Provide which models you are talking about. Regardless of brand, you do NOT want a flat-bottomed hood; you want a canopy-shaped hood for better capture. Also, since you are looking in the price range of Miele, you want a hood with baffle filters not mesh filters. Unfortunately, both of those brands make a lot of mesh-filter hoods.

Before hood-shopping, do you know what the size of the duct is that goes from the hood to the outside? Hoods specifications will list which size duct they require.

Miele DAR 1220-3, DREB XL Blower (700 CFM)

Zephyr 30" Tidal II Wall

Both are canopy-shaped with baffle filters.

Need to double check our duct size. both require 8"

I don't have the Zephyr that you are considering, but I have a 2 year old 48" 1200 cfm Zephyr chimney hood that I love (10" duct). I also have a Zephyr beverage center that has been just perfect.

so the duct size is somewhere between 6 and 7 inches. both hoods recommend 8 inch round ducts. What will happen if I'm using a smaller duct?

Lots of noise, and you will not get the cfm that you bought the hood for.

We can't be change the duct. Is it worth looking for a similar hood made for 6 inch duct? Or will they all perform the same with a smaller duct?

You need a HVAC person to determine the resistance of the duct you have, and then, with some guesswork about the flow resistance of the hood filters, find a blower that at that resistance achieves the flow required (see other threads here). Basic concept:

You will never get the "maximum free [air] delivery," shown above, but this is what is stated for blower performance. Hence the need for analysis along with a "fan curve" for candidate blowers. So you can make up for constriction with a suitably tailored blower, if available, but as commented noise will certainly rise.

@kaseki, thanks! more to look into. Can you share the links to the other threads?

A 6" duct is half the size of an 8" duct but the throughput penalty is greater, especially with a lessor blower. A 600 CFM hood can easily be reduced to below 100 CFM with inefficient ducts.

Zephyr claims 1300 CFM for this hood. In an air handling system that's an 18" duct. In a high velocity commercial range hood that's a 14" duct. An 8" duct is 1/3 the size of a 14" so would require AT LEAST 3x the velocity of our commercial hood. That ain't happening.

OP, let's step back a bit and see if we can figure out what you need and then see if there's a good way to do it with the duct you have in place.

What range is the for? What styles of cooking do you do? How often do you cook? How often do you entertain?

Where is this? How often do you have a lot of windows in your house open? When was your house built?

“What will happen if I'm using a smaller duct?”

It will be noisier than if you had the size of duct the hood specified. It will also be less efficient than if you used the size of duct specified. But here’s the thing—if you instead bought a hood that specified the 6” or 7” size of duct you have, you would still have a noisier less efficient hood than if you have an 8” or 10” duct. You cannot overcome the laws of physics. Pulling air through a smaller pipe is going to be noisier and more difficult than pulling air through a larger pipe.

That is why many people doing a kitchen renovation install a larger exhaust duct while the walls are open. Also, many people stuck with smaller ducts simply live with the louder hood noise and move on with their lives.

Ideal duct size if exposed to cold air in an attic, say, is that which keeps the air velocity between 1000 and 2000 ft/min (actual CFM / duct sectional area). This balances grease collection due to impingement (a high velocity effect) against grease collection due to condensation (a low velocity effect). Typically in existing homes, a 10-inch duct is about as big as can be easily fitted, unless the home is architecturally convenient for large duct installation.

So what happens at lower speed settings. To keep condensation down, pre-heating the duct by running the hood a couple of minutes before generating greasy effluent is a good idea.

Duct length is also important. A short duct will have less impact on blower flow rate than a longer one.

@opaone, thanks for the detailed reply. I'm planning to install a Miele HR 1124, 30 inch all gas range.

My usual food rotations are pizza, pasta, stews, roast chicken, quick stirfrys. I use our current electric oven broiler for meat, fish, sheet pan veggies. The current hood isn't cutting it for pan searing meats or stir frying so I have been doing that alot less often.

We live in an apartment, built in the early 2000s, won't be able to change the duct work. Have a balcony door around 18 ft from the range that we leave open when it's needed for cooking. That really isn't ideal since it can get super windy in the winter and we don't want the door breaking when it slams shut. We don't entertain a ton. Maybe 4 times a year.

The current range hood is a microwave combo with exhaust. It's just not powerful enough to capture everything and without the door open we sometimes set the smoke detector off (super sensitive nest).

@RoyHobbs, thanks. We are unfortunately stuck with the 6 inch duct as we live in an apartment. What you are saying about hoods and duct combos is what I am trying to figure out. Figure a hood rated at 700 cfm on an 8 inch duct isn't going to perform as well on a 6 inch, but how much worse? Is it half because the area is that much smaller? I'm less concerned about noise, performance and reliability are the main focus.

