Hood FAQ

February 27, 2018
last modified: February 27, 2018

I wrote the following for our interior designer who did not understand why we want a large deep hood. I thought others might find it useful. This is based on a number of conversations with people at CaptiveAire, Greenheck, and Braun as well as posts here from @Kaseki and others.

Please add comments and I will update this as I have time.


This morning I returned home from breakfast and could still smell the less appealing bits of last night’s dinner.

We currently have a Vent-A-Hood with a proper make-up-air system that has overall proven quite inadequate and as I’ve recently learned is actually not a good hood option despite what sales folks proclaim. They are poor at capturing and exhausting effluent, are not quieter than other hoods, noisier than some, and are a major PITA to clean. The one thing I can say for VAH is that it does seem to do a good job of removing grease from the air stream before exhausting it - if only it collected and exhausted more of the effluent.

Standard residential options from Wolf, Best (Braun), ModernAire, Zephyr and others are only marginally better than VAH. This is perhaps due primarily to a common flaw of all of them in having near zero containment area along with a light bar that reduces the open aperture.

The Problem - Effluent

Hot effluent from cooking (odors, grease, steam, heat and by-products of gas combustion) rise from the cooking area. As it rises it spreads out and has quite varying velocity with erratic bursts, sometimes in excess of 4,000 CFM.

For us our primary concern is our health and getting rid of the by-products of gas combustion. Second is reducing cooking odors as much as possible for ourselves and when we have guests. Third is reducing the heat in our kitchen when doing a lot of cooking and Fourth is reducing grease build-up on kitchen surfaces.

The critical bits to capturing and exhausting this effluent then are:

1 - Hood Aperture. The overall opening or aperture at the bottom of the hood must be large enough to capture the effluent as it rises and spreads out. Generally, for non-commercial kitchens, the hood should extend 3-12” beyond the cooking surface on each open side. So, for a 48” x 24” range we’d ideally want at least a 60” wide by 36” deep hood opening. This will still leave some effluent uncaptured but is much better than what is typically installed and beyond this we’re encountering very marginal gains.

CFM cannot make up for too small a hood opening. It takes about 7 times as much airflow (CFM) just to make a 27” deep hood perform like a 30” hood. So, rather than 600 CFM you’d need about 4200 CFM. That is not practical.

The consumer appliance industry focuses almost exclusively on CFM. CFM alone is actually a quite poor indicator of hood performance — but does make for good marketing.

It is also critical to understand that we are talking about the opening to the containment area, not the overall hood size. A 27” deep hood with a 4” light bar along the front edge will actually only have about a 23” opening.

2 - Containment Area. This is where most consumer hoods fail. Effluent from cooking does not rise evenly but in spits and bursts. These can overwhelm the exhaust system and when this happens the odors and grease roll out from under the hood and spread throughout the kitchen. Hoods need a large open volume of area to contain these bursts until the exhaust system can exhaust it.

For example, you have a 1200 CFM Modernaire hood and are pan frying a steak. As it cooks the effluent velocity is varying between perhaps 200 and 2,000 CFM. Any effluent over the approximate 800 CFM that the hood can immediately exhaust will not have anywhere to go and so will roll out from under the hood and spread throughout the room. When you turn the steak over and it hits the pan you get a quick 3,000 CFM burst of greasy smelly effluent that hugely overwhelms the hood’s exhaust capabilities. With the exhaust blower overwhelmed and no containment area the odors, grease, heat and steam spread throughout the kitchen (and house). And then largely remain there until all of the air in the house has been exchanged a few times.

This is why commercial hoods are always so big and empty.

3 - Exhaust (CFM). With a properly designed hood the exhaust system need only keep up with the average effluent produced as the volume of the hood itself contains the bursts until they can be exhausted. A properly designed hood can actually use fewer CFM’s so long as the velocity is enough for the filter baffles to remove grease.

To be continued...

Comments (32)

  • opaone


    For a kitchen that is more show than use and that does not have a gas range or cooktop this is not much of a problem and almost any hood will suffice. Otherwise we’ll want to carefully consider our options.

    Also keep in mind that it’s best to eliminate effluent as close to the source as possible. Once spread out it it much more difficult to remove*.

    A - Downdraft. As mentioned above it is just about impossible to overcome the force of hot rising effluent. Downdraft is nearly useless and will only exhaust about 0 and 5% of effluent.

