There are a lot of ways to cook, starting with a campfire. If you want to cook on professional-like equipment and keep your house clean, then there are costs (performance, aesthetic, and monetary) that have to be accepted. Further, since the subject is technical, you have to study it or allow people to tell you what to do. Not all such people will be working for you, and some may also lack understanding. This site has a wealth of information you can tap.

A hood will poorly capture and contain rising and expanding effluent when the hood entry aperture does not overlap the plume. The CFM levels in use by residential cooking hoods do not establish a peripheral velocity to entrain effluent from more than an inch or two away from the aperture, and then only if the effluent has a relatively low velocity. Hot cooking can generate plume velocities over a meter per second. So using a hood that doesn't overlap the plumes limits hot cooking to the back burners if one wants the kitchen air free of grease and odor.

In lieu of your hair photo, here is a schlieren photo of a commercial hood and griddle where insufficient air flow rate allows the effluent to escape even with an overlapping hood. You might be able to imagine what would happen if this hood were only half as deep front-to-back.

I am unclear what part of your message is actually a hood specification question and which part describes a fait accompli (done deal). There are hundreds of threads here with instruction on sizing the hood, sizing the air flow, and means of quieting the hood system.

Also, you will need to define your make-up air (MUA) system to go with your hood system unless you have particular circumstances that allow you to not have to use an actual system.

Thanks. (I think).

First, here's some info for you to munch on: https://bamasotan.us/range-exhaust-hood-faq/

You DO NOT want a vent-a-hood. They are loud and the frequency spectrum is especially irritating.

The quietest system will be a commercial system like this: https://www.houzz.com/discussions/5745986/commercial-range-hood-install. For 90% of use it's totally quiet and still does a better job of effluent removal than any consumer hood available. It can also be placed up to 7.5' above the floor so it's totally out of the way. When on higher speeds the frequency spectrum is also much more pleasant than consumer hoods that are a higher pitched whine.

Next best for both quiet and effluent removal would be something like a Wolf hood with an inline blower and a silencer (Fantech) placed in the duct between them.

Thank you. This is what I am going with. Tell me what you think: I am going with a 40" ZLline 698- RD-40 hood insert with a T-85 internal remote blower to be installed in my attic as opposed to an outside wall.

Then, of course, a damper for the exterior opening.

What range/cooktop will this be over? How much do you cook and what styles?

At only 18" deep it will provide exceptionally poor exhaust performance for the front burners and quite poor for the back. From a health standpoint I'd not put this in my house. It really should be at least 22-30" deep.

I cook daily, and wok often. I am getting the 21” deep liner and have the cab. for the liner 23” deep. Wolf 36” dual fuel range.

The 698-RD40 is listed as 18" deep overall but only 16.5" aperture. Even if it is 21" that is too shallow to be very effective unless you never use the front burners.

Let it go, Kaseki. This is not that kind of forum.

Sorry, I apologize for any misunderstanding I have with respect to the kind of forum this is.

@Jacqueline Williams, @kaseki is one of the most knowledgable people you'll find for range hoods and kitchen ventilation. He is trying to help you. Which is indeed what this board is about.

No, his knowledge may be impeccable. I need that, but not judgement, ie. snide comments as to what I am asking. It is not your place to comment one way or the other, unless you are commenting on my question.

Stay away from Ventahood. They are quite loud and not worth the money.

Only you can decide what is and is not worth it to you. At the upper end (Accurex) you get the best, most energy efficient and quietest removal of effluent in a hood that's out of the way overhead and with very good lighting. That's perhaps $6k installed. For a bit less you can get a Wolf hood w/ inline blower (and silencer if space allows) that isn't as quiet nor as good for effluent removal, nor overhead, etc. Still a good system for perhaps $4k.

Below that you get more noise and slightly less effective effluent removal.

How important are these things to you?

Hi all - piggybacking on this forum in the hope someone can help with a simple question I’ve been unable to answer elsewhere. I am remodeling and planning to install a 6-burner 36” “pro-style” gas oven/range with 90k BTUs total output from the range. I am planning to buy a 42” hood that will be vented outside. Both will sit on an external wall to the side of my house where there is minimal foot traffic. I was planning to get a Best hood with a 900-1200 cfm remote/external blower to reduce noise, but the sales rep at my kitchen supplier said that a remote/external blower would need to be at least 15’ feet away from the hood to meaningfully reduce noise. Since my blower will be basically on the opposite of the external wall, I wouldn’t see the noise reduction and it will be a waste of money. I’m willing to spend more money for noise reduction but don’t want to be stupid about it. Was hoping someone here could either validate her advice or explain why she’s wrong. Thanks all.