@kaseki, reading materials were great! thank you. I'm not sure how much I'm going to be able to do in our apartment to establish proper MUA.

At this stage I'm looking for a proper hood with that has containment volume, is well built and reliable and can work with the limits of our current kitchen.

It looks like prestige / bluestar (both are made by Prizer) makes a 30 inch hood with a smaller 600 cfm blower spec'd for a 6 inch duct. No one gives specs on capture area, but based on the blower's spec to work with a 6 inch duct, will this perform better than the Miele or zephyr spec'd for an 8 inch duct and rated to run up to 700 cfm?

For capture area use hood width times front-to-back dimension, less any particularly large lamp bar mounted at an edge.

Performance of any blower on a six-inch duct requires knowledge of the duct length to determine pressure drop vs. flow rate, knowledge of the pressure drop vs. flow rate of the baffles, pressure drop of imperfect MUA vs. flow rate, and then applying the combined pressure drop vs. flow rate to the blower's fan curve.

spent sometime at an appliance show room looking at the different hood options. It looks like a bunch of the consumer grade hoods put the baffles at a much lower angle or even parallel to the cooking surface. It seems like this limits containment volume as the baffles block airflow, but maybe i'm misunderstanding the science. Would you still count the space between the baffles and an internal fan/duct as part of containment volume?

All else being equal, is containment volume an important comparison for hood performance?

This prestige bullnose wall hood angles the baffles to the center. Lighting in center is also nice for pushing the baffles to the front and back edge and providing even lighting of your cooktop. Angled baffles add containment volume, but the metal pieces (circled in red) from either side subtract.

Some photos of the hoods I looked at for comparison. Photo angles weren't consistent, so it maybe difficult to tell how much space is under the baffles.

Miele DAR series wall ventilation hood - really sharp angled baffle placement. had the most space below baffles. control and lighting bar width similar to bluestar

Bluestar proline wall hoods - much shorter angle for baffle placement. expect the additional steel grill to limit airflow, but may also help with grease capture.

Zephyr - baffles sat almost flat with the hood. There's also a large metal strip in the back, vs the Bluestar and Miele. It seemed like this made the baffles smaller, but i didn't measure baffle width.

Fisher & Paykel Professional Wall Ventilation - higher baffle angle than bluestar, shorter baffles. Angled light bar may give some additional containment volume.

A few comments:

1. Pipe diameter can be used to find the VOLUME or CAPACITY of the pipe. Saying a 6" exhaust duct, assuming round here, is half the size of an 8" is not correct. It has half the VOLUME of an 8". Remember grand school geometry pi x r-squared give the area of a circle. So the inference in saying the 6" is half the size really is referring to it's 2d AREA.

6" round exhaust hood has an AREA of 3x3x3.14159 = 28.2743 square inches of area

8" round exhaust hood has an AREA of 4x4x3.14159 = 50.2654 square inches of area.

10" round exhaust hood has an AREA of 5x5x3.14159 = 78.5397 square inches of area.

When talking about exhaust venting for a grease carrying duct (which is what kitchen hoods are) ASHRE says a MINIMUM velocity of 500FPM should be used. This is also the most efficient velocity. Faster velocities are less efficient, require larger motors and do not capture grease as well, but are acceptable up to about 2000FPM. Lower velocities allow grease to condense and can cause fires, reduced capture and poor performance.

So lets see what the velocities are for the 6" and 8" range hoods (We have to convert the are of the pipe from above from square inches to square ft):

700 CFM

6" - 700 / (28.2643/144) = 3566 FPM

8" - 700 / (50.2654/144) = 2005 FPM

10" - 700 x (78.5397/144) = 1283 FPM

12" - 700 x ( 113.0972/144) = 891 FPM

14" - 700 x (153.9379/144) = 654 FPM

16" - 700 x (201.0617/144) = 501 FPM

So as we see above the CORRECT duct size should be 16" for a 700 CFM blower to achieve the ASHRE recommended 500FPM minimum velocity for a grease carrying exhaust duct. This would also be assuming the duct run is very short and has minimal to no friction losses due to it's short length and no bends.

If the run is LONG or has a lot of bends then there will frictional loss in the pipe. These losses are fairly well defined based on pipe size and length and can be found online with simple searches. If the exhaust pipe is not round then the math for area is simple, but the frictional loss calculations become more complex. There are charts to get a rough estimate.

When running the fan at LESS than it's maximum speed then we need to figure out the correct duct size to achieve 500 FPM. Keep in mind that this will mean that we will have a higher FPM through the same size duct when running at a higher speed. To do this calculation we need to know what the total static pressure the hood fan is capable of achieving. For Zephyr models they list the total static pressure as 1.25 w.g. at the specific hoods maximum listed CFM.