    B - Recirculating. Potentially a good solution though none currently on the market actually work and should be avoided. To work properly they need a system that effectively filters the air to remove grease, moisture, odors, gas by-products and heat — the purpose of an exhaust hood. The filter system to do this requires about a 3’x3’x5’ space, costs about $9,000 and uses a lot of energy. Even being energy conscience and not liking the idea of exhausting conditioned air and bringing in outside air that needs to be conditioned, this is not a good investment.

    C - Vent-A-Hood. Will capture and exhaust about 20-50% of effluent. However, these are noisier and more difficult to clean than standard consumer hoods.

    D - Wolf, Zephyr, Bluestar, Best/Braun, ModernAire. These consumer hoods suffer from poor design. They lack sufficient open aperture and containment area, have baffles at not sufficient enough angle to drain off grease (so must be cleaned more often and don't work as well between cleanings), and often lack sufficient air distribution behind the baffle filters. They will capture and exhaust about 20-50% of effluent. The remaining 50-80% of steam, odors, grease and by-products of gas combustion will spread out in the kitchen and house until the air in the house is exchanged enough to reduce them.

    Note. There is a popular video of a sales person placing a smoke bomb in a pan to show how good one of these works. This is quite deceptive. The deception is in using cold smoke. Hot bursty effluent that results from cooking reacts much differently and will not bend in to a hood the way the cold smoke does.

    E -

    F - CaptiveAire, Greenheck, HoodMart. These are commercial hoods. They have sufficient open aperture, containment volume, filtration, and exhaust to properly handle the effluent.

    Solutions Part II - The Gap

    Solution E above is not a mistake. There is currently a huge gap between the minimally effective consumer hoods (C & D) and commercial hoods (F) such as CaptiveAire.

    Hoods in C & D, along with a proper MUA system are likely sufficient for many people who do not cook much, do not produce much in the way of effluent or do not mind lingering odors. We’ve had a Vent-A-Hood over our all gas Wolf range for about 15 years and while we do want something that works much better, we have done OK and could certainly go another 15 years if we had to.

    A commercial hood however is large, expensive and likely overkill for most.

    A good solution for many people (E) is somewhere in the gap — a hood that has a sufficient and fully open aperture of 27” to 36” deep, a proper volume of containment area, effective and fairly easy to clean filters, and a multi-speed inline fan. Hopefully Bluestar, Wolf, CaptiveAire, ModernAire or someone will begin offering something like this.

    To be continued...

  • opaone

    System Design

    A few miscellaneous bits to keep in mind.

    CFM - The CFM ratings of any exhausting device are based on what is called free air flow. In actual use these fans produce lower CFM’s (and sometimes much lower) due to static pressure losses. Lack of make up air (that requires the fan to suck air in through gaps in walls), too small of ducts, bends in ducts, long duct runs, exterior caps and other exhausting appliances will all increase static pressure and reduce the effectiveness or CFM’s of the fan.

    How Many CFM's? - @Kaseki has calculated that about 90 CFM per sq ft of aperture opening s/b sufficient. It is also important to check with the hood manufacturer to make sure that you have enough flow rate over the baffles for them to effectively remove grease as well as not too much flow rate. The 90 CFM flow rate should be correct for most hood designs.

    Fan Curve - This goes along with CFM measurements. The fan curve tells you what CFM the fan will produce at what static pressure (measured in inches of water). By adding up all of the static pressure losses (hood, ductwork, exhaust cap, interior air pressure from lack of MUA) you can determine what the actual CFM of your hood system will be.

    Duct Design - Duct size, length, bends and exhaust caps all have an impact on performance and on noise. A duct should ideally be perfectly straight but this is not always possible. For reference, each 90° bend is the equivalent of about 11’ of duct. A 45° bend is the equivalent of about 4’ of duct. Flex duct about 5’-25’. Besides reducing performance, each bend CAN also increase noise. I say can because a bend between the hood and fan will increase noise itself but may actually reduce overall noise as it acts as a baffle between the inline or exterior fan and your ears.

    Duct Velocity - The duct should be sized for a minimum of 600 ft/min and a maximum of 2000 ft/min. Velocities above 2,000 result in increased noise and lower efficiency. Velocities below 600 can result in particulates in the effluent, primarily grease, settling on duct and other surfaces.

    Inline, in hood, or exterior fans - The fan (or blower) may be located in the hood, inline in the duct, or outside. A large exterior blower is usually the quietest because the blade rotation that causes noise is fairly slow. These are expensive and not too aesthetically pleasing though.

    @Kaseki has mentioned that an inline or external fan with a duct silencer between the fan and hood is the best solution for quietness and based on my research this is likely correct.

    A fan in the hood itself will be the loudest.