@Gene G, for noise reduction you do need some distance but nowhere near 20'. Is you plan for an external blower to be directly on the other side of the wall from the hood? E.G., the blower will be about 8" from the hood? Or will you have some length of duct inside before it turns to go outside?

A blower in a hood is the loudest. This can vary greatly though. Kobe and Prizer both have much quieter blowers than Vent-A-Hood for instance. More: https://bamasotan.us/range-exhaust-hood-faq/

A good quality and properly installed external blower, even directly on the other side of the wall, will be quieter than an in-hood blower. How much is difficult to say.

The more duct distance between the hood and blower (external or inline) the better, though there is a point of diminishing returns. A silencer in the duct run between the hood and blower has a very significant effect.

So, even though you're on an external wall there may be some benefit to running some duct inside so that you can place a silencer between your hood and blower.

In order for an external blower to be perceived at the cooktop as having reduced noise when there isn't room for a silencer in the path, the blower should be oversized and operated at partial power so the blade tip speed is lower than its maximum power design speed. This will reduce the nominally highest source of hood noise: blade tip turbulence. Oversized blowers are difficult to package into hoods, so this is a reason for using an external blower.

In any case, the blower size (or in this case operating point) needs to be such that when cooking with hot oil (wok cooking or meat searing, for example) the average air flow into the hood yields a velocity at the hood entry aperture of 90 ft/min (which seems to be sufficient for residential hood configurations).

So a 3.5 x 2 ft hood aperture (7 sq. ft.) needs to flow 7 x 90 = 630 CFM. This will require a blower rated at around 900 -- 1000 CFM, but if you go oversize for lower noise, then perhaps a 1200 CFM rated unit will be suitable. Broan/Best, Wolf, and Abbaka, among others, sell these for down-roof and vertical wall applications.

One other feature of these external blowers is built-in dampers, so along with the hood damper there are two in the path significantly resisting external wind pressure.

This is extremely helpful. I expect there will not be room for a silencer given the short length from the hood to the blower, but it makes sense that an oversized remote blower operating at partial power would have advantages over a smaller blower since it reduces blade tip speed. Regarding the blower itself, our sales rep said that the Best “IQ” system is considered to be the quietest on the market, and their info sheet says that the external blower will be even quieter:

https://avblinq.avbportal.com/assets/products/documents/Best/PKEX2245/PKEX2245.pdf

So it seems that going with the Best EB12 external 1,200 CFM blower is the way to go, even if it’s mounted on the adjacent wall.

I don't have a Best IQ system, so I can't comment on it other than to question whether it develops flow rate as a function of sensed heat; if so, it may be unsuitable for induction cooktops and ranges, and also might fail to provide early turn-on to allow duct heating in cold weather and (as some find relevant) establishment of the air flow in the residence. My Wolf (Broan) 1500 CFM roof blower is slow enough to come to full speed that flow establishment seems to keep up in my configuration. Duct pre-heating in a cold attic can be important for minimizing grease condensation, particularly at lower power settings where the duct velocity is significantly lower than 1000 ft/min.

When you say “lower power settings” do you mean that the blower is configured manually to operate at partial capacity at all times or that it is operated at a a lower setting day-to-day using the standard controls on the hood? Trying to understand if I need to tell our contractor to make any specific configuration changes if we install a remote blower with higher rated capacity than we need.

I meant in this case just turning to blower power down with the control. Example bacon simmering. Plume velocity is low and hence containment won't require 90 ft/min at the hood entry aperture. But the baffles still need some centrifugal extraction capability so the flow velocity will probably want to be a bit higher than just the level needed for successful containment. But at that flow rate, the flow in the duct will be slow and condensation will be enhanced.

Note that most residential cooking conditions are grease-wise sporatic, not commercial like, and deposited grease is likely to rancidify (harden) onto the duct without much hazard from significant grease build up.