Static pressure is the resistance to air movement. This could be from restrictions in the exhaust pipe, lack of available air to put in the pipe, etc. This is where make up air (MUA) comes into the picture. Some new homes are built with a vapor barrier (tyvek or plastic sheet) that "seals" the house so well that air does not leak in around windows, cracks, etc. If an exhaust fan is running it is removing air from the building. When this happens the air pressure inside the building goes down. The exhaust fan has to work harder to continue to move more air. Once the pressure goes beyond the design static pressure for the fan the air flow slows down. In commercial building a MUA unit will have a fan and a way to heat or cool the air. This unit is controlled by a static pressure sensor that meaures the pressure difference between the outiside and incide the building. When the pressure drops inside the building the MUA fan speed increases to supply more air and equalize the pressure.

In a home the MUA could be opening a kitchen window. When the exhaust hood/fan is on the make up air will be drawn in through the window. Or a system, like what a commercial building uses, can be installed.

NOW all that explained we want to know what the correct duct size should be for a Zephyr hood that has multiple speeds. I will use the Typhoon model. It has a CFM range of 250-850.

Duct Size (FPM)

CFM 6" 8" 10" 12" 14" 16"

250 1273 716 458 318 233 179

850 4329 2005 1558 1082 795 608

So what did we learn from the above chart? For this particular hood we should have at LEAST an 8" duct, but I would argue that a 10" duct should be the minimum. The max working static pressure of most Zephyr hood is about 1.25" w.g. That works about to about 1500FPM in duct before the fan runs out of oomph to push more. Add in bends or other restrictions and 10-12" would make the most sense.

Now if you you are venting directly out the back through the wall and there is no real ductwork, then all of these numbers can go out the window as there is not realy duct to worry about.

The above is useful, but needs to consider both grease accumulation due to impingement, minimized by low duct air speeds, and grease accumulation due to condensation, minimized by high duct air speeds, as well as practical limitations of residential kitchen ventilation systems. Also, the use of the term VOLUME in the first paragraph seems to be misguided, or missing something.

Can you provide a reference for the ASHRAE value of 500 ft/min?

Cold ducting in cold climates is better operated above 500 feet per minute to avoid condensation accumulation. The last report on that that I read suggested 1000 ft/min. Impingement is best avoided by staying below 2000 ft/min.

Also, persons suggesting larger than 10-inch duct for most residential applications need to wrestle that size in an attic to get perspective. Suggesting even larger duct sizes will seem to be a good idea, but in general they are impractical in residences where a duct has to go into an attic and then angle to a desirable roof position for exhausting. Incorporating larger sizes in initial building construction is easier than retroactive installation during kitchen renovation.

The residential hood companies such as Wolf, Broan, Abakka have 10-inch duct interface hoods and blowers as their largest sizes. Above that one can consider in-line blowers from, e.g., Fantech, or commercial up-blast roof blowers.

For hood systems I have looked into, given proper MUA, the largest source of static pressure at full power is the baffle assemblies. Ducting would be second. MUA can dominate, however, if based on building leakage, or in some cases even when passive (no blower). In any case, MUA must be provided if there is any chance of back-drafting combustion appliances.

Residential blower fan curves are monotonic. More resistance to flow leads to less flow rate. Given a fan curve for a particular blower at a particular rpm, the actual flow rate will be the point on the fan curve where the baffle pressure loss curve plus the ducting pressure loss curve intercept it. See generic example.

In my personal case, with an 11 sq. ft. hood entry area (nominal) and goal of 90 CFM/sq. ft. at full power, the flow rate has to be 1000 CFM. My 1500 CFM Wolf blower will be restricted to 1000 CFM at a static pressure loss of a bit more than 1 inch, w.c. This can be allocated to the baffles, ducting, and MUA. I have 10-inch ducting (1800 ft/min @1000 CFM) and actually achieve this flow rate (approximately). Duct physical length is probably 20 ft, with maybe two 60-degree bends and one 30 degree bend (nominal).

I am unclear what the meaning is of "do not capture grease as well" in the context of ducting. The baffles capture (more or less) the grease at the larger end of the particle spectrum, and will do so more effectively as the velocity in the baffles increases. The hope is that the smaller end of the particle spectrum will be exhausted with little being left coating the duct. Efficiency, in the sense of moving air at a given cost, is not a driver for residential cooking ventilation component choices due to the modest amount of air flow and the limited time that it is operated at full power.

As @opaone has shown, if one want's to operate at lower CFM and still capture and contain cooking effluent, a commercial style hood with a larger containment reservoir below the baffles is needed.

tl;dr: practicality of duct size is a significant consideration.