    Much noise is due to vibrations traveling from the fan and duct in to the structure of the house. To reduce this the fan and nearby duct should be mounted using isolation dampers.

    Wherever your fan is located make sure that it is accessible for cleaning and that you check it frequently and clean it when necessary. A duct fire from built up grease is not something desirable.

    Duct Silencer - A duct silencer such as those available from Fantech may help to decrease noise.

    Make Up Air (MUA) System. This is about as critical as the hood itself, particularly in newer houses that are more sealed and energy efficient.

    This has a significant affect on how well your hood works and on preventing backdrafting of other non-sealed gas appliances. The MUA CFM should match that of the hood and ideally that of each ‘speed’ to prevent having too much or too little MUA.

    Due to static pressure losses from lack of make up air, a 600 CFM range hood in a moderately well sealed house may only function at about 100 CFM.

    In a well sealed house it may also be a good idea to consider other exhausting appliances as well:

    Clothes Dryer - 300-600 CFM

    Central Vacuum - 100-200 CFM

    Bath Exhaust - 20-200 CFM each

    Wood Burning Fireplace - 300-1200 CFM each

    This is an area where building codes have not kept up. Codes still assume a fairly leaky house with an ACH50 of about 4.0 or higher. Newer homes in northern climates are almost always under 3.0 and many under 1.0 which means that exhausting appliances cannot pull sufficient air in through leaks and passive intake.

    Ideally a Make Up Air system should talk to each exhausting appliance, know how much air they are all exhausting combined, and then bring in that exact same amount of fresh outside air. It should then condition it (heat, cool, de-humidify) if necessary. Too little make-up air reduces the effectiveness of exhausting appliances, sucks air in through leaks that can be uncomfortable, and can cause some to backdraft. Too much make-up air isn’t good either as during cold winters it will force warm and moist indoor air in to wall cavities creating mold problems.

    * Toilets are like Ranges and exhaust fans like hoods.

    Dealing with odors as close to the source as possible applies to other areas as well. Think about the ceiling exhaust fan in a bathroom. In a 3/4 or full bath the ceiling exhaust is necessary to remove hairspray or the extremely humid air from baths and showers. But think about other odors. We create them and then the ceiling fan sucks them up past our nose before exhausting just a little bit of them. However, if we have an exhaust located behind the toilet it will exhaust much of the odor before it reaches our nose or spreads out in the room. We did this using a 140 CFM inline duct fan in a remodel about 15 years ago and it works extremely well.

    But wait, there’s more (RIP Billy Mays). An exhaust behind the toilet takes care of maybe 60% of the odor. Sometimes that’s still an issue. A Toto Washlette with an air freshener that exhausts out of the back will by itself take care of about 50% of odor. The combination though, in practice, is nearly completely effective and is what we, thankfully, currently have.

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  • opaone

    Here's a section of a commercial hood from CaptiveAire. Note the large containment area, baffle filters at sufficient angle to drain off grease, grease catch, and the open area between the baffle filters and exhaust riser that will allow adequate distribution of air across filters. Equal distribution of exhaust air across the baffle filters helps them to be more effective and for the hood to be quieter.

    Here's the same hood with an overlay showing a typical consumer hood in blue. Note the lack of containment area below the filter (red).

    The blue box towards the front is the light/control bar that many of these hoods have and that decreases aperture. So, while this is overall 27" deep, it actually only has an aperture opening of about 23".

    The red line is the baffle filter in typical consumer hoods. It is too flat for grease to drain off. Buildup will reduce effectiveness and increase noise as well as require more frequent cleaning.

    Think about pan frying a steak on the front burner of the stove with the effluent rising and spreading out. Which hood will do a better job?

  • kaseki

    I see I have a project here for later today.

    There was a Garden Web FAQ for this long ago. As I vaguely recall I thought it was not wrong, but not as thorough as it could be, nor really up to date. Looks like youall are getting some good stuff out there, and I will be happy to help refine it as soon as I can.

    opaone thanked kaseki
  • kaseki


    This FAQ is now so long that it is not practical to copy the whole treatise and annotate it. So let me make comments here that can be considered as potential sources of editing by the primary authors. To not lose all my work I will comment in parts.

    I start with the attribution to me. I believe my view (and hopefully whatever I've written the hundreds of times I written) is that an external or in-line blower with sufficient duct space between itself and the hood can be quieted by an in-line silencer. The key factor for inclusion is that the silencer is larger in diameter than its mating duct and fairly long. Dimensions can be found at Fantech's website.