Keep in mind that the plume has a spectrum of grease particle sizes, and the smaller ones are not removed at the baffles, but are to be ejected into the outdoor air. This is an imperfect process.

So to beat the point to death: One may have an oversized blower for which one never needs to turn the control to full power, but which one will still want to adjust the flow to whatever is needed for containment without being unduly high and loud. HTH

Understood. Very informative! Thanks for the thoughtful reply.

Very insightful, thanks all. My kitchen remodeling situation: 30" 4- burner gas range - lots of cooking with oil, heavy spices, wok frying and I'm concerned about smell going into/staying in the kitchen/living/dining area + noise (we have an open floor plan). Considering an external blower. Based on reading so far, I'm thinking a 36" hood with 1200 CFM external blower should work. Have a flat roof right above the kitchen, so we may be able to get 8' of duct. Will appreciate your thoughts on this configuration, plus any specific brands you recommend. So far I've looked at Best and Zline - but have no prior experience with them. TIA

Here is the combination I am considering - will appreciate your feedback and alternate suggestions https://www.airportappliance.com/cooking/ranges/professional-gas-range/PRG304WH/

https://www.airportappliance.com/cooking/hoods/vent-hoods/PKEX2239/ with https://www.airportappliance.com/cooking/hoods/venthood-accessories/EB12/

Cooking with hot oil drives a suggested entrainment air velocity; odor capture requires a hood that overlaps all pans that will be in use. We need to solve the second requirement before the first requirement.

I imagine pots, pans, woks, etc. can easily extend to the range top edges. The plume will grow as it rises at nominally 7 degrees. So for a 36-inch hood height (likely needed due to its front-to-back depth) the hood width should be 30 + 2 x 36 tan 7deg = 39 inches. I would go for a 42, but a 36 may work if there are cabinets on each side providing some side skirt effectiveness.

The front to back depth should be 24 + 36 tan 7deg = 28 inches. I am only aware of custom, outdoor, and commercial hoods being available this deep or deeper. Wolf has a 27-inch wall hood, but this includes some light bar space at the front edge that will only provide partial capture area. It could be spaced out. I haven't searched Best for hood depths.

In any case, working from the requirement as an illustration, 39 x 28 / 144 = 7.6 sq. ft. Using 90 ft/min as a goal (for 1.2 m/s peak center gas cooking plume velocity -- seems to work), then the required air-flow rate would be about 700 CFM at the pressure losses that will be present. Without getting into all of those, we can punt and use a conservative factor of 1.5 for the blower's rated CFM, or in this case 1050 CFM. 1000 CFM will do. 1200 will be fine and could be run a bit under max for some noise reduction.

You will want a Fantech silencer in the path between hood and roof blower.

A damper is needed at the hood and in the blower assembly, or just below it. Wolf / Broan roof blowers have one on the outside that opens when the blower operates. Your selected Best EB12 does also.

Note that the fan curve for the EB12 crosses 700 CFM at about 1.1 inches, w.c., so baffles, ducting, and make-up air path must not exceed this value. Make-up air needs to be discussed separately.

The installation guide should be consulted for distances to combustibles, which include the contents of walls all the way to the other side. The riser shown may not be the one needed in some circumstances. Burner BTU seems to be missing from the spec so some research is needed.

Thank you so much, Kaseki. I was concerned about the hood depth as well but I couldn't find one deeper than 22" at my local store, will dig some more online, will appreciate any links, Burner BTU spec is - Front burner - up to 18,000 BTU, Bake Power 20,500 BTU, Broil Power 17,500 BTU - more here - https://www.thermador.com/us/products-list/ranges/30-ranges/PRG304WH#/Tabs=section-technicalspecs/Togglebox=-960854172/Togglebox=-172343114/

I also rechecked available duct length. I have 8' ceiling height, and 1' of flat roofing frame above that. Since the range is 36" high, and the hood installs 24" above that, that leaves 4' of straight duct length before it opens out from the top of the roof. I remember reading in this forum that we need 6' or more of duct height for noticeable sound reduction -- so I'm assuming my option is to build a 2' chase?

Whether the duct is 2 ft of 6 ft is pretty inconsequential to blower fan tip turbulence noise. I thought you had an 8-ft attic space. So for low noise with a residential blower on the roof you will definitely need a blower running at part speed, because the 10-inch Fantech silencer is at least 3-ft long.

