25x10 area at 28sq?? thank you!

Brendan Kraus wrote: "Water would just increase pressure in that small section to compensate, maybe air is different?"

A water pump, faced with an increase in pressure, would have reduced flow rate. See the flow vs. head plot for any modern swimming pool pump -- very similar to fan curves for air blowers. Exceptions to both of these require positive displacement pumps, such as Rootes blowers or air compressors. Even then thermodynamics has to be accounted for, and the hotter exit air with increasing back pressure will have lower mass flow rate.

I know this will sound crazy but I need serious advice. I'm installing commercial appliances 36in gas stove and above it from the same company a range hood thats 36in with a 750 CFM blower. The way my kitchen is positioned in my house and the way my 7.5in beams run, I cant fit a 10in duct that the company wants. I was thinking of using special engineered plates to reinforce my beams and do a 4in with a traveling distance of 6 feet with only one elbow. I know thats a crazy reduction and I"m started to wonder if its worth getting a 3k range hood? On low power would it still preform somewhat? The other option I have is to go along the beam and get a 6in duct but the travel would be 30 feet. If I'm gonna be stubborn which out of the two would be the better option. Any advice or suggestions?

I just reviewed this thread and I think the answers to your question are already addressed. Please review this and many of the other hood threads, determine an appropriate hood size for capture (overlapping the pans enough for the expanding cooking plumes), calculate the flow rate needed to achieve 90 CFM/sq. ft. of aperture, multiply by 1.5 for a blower rating (this assumes adequate duct and MUA), and then calculate the duct size that provides 1000 to 2000 ft/min velocity at the needed CFM. With less than this we can predict that the ventilation will be inadequate.

Bottom line from this thread: A 4-inch duct of any length will be inadequate for any cooking if cooking effluent is to be captured and contained. A long 6-inch duct would only be adequate if the cooking is always low temperature and the hood is over a 30-inch range.

Also seriously recognize that quasi commercial cooking appliances, if they can even be used in a residence, require comparable ventilation systems. Review the Greenheck Guide, which may be found here: https://www.tagengineering.ca/wp-content/uploads/2015/02/KVSApplDesign_catalog.pdf

I am installing a new 36" range hood with a factory 6" outlet. If I installed 8" duct.( straight up about 5 foot run, and through the roof to a roof cap) . My question is I know you lose efficiency if you make your duct smaller but does it hurt if I use an adapter from 6" into 8" duct?

Short answer: No. In very cold climates without any duct heating from preliminary air flow, a larger duct might slow the airflow velocity to a low enough speed that there could be more grease condensation. I wouldn't worry about it for a 5-foot run. In your case the larger (but albeit short) duct will allow the fan to flow more air than with a 6-inch duct. This will likely be very slight due to other factors being dominant. If possible, use as an adapter a proper transition duct and not an abrupt change in diameter.

My builder installed a 6-inch foil flexible duct to the roof cap (roughly 4 feet). The installed GE Monogram vent hood calls for an 8-inch duct. Rather than replacing the duct, our builder installed an 8-inch to 6-inch adapter. I know this isn’t ideal, but is this a satisfactory solution? Also, should I be concerned about the foil flexible duct as opposed to flat galvanized steel?

It's neither to building code (fire hazard) nor does it vent adequately. Replace with 8" rigid steel

That is irresponsible of your builder and is a warning sign about their knowledge, quality and integrity.

Reduction from 8" to 6" alone is a problem. It will reduce air flow and so the effectiveness of your hood and it will increase hood noise as well as the amount of electricity used. So increased cost, more noise, less odor, gas combustion by-products and other effluent removal.

Flex duct creates two problems; It's a major fire hazard and it increases static pressure (reduces air flow). From a static pressure standpoint 6" flex is roughly the same as 4" rigid in most installations.

At a MINIMUM you should have 8" rigid for your entire run if that is what your hood calls for (Many US hoods undersize this). If you have many bends then you should increase that to 10" to keep static pressure down.

Note to readers: Whether your builder is I. M. Pei or Fly-by-Night Enterprises, you have a duty, at least to yourself, to review every device's installation instructions to ensure that they are being met in both the architectural design and in the construction execution. Your Code Enforcement Officer should not be expected to detect all failures in meeting installation requirements.

Code enforcement folks are quite often not the brightest bulbs in the bunch (some are quite good though). On numerous occasions I've waited until they'd completed an inspection to ask them about the numerous genuinely important items they missed after they've nitpicked on stuff that really doesn't matter.

And keep in mind that CODE IS THE WORST THAT A BUILDER IS LEGALLY ALLOWED TO GET AWAY WITH. Code minimum is not necessarily good, it's just not awful.

I have a related question to the reducing from 8" to 6" duct. In my application the problem is the location of my hood and vent. I have a chimney-style hood. Unfortunately, it was installed with an undersized insert that just does not do the job. I want to replace it with something that goes up to at least 600CFM. That requires an 8" duct. My existing vent is 6". The good thing is that it vents out of the wall that the hood is mounted on so the duct is only about 2' with a single elbow out. I don't mind replacing the ductwork with an 8". The problem is that there is a 2x4 in the wall frame directly in the center behind the chimney. I cannot fit an 8" duct between the 2x4 and the edge of the chimney. How can I remedy this?

First, you need to know what the 2 x 4 is accomplishing, so you can determine whether some other means, such as a metal strap or thicker steel inlay would do. Second, if this 2 x 4 is there by decree of heavenly powers, then with some loss in hood performance, a pair of 8 to 6 reducers, 'back-to-back' could be used to smoothly taper the duct at the point of the 2 x 4 and then re-expand it. Frankly, your description is inadequate to opine further. How about a photo?

I’ll send a photo when I remove the chimney, but the 2x4 is in the exterior wall where the vent is. So the reduction would be to the wall cap. So it wouldn’t need to re-expand. At that point it vents out.

Perhaps the 2 x 4 can be moved, if just providing some wall rigidity. Alternatively, a transition from 8 inch round to 6 x 10 inches rectangular to a suitable wall cap might work.

Can it be installed vertically? I can't seem to find a 6x10 wall cap that vents downward lengthwise. What about a 8x6x6 Y? Do those exist? Would that work?

Sheet metal shops, HVAC distributors with fabrication shops, and many HVAC/plumbing contractors have the means to make almost any realizable configuration. You will have to ask.

OK. Thanks.