    The 4000 CFM value needs some reference. I would expect that to be the performance of a bonfire. The largest velocity I've seen (from the Finnish study I have referenced in the past -- and can re-reference if desirable) was 1.2 m/s (4 ft/s). And this drops off with angle. The diameter of the largest residential pan I think likely is 16 inches, so at the hood the brunt of the effluent diameter would be around 24 inches. Even if constant velocity exists over this area, the flow rate would be (pi/4) * 4 ft/s * (2 ft)^2 * 60 s/min = 188 CFM. The specific flow rate is 4 ft/s * 60 s/min = 240 ft/min = 240 CFM/sq. ft. Hence, even if the entire cooking surface is accomplishing this specific flow rate, as with a commercial griddle, the hood only has to handle the hot surface area times 240 CFM/sq. ft. [So 4000 CFM seems to be too much for realism unless a large commercial grill is envisioned.]

    The presence of capture volume, hood aperture area extent, and baffle aerodynamic behavior along with hood configuration, I suspect, lead to not needing the blower to induce this specific flow rate over the entire hood aperture. Greenheck calls for 85 CFM/sq. ft. for "medium" cooking appliances (Figure 4, page 9 of the Greenheck Guide for what that encompasses) with a 1.2 factor for island hoods (page 12 figure 8). I have suggested 90 CFM/sq.ft. in this forum because that is what I estimate I have with my Wolf Pro Island hood that works successfully with induction wokking in a peninsula layout. Also, for most people, even with 48-inch ranges, going beyond 1500 CFM hood blower rating and counterpart MUA will be their practical limit.

  • kaseki


    "For us our primary concern is our health and getting rid of the
    by-products of gas combustion. Second is reducing cooking odors as much
    as possible for ourselves and when we have guests. Third is reducing the
    heat in our kitchen when doing a lot of cooking and Fourth is reducing
    grease build-up on kitchen surfaces."

    While I am a fanatic about odor removal, I would order these as

    1. combustion products (not relevant to induction)
    2. grease buildup on walls, fabrics, and other surfaces
    3. odor removal (tends to be attached to the grease in many cases)
    4. heat removal

    o Hood aperture: Here I would argue for taking the contour of the largest pans to be used on all of the burners, and increasing that boundary by the tangent of 10 degrees times the height of the hood over the pans. This is less "conservative" than your dimensions, but probably more practical for likely hoods that will be purchased by most. It is the hot surfaces and gas sliding around the pan edges that is expanding, not the dimensions of the range. Of course, larger is better, but has a penalty of requiring more hood system blower flow rate and more MUA system blower flow rate.

    o " effluent velocity is varying between perhaps 200 and 2,000 CFM" I appreciate the idea here, but I have never seen this flow rate quoted in any material, so a reference might be helpful. A 2000 CFM "breeze" above a steak would be quite noticeable -- and remarkable. In other words I have my doubts about the actual values, not the point.

    o Also, a FAQ must always use the correct dimensions for parameters that are touted. CFM is a flow rate. Feet/minute is a velocity.

    o Point D. Actually my Wolf hood does pretty well at effluent capture and containment, at least for our cooking, including searing steaks on a hot cast iron griddle pan (so long as my wife turns on the hood). However, we have only induction under the hood -- no gas grill.

    o Duct velocity. The higher end of the range (2000 ft/min) should be avoided if the duct will be cold (even flowing air) because turbulence induced impinging grease tends to stick, and turbulence, especially around bends, increases with velocity.

    o "Ideally a Make Up Air system should talk to each exhausting appliance,
    know how much air they are all exhausting combined, and then bring in
    that exact same amount of fresh outside air." Alternatively, the pressure differential between inside and outside can be used in a control loop to establish the MUA blower power.

    A less complex system is often used in commercial buildings where a bypass duct is constructed around the blower such that a controlled damper allows air to bypass the main MUA feed if the building pressure is too high. Such as scheme could be adapted to a residence although I don't know of any organization doing this.

    It may be a decent approximation for the MUA blower to track the vent blower. I think the Fantech MUA system works this way, but I haven't checked it out. Except for fireplace operation, this may be close enough for adequate hood compensation.

    The safety issue has to be met however. And this is an interior pressure drop issue where gas combustion appliances do not have their own sealed MUA. Here, the data in the chart following should be incorporated into the FAQ.

  • kaseki


    I think these two images can help illustrate the containment aspect.


    no spillage due to enough flow rate

  • kaseki


    The toilet discussion might be off-putting to some. Significantly condensed, it could lead into a view of a Captive Aire commercial system with the MUA introduced over the cook's area in front of the hood. However, in restaurants, the usual design is to supply the kitchen area with slightly less than optimal MUA and put the rest into the seating area so that a breeze flows from seating to preparation further keeping kitchen odors out of the seating area. Often, only the seating area needs to be air conditioned.