Alternatively, one could build a roof pedestal acting as a chase and support that was high enough to embed a silencer, then use a modest sized up-blast commercial blower, or even a residential blower on a sloped surface at the top of the chase.

A typical setup is 3 ft to the counter top, 2.5 to 3 ft to the hood, 1.5 ft hood height, and whatever is left covered by a duct cover. Most pro hoods are put at 30 to 36 inches above the cooktop. The deeper the hood, the higher it has to be to provide clear sight lines to the pans at the back, depending on your height.

I suggest you draw a side view -- to scale -- of the cabinet, counter, hood, etc. with a stick-figure cook. Examine sight lines, head bumping issues, and keep in mind that people typically bend at the waist.

Burner power levels are high enough that I would certainly review the range installation guide before ordering anything to determine what the combustibles requirements are.

Thanks, Kaseki. I drew a side view with a 5'6" person in mind (we are short :), a 36" silencer connected directly to the hood will stick about 2" out of the roof. Assuming we can use a 90-degree elbow, I have the blower installed to the side of the wall (will need some framing to stabilize it). Here is how it looks - please let me know what I may be missing here.

You will need to allocate room for duct clamps for the silencer and perhaps a damper for the interface between hood and duct. A smooth 90-degree bend will have some height to it; more than a little for 8 to 10-inch duct. If you don't have a good reason for sending the effluent down the wall, I would plan on putting a chase or pedestal on the roof and operating the blower at a 20-degree angle or so. This will avoid an undesirable ell bend. Note that the silencer is larger in diameter than the duct. Visit Fantech's site for physical dimensions. The hood interface needs to be determined to ensure that if the hood is mounted to the wall, the duct hole at the top is far enough away for the silencer to fit.

The reason I mentioned the drawing was actually different; it was to check sight lines from cook to pan with the hood at installation height. Evidently, you won't have a problem with a 24-inch deep hood, and maybe not for an even deeper one.

Your drawing is not to scale. With the dimensions shown, there will be only 12 inches between the hood top and the ceiling. Expect the chase to be more than 1 ft high.

Another issue is that with a flat roof, one cannot just depend on gravity and shingles to keep water out. Whatever techniques are in use with the present roof to seal it must not be degraded by the installation. A pro for that type of roof should be consulted or hired.

I would consider a commercial pedestal for a flat roof for this case, and look into a modest commercial blower to mount on it. This would save some added duct height needed to put a 20 degree or so interface to a down-roof residential blower so it can perform as expected, rain rejection wise. You would want an induction motor compatible with the hood controls.

Don't forget MUA.

Very quick thoughts...

- Shouldn't need 1200 CFM for that small of a hood.

- Providing MUA to the outer rooms so that it flows through these rooms and across the kitchen to the hood might help keep odors down. Commercial kitchens are often kept negative pressure to the dining areas to accomplish this.

- Proper containment area would help a lot.

- Is this a single level home/building?

Thanks, kaseki and opaone. I have no idea about MUA so will research :) -- a few updates from last couple of days. We are going with a 36" range as opposed to 30" - https://www.airportappliance.com/cooking/ranges/professional-gas-range/PRG366WH/ -- so planning to go with a 42" hood - https://www.airportappliance.com/cooking/hoods/vent-hoods/WP28M42SB/ as I understand a 36" hood will not have adequate containment - correct?

I'd like to avoid building a chase or pedestal -- so I was wondering if it would work if we lay the silencer horizontally on the flat roof and mount the blower on the wall - here's my rough sketch - thoughts?

I would ask Fantech about using it horizontal for range exhaust. There could be an issue w/ grease pooling in it. They may advise against it, may say it's OK or may say it's OK so long as you check/clean it once a year.

thanks, I shared this with an HVAC installer and they are suggesting that we move the remote blower to a location "as far away as possible" and apparently that will avoid having to use Fantech silencer. I personally would like to have the option of using a silencer, but in this configuration, that would require using two 90-degree elbows - one as shown in my diagram so I can run the 10" duct along the flat roof up to a parapit-type wall, and the other to run the duct vertically along the wall so I can install the silencer along the wall and install the blower on the wall - does that make sense?

something like this

"as far away as possible" can introduce a lot of static pressure and reduce the effectiveness of your hood. Silencer's are nice because they do a great job with no more static pressure than that same length of duct.