    MUA needs a lot more description. Topics to address are heating and cooling while accounting for any pressure loses induced. Filtering (eagles, sparrows, butterflies, mosquitoes, gnats, lint, dust -- where to limit the ingress of undesirable stuff -- and what does it cost in pressure loss. A 4-inch pleated 3M 2-ft square filter at 1500 CFM might have a pressure loss of an inch of water column. This can't be used in a passive system without depressurizing the house; instead, a blower is essential.

    Bottom line: The MUA system needs to be engineered just as the hood exhaust system has to be. Screened passive may work for some so long as there is no issue with combustion gas appliances back-drafting.

    The end of the FAQ will need a reference section that I will help with once the organization of the main body is complete.

    Fan curve needs an illustration and a bit more explanation. Here is an example curve (from Broan).

    B. There is a VaH recirculatiing hood that at least one forum member owns and claims is adequate. I forget the model number (which I am certain I collected for just such purposes as this, but most recently jwvideo commented on it so it should be searchable.)

    A description of baffles vs. mesh filters is also needed. I'm not up to that at the moment. Probably an outline of the FAQ needs to be stabilized, then the existing writing stuffed into it, and then the holes filled. I'll leave the outline to you.

  • kaseki

    Last, at least for tonight,

    (a) we need to know from Houzz what they need in terms of permissions from source material owners to use images, and who has to get the permissions. It may depend on whether this forum is considered educational or commercial.

    (b) it would be good if the original GW FAQ was found to be retrievable, as it may have some text worth incorporating.

  • opaone

    Great stuff. Thanks. I'll work on incorporating over the next day or two.

    BTW, image use s/b covered under creative commons. We should include attribution, original source if possible, whenever we can.

  • kaseki

    I'll provide those for any I suggest in the next pass once you decide which ones you might want to use.

  • opaone

    Finally getting back to this... In no particular order.

    The 4000 and 2000 CFM numbers are guesstimates by one of the engineers. Accuracy isn't important as they are only representative to make the point that effluent is bursty and can easily overwhelm almost any blower. Maybe needs to be worded better?

    Better info on pressure balancing would be appreciated. I didn't mention pressure differential systems because those systems have historically been quite expensive, need numerous sensors on various sides of the house due to pressure differences on windward & leeward sides (and these must be maintained) and from my understanding are not really that effective for single family residential vs simple CFM for CFM balancing. And that exhausts my knowledge.

    I think a more detailed discussion on hood sizing would be useful. I was trying to keep the main body fairly simple and leave more detailed discussions for further down in the thread.

    More to come...

  • kaseki

    Yes, the entire MUA topic needs more. The problem is that MUA for a restaurant, while higher flow rate than for a residence, is vastly easier to configure. One reason is the hoods run at constant flow rate. Another is the construction aspects/architectures. A third is the aesthetics and comfort requirements. And a fourth is much broader MUA support in the commercial HVAC industry.

    Take your time.

  • kaseki


    What we need first is an outline (topic headings and subheadings) that we can pack the descriptive material segments under. Are you developing one or shall I try my hand at it?

    BTW, I did find the old FAQ at https://www.gardenweb.com/discussions/2766920/overview-of-vent-hoods.

    It lacks any info on how to size the important elements and the requirements that drive the sizing.

  • opaone

    Good idea. If you have time go for it. Otherwise I'll take a stab at it sometime in the next day or two.

    My thought is maybe 3 levels of detail; 1) Introductory stuff that every reader should know, 2) Moderate detail that more serious cooks should know to oversee proper design & installation by others, 3) Full detail for DIY, people who want to know all of the why's for stuff or people who just like lots of detail.

  • kaseki

    Here is my preliminary suggestion, but I still have to take the time to consider it further:

    • 1. Introduction: [What this FAQ addresses and why it has been written]

    • 2. Definitions: [This covers your defs above and any others we need, including any non-redundant defs from the existing Appliance Forum FAQ. We may have to add to this as we expand the detailed sections.]

    • 3. High level summary [your part 2; could stand a better name]

    • 4. Ventilation Systems

    • 4.1 Requirements

    • 4.2 Capture area

    • 4.3 Containment air velocity

    • 4.4 Duct sizing

    • 4.5 Silencer application

    • 4.6 Blower Sizing [fan curves and such vs. pressure losses; maybe placed after Blower Options]

    • 4.7 Blower Options [internal, in-line, external (wall or roof), down-slope vs. up-blast]

    • 5. Make-up Air Systems

    • 5.1 Requirements [includes the safety negative pressure limits, low turbulence around the hood, filtering, heating, etc.]

    • 5.2 Components [blowers, filters, heating, ducting, silencer]

    • 5.3 Control options

    • 6. References

    • 6.1 Cited

    • 6.2 General Reading

    • 7. Example Exam [just kidding]

    Once we agree on an outline, one of us should pack it with what we have and pass it to the other. We may want to do this by email or PM until it is ready for others to critique. I don't want to be appending an ever growing FAQ to this thread every time we make a change, and two cooks is enough for the initial broth.

    We will also need to determine what development/writing tool we are going to use that also copies nicely into the Houzz formatting scheme. It would be nice if we could just edit the thing in Houzz format on some houzz.com link independent of the thread formatting system.


  • opaone

    Overall looks great. One primary thought...

    85% of people read only the first paragraph. Each additional paragraph looses some readers.

    So, I think definitions either after the higher level summary or at the end. And, we need to make the first few paragraphs and pages as meaningful for as many people as possible. Pack in the most meaningful stuff for non-technical people as appropriate and then delve in to the technical.

    Add something between 4.3 and 4.4 about CFM in general (your 90 / sq ft, etc.) and then delve in to what's needed to achieve that CFM?

    Are you thinking static pressure calcs in duct sizing or elsewhere?

  • kaseki

    I don't object to the content I listed for section 3 (your section 2) being first, if it can say enough to be helpful, and preferably capture the reader. Most won't want to read the FAQ to begin with; they want a simple answer to a question that they don't realize is actually complex. Imagine the counterpart difficulty of defining the assemblage of parts for a do-it-yourself range with thousands of options. This is what integrating a hood system into a residential architecture and matching it to a cooking appliance is equivalent to. (And this is why professional HVAC companies define the systems for restaurants.)

    Many FAQs on the Internet answer specific questions. However for kitchen ventilation a question:answer format is a poor choice due to the degree of explanation needed and the interaction among all the parts affecting the efficiency of the whole. In my view, this FAQ is better tailored toward readers using it for education. It can't be used to answer 'What hood should I buy' but only 'What do I need to know to ascertain whether a particular hood [ventilation system] is suitable,' or perhaps 'What do I need to know to cull through thousands of ventilation components to find a set that successfully captures and contains for my particular conditions, is affordable, and looks good.'

    I intended 4.3 would handle the '90' subject.

    I also hoped to avoid writing the relevant contents of multiple ASHRAE Handbooks into the FAQ, so providing details such as losses in myriad duct forms without providing the equivalent levels of detail for other subjects may not be as useful as one might hope. Some basic duct loss info may be useful as indicative of other losses, but in the order of things that interfere with flow rate, I would rate MUA restriction first (for many situations), baffle/filter restriction second, hood internal transitions to the duct third for short ducts, and duct loss third for longer duct lengths and numbers of bends.

    But baffle loss data for residential hood baffles isn't available and hood transition losses for residential hoods is not available. Sometimes we don't get fan curves to make use of the pressure loss data. And due to its counterpart complexity, the actual performance of MUA systems is also ill defined, or at least difficult to quantitatively define. Thus, qualitative comments may be the best that can be made about dominant loss mechanisms, and leading the reader toward rule-of-thumb characteristics deemed sufficient for successful capture and containment most of the time may be the best we can do.

  • nanj

    The beauty of internet forums is the generosity of knowledgeable people who share their wisdom for the benefit of others. Thanks to you both!

  • PRO
    Neptune Power Washing

    With all this, I have to ask: Is there such a thing as a heater that is inside of a duct with a fan that comes from the outside to preheat the make up air so that, in winter, it is not brutally cold? Something that is prefabricated and a handy homeowner or HVAC technician can install out of the box? I suppose it would be electric, but if only run when the exhaust fan is, probably not that much of an expense. I also had the idea of, for a passive system, having a duct that took a long way around a house or a basement to preheat the air and maybe stop drafts but that idea was turned down here.

  • kaseki

    While I use a hydronic boiler loop to a heat exchanger (radiator), others have used electric, which is generally simpler depending on electrical supply factors. Greenheck sells electric ones for commercial systems, and Fantech for residential systems. Broan may have something. Forum member Cooksnsews showed an electric one in a long ago thread here.

    Blower is white; heater is aluminum

    My notes indicate the link is/was:


    Unfortunately, the images in that link are now defunct due to a change in Photobucket's policies.