As a very rough calc for 8" duct a wide 90° elbow is roughly the equivelent of 10' of straight duct. A short 90° about 16' of duct.

I think what you've outlined above should work.

I don't believe that tip turbulence and duct turbulence induced noise will be attenuated enough by any practical separation of hood and blower.

A 36-inch hood over a 36-inch range will likely have inadequate capture when cooking with hot oils/grease at the smoke point on edge burners. What is captured will be contained if the air flow rate is high enough to immediately (in the case of residential hoods) entrain the plume into the baffle flow.

Note a 24 x 42 inch hood capture area is 7 sq. ft. requiring 630 CFM from a blower rated at 900 - 1000 CFM at zero static pressure.

Thanks again opaone and kaseki - we just got demolition done and discovered that our ceiling height is actually 9' and not 8' (yay!). Also, with our cabinet design, looks like trying to fit a 42" hood is not going to work. So, we are leaning towards the following configuration:

36" cooking range - https://www.thermador.com/us/products-list/ranges/36-ranges/PRG366WH

36" hood with a 1000 CFM blower

https://www.thermador.com/us/products-list/ventilation/ventilation-pairing-ranges-36/PH36HWS

https://www.thermador.com/us/products-list/ventilation/hood-modules/remote-blower/VTR1030W

in the following configuration - the top end of the silencer is going to stick out about 4" to 6" above the roof, but we have to build a curb anyway to install the blower at a slope, so the curb can cover any duct sicking out of the roof. Thoughts?

My instinctive thought is that the proper phrase is: With my 42-inch hood design, the cabinets will be designed to accommodate it. However, you are responsible for all trade-off results, as only you can determine the weighting factors.

Otherwise, you seem to have found a good way of getting a silencer into a short duct space.

Good luck.

Haha.. totally understood, kaseki, I wish we could make the cabinet design work, but unfortunately couldn't, hence the trade-off. Figured we could keep the cooking with hot oil / grease to the middle burners for the most part. My plan is to send this to our GC, HVAC and roofing people and see what they end up with. Either way, thank you so much for all your help -- this has been a great educational (and dare I say fun) experience. Hope I can come back soon and post pictures after the installation is done :)

btw - my GC recommended getting a different type of blower that will be much easier to install on a flat roof - https://www.greenheck.com/shop/roof-mounted-fans/hooded-axial-supply-fans/as-12-433-a4x-qd ( as opposed to building/installing a curb, etc)

Any word of advice/experience with this type of setup? I thought you needed to get the blower and hood from the same brand for compatibility. I'm all for making the roof installation easier, and not having to mess with the roofer's warranty but I would like to make sure these blowers work with different hoods. Wish there was a standard configuration they supported that one could check in the specs and be sure, but I don't see it is that straightforward.

It is convenient that the blower's control scheme match that which the hood manufacturer built into the hood. If this is not achieved, then a different control scheme will be needed either on a wall panel or integrated into the hood, if space allows. You are getting into the EE / Etech skill set in some possible combinations. The motor manufacturer may be able to provide a control that you can make work.

Also, a down-blast blower as shown should be fine so long as it isn't buried in snow. With a proper pedestal mount, that should be easy to avoid (most places). You need to compare the fan curve shown against whatever the hood manufacturer's recommended blower's fan curve is to be sure that there is enough flow at relevant pressure losses.

Also MUA has to be defined before you are done spec'ing the blower.

Thanks. Also, noise-wise, are the hood manufacturer's blowers any less noisy? The one my GC recommended from Greenheck shows a noise rating from 11 to 15 Sones, I wasn't able to find the noise rating for the remote blowers from Thermador, Broan, Wolf, etc - so just curious how they compare to this.

I wouldn't want to get noise complaints from the neighbors because I think 11 Sones will be pretty loud :)

It might be better to get a higher powered model and run it at partial power. This is always a good ploy, where space and $$$ allow, for reducing noise down the duct, and certainly toward the neighbors.

Frankly, I haven't got data for the residential external blowers, including my own, nor have I ever noticed noise from roof-mounted commercial blowers, all of which are much larger than this modest Greenheck device, although surely there is some.