    Heating in a basement is certainly doable, given something like a Modine heater that warms the basement air on demand and can manage the 40k BTUh that may be needed (well over 20 kW for higher flow rate, colder climate conditions; 1 BTUh = 3.4W). It is necessary that the air flow path from basement to kitchen be low pressure loss, and in all cases, combustion appliances require minimal (see list farther up this thread) pressure loss relative to the outside.

    Relationship of MUA heating required vs. air flow rate and differential temperature.

    It would be best to not burden this thread with questions unrelated to the FAQ itself. Questions of this sort would be better asked in new threads. I will interpret this question as a reminder to include information on the subject when the FAQ proper is written.


  • opaone

    Here is a good visual of how poorly consumer hoods function and why better hoods are needed. Typically effluent contains grease, steam and odors but not so much visible smoke. In this case there was visible smoke so it's easier to see what's happening.

    This is a Ventahood MagicLung running at full power (both blowers on high). Make Up Air (MUA) was running at full power of 800 CFM and no other exhausting appliances were running so there was sufficient make up air. The duct for the hood goes straight up 4' + 90° wide el + 6' straight horizontal to where it exits the side of the house. Static pressure for the duct system is below the maximum specified by VAH. The hood and blowers were fully cleaned about 4 weeks ago.

    My guesstimate is that about 30% of effluent was being exhausted by the hood. There was considerable effluent rolling out from under the hood and spreading throughout the kitchen indicating that the hood was not able to exhaust even the limited amount of effluent what was captured.

    From in front it appeared that there is a dead spot in the middle of the hood where most of the roll-out was occurring with the effluent rising towards the left & right sides seeming to be exhausted better though not fully. Given the interior baffle design this was surprising.

    This hood is much louder than commercial hoods. The ambient noise level in this kitchen was 44 dB(a). The hood increased this to 54 dB(a) on 25% power @ 5' from in front of the hood, 62 dB(a) on 50% power and 67 dB(a) on 100%. A commercial Captiveaire hood is 52 dB(a) @ 5' from in front of the hood (at a D'Amico & Son restaurant). This commercial hood exhausts nearly 100% of effluent and is much quieter.

    FWIW, the EU recently released its latest noise study that recommends exposure to levels above 53 dB(a) be limited to no more than 4 minutes.

  • kaseki

    Another demonstration of why the hood capture aperture has to extend far enough to encompass the plume size at the hood height. Thanks opaone.

    For the record, what is the range size, what is the entry aperture size, and what is the hood height above the range?

  • malokd
    Any idea what it would cost to get a captiveaire or similar exhaust in a residential kitchen?
  • opaone

    For ours the hood itself (60"x30" insulated hood, filters, grease catch, LED lighting) is about $3k and the direct drive blower (1500 CFM @ -1.2" wc) about $1k. Plus ducting & installation by our HVAC contractor of about $1k.

    We may also get a Captiveaire MUA w/ gas fired heater which all together is another $9k. This includes MUA blower, gas furnace, temp and air pressure sensor system and a control system that keeps MUA perfectly matched to the hood exhaust. Still debating this one though.

  • kaseki

    As I have mentioned in several threads, but perhaps not above in this one, one tricky aspect of MUA, especially if introduced after house construction, is "layout," that is, where it is taken from, how is transported to the interior, to what degree is it filtered, to what degree is it air conditioned, and where it is introduced to flow to the kitchen. In particular, one doesn't want the MUA to cause a draft that displaces the rising cooking plume from being captured.

    Available volumes for using ducting for this may be lacking in many architectures. It may be necessary to introduce the MUA into a basement or mudroom and heat it there as it passes through to the kitchen via the house interior.

    Ceiling diffusers, floor diffusers, and other duct related approaches should introduce the air far enough away from the hood, or close by but aimed away from the hood so that the MUA velocity is reduced as it expands into the room and the flow is as laminar as feasible as the MUA approaches the hood.

  • Dormelles

    Suggested Additions:

    #5.4 - Local Building Code Compliance and Permits: A majority of cities in the United States will have adopted the International Residential Code (IRC) for all aspects of construction and renovation. This code states as of the most recent 2015 version that any kitchen exhaust that withdraws a capacity of 401 CFM or higher must have a makeup air system that meets certain requirements. Therefore, exahaust fans with a capacity of up to 400 are acceptable without makeup air. Building permits are required by most cities for installing new ducting when replacing a non-ducted recirculating hood with a ducted hood. Some cities will have adopted amendments or exceptions to the IRC, making it all the more important to check local exhaust requirements as some cities will require makeup air at a lower threshhold such as 350 CFM or may not require makeup air under certain exceptions such as up to 600 CFM.