Note that noise from a point source decays as distance squared. How close are your neighbors?

We are in an urban setting, so neighbor's houses are adjacent side to side touching the perimeter walls.

The Greenheck site had more powerful blowers in this configuration, I thought the one I picked which goes up to 1440 cfm should be powerful enough for my 36" hood

Yes, it appears to be powerful enough. What I am bringing up is that some noise abatement can result from running blowers at partial power. If the blower is more powerful than needed, one can have lower noise and sufficient flow. But I don't know what your pressure losses are, particularly in the hood itself, and particularly in the not defined MUA.

Thanks, our plan for MUA is to open a close by window (6' to the left of the range), while running the vent.

As for the pressure loss, with my new design, I only have a little over 3 feet worth of straight duct run between the hood (18" high) and the blower on the roof - so I'm assuming pressure loss is going to be fairly minimal.

So -- I'm leaning more towards a 1200 CFM fan of the same brand as the hood (either Thermador or Broan) to avoid the potential issue of matching the electrical control scheme you'd mentioned earlier. I'll also look into a 1500 CFM model so it can be run at lower fan speeds to reduce noise. Our GC prefers the Greenheck model because it is easier to install on a flat roof. He is trying to avoid having to build a curb to install the Broan or Thermador model at a slope as mentioned in their installation guides. Apparently this will be more work to put roofing material around the curb to properly seal and waterproof.

Anyway, I'm using your earlier calculation formula to estimate the required CFM.

• ideal hood width at a height of 30" from a 36"x24" range = 36 + (2 x 30 tan 7 degrees) = 43.4" (but I am compromising here with a 36" wide hood instead)
• front to back depth = 24 + 30 tan 7 degrees = 27.7"
• capture area = (43.4x27.7)/144 = 8.3 sq ft.
• goal: 90 ft/min
• required air flow rate at zero static pressure = 8.3x90 = 751 CFM
• using a 1.5 factor for pressure losses = 751x1.5 = 1127 CFM

So I guess a 1200 CFM model should be adequate, although running a 1500CFM model at lower speed will be less noisy.

Does that sound about right?

In so far as it goes. However, code (most places) requires deliberate MUA when over a rated 400 CFM. In northern climes, it has to be heated. There would be pressure loss even through a screened window at 751 CFM, unless the window screens covered a large area. And at that flow rate, your room heating will not suffice. In any case, the hood baffles will be the dominant pressure loss, given low pressure loss MUA.

Thanks - just curious what would be my options to put a deliberate MUA in place in this scenario? Is that a separate unit installed through an exterior wall of the kitchen that draws air into the room? Will appreciate it if you can provide a suitable example

MUA systems can vary all over the map depending upon requirements. For example, assuming that the house (a) has no combustion appliances, or (b) combustion appliances have their own MUA, or (c) combustion appliances are in a part of the house not connected -- air wise -- with the kitchen, then one may get away with a duct to the outside. This should have a motorized damper triggered by something related to hood operation. It preferably should include a filter, such as a furnace filter. Air flow should be directed such that the air that makes it to the hood is relatively non-turbulent. In this example, one is providing MUA with a pressure loss of mainly the filter. In colder climes, a heater can be added. If electric it probably won't have much pressure loss itself, like electric dryer coils if you have ever looked at the dryer guts. If hydronic, then one has the air flow pressure loss of what is essentially a car radiator.

For this system type, which I call passive due to not having a blower, the pressure loss is low enough that there won't be much air infiltration through walls, and if the hood blower is properly selected, the hood CFM will be as desired. All this requires some calculation against the hood blower's fan curve.

The alternative is an "active" system with a blower. In this case one needs a way to control the MUA blower vis-a-vis the hood system blower. Control schemes vary. Mine, when finished, will utilize the pressure difference between my kitchen area and the outside. Others may work on flow in the hood duct, or current to the hood system blower, or other schemes.

You may not have read all the references I've scattered about this forum. Here are some reminders.

(1) Go to the Fantech site and review what they have for active MUA systems.

(2) Visit the site of Electro Industries. I think @opaone uses this brand. https://electromn.com/

(3) Read the first 20 or so pages of the Greenheck guide, available at: https://www.tagengineering.ca/wp-content/uploads/2015/02/KVSApplDesign_catalog.pdf