    * * * * * *

    My personal observations about the FAQ thus far:

    1. Thanks for this great advice. I found it only after weeks of research and within minutes of almost ordering a hood which may not now be my first choice due to limited aperture and containment area. I wish this post had been easier to find. I've read many posts about hoods on Houzz by searching for specific brand reviews and happened to stumble across this linked from a different thread. At least half the information here I had never read before!
    2. CaptiveAire, Greenheck and Hood Mart are recommended as suppliers for adequate hoods, but the barrier to entry for most homeowners with these systems is quite high due to cost exceeding $3K, mounted weight of the units which can quickly reach several hundred pounds, and limited shopping options for ordering online or finding local dealers. For $3K or less, a homeowner can get an outstanding hood from a top residential brand with plenty of ordering and lighter-weight mounting options. The original poster left "Option E" blank, recognizing that a gap exists in the market for something between residential and commercial hoods, but until that gap is filled, perhaps we should create a dedicated section for advice to homeowners on how to maximize choices among residential hood offerings.
    3. Following up to point #2, not only are commercial hoods going to be prohibitive for many homeowners to consider for various reasons, but the makeup air requirement is also going to prohibit many homeowners from selecting a model that exceeds 400 CFM. That's why there are so many hoods available that advertise being "code compliant" with lower CFM ratings. The larger the hood you select, the less likelihood there is that it comes in a 400 or lower CFM rating. In the southern United States and in older homes, makeup air just isn't all that practical since weather is temperate year-round, and a window can be opened when the fan is running. Also, many homes aren't so airtight that a maximum 400 CFM hood with 3 speed settings typically run briefly on lower speeds would demand makeup air even with windows kept closed. With so much of the United States being home to middle-class income earners, a lot of homes that are candidates for kitchen vent hood upgrades are going to be older and not nearly so airtight as newer, expensive homes. This may shift as more homes are built using better insulation standards.
    4. In closing, while I really appreciate all this information as will many other readers, it does leave me somewhat confused as to where to go next. I'd like an effective hood but may have to launch out into uncharted territory to get one as it seems few homeowners are shopping for this type of setup over the standard 30" range. More specific help as to model numbers, brands, and cost would be really helpful for homeowners with the typical 30". Thanks!
  • kaseki

    Let me submit my personal apology for not being able to take the time to re-organize the FAQ and add missing portions as perhaps evident in the material above. When I might be able to allocate the time, there is still the issue of how it gets edited by multiple submissions, because the format here is better suited to short commentary, not long treatises that get repeated in-line as material is added and corrected. One doesn't want the effort to be too dictatorial.

    Perhaps when I can reorganize it, we can proceed by adding comments, the comments are incorporated into the original first message via editing, and when happy the correction submitter deletes most of his or her comment to a succinct statement of what was corrected. That way the thread could be kept to a length that at least the most inquisitive would read.

    A process of this sort may require some experimentation; I am unsure just how much of a long initial message will be tolerated, and how easily it could be mass replaced by editing, because the only sensible approach to maintaining a lengthy document is as an editable document that can be pasted whole into a forum thread.

    So, questions for site maintainers:

    • What limit is there to an initial post length, number of characters, images, etc?
    • What limit is there to partial or total replacement by editing of an initial post?
    • How long does an OP have before no further changes are allowed?

    It doesn't appear that I can flag my own message to get a site admin to comment, and in any case, the flag response is a menu of undesirable options. But maybe it can be used. Opaone, if you see this, feel free to flag it.


  • Dormelles

    @kaseki What you are asking for is a documentation section that can be updated by multiple contributors with a change log showing all previous versions. I am sure Houzz has the software capability to host such a section on its website; whether it will ever choose to do so is another question. Threads such as this one would be tough to edit in that way. We could re-start a thread occasionally with updates and link from the old to the new to clean out content. My problem was not even being able to find this thread in the first place during the intensity of my research, so having a resources section where information like this is easily found would be valuable. Houzz is a repository of outstanding information, but it is so piecemeal that you really have to search multiple threads to hope you find what you need.

  • kaseki

    There was an original Garden Web Hood FAQ that I found again here at Houzz not long after the GW-Houzz merge. I don't know where it is now. So such can exist, if not be obvious to new members. The original FAQ should be incorporated into the new one, as appropriate..

    But I need the answers to my questions before I will even consider starting the re-org and fill in of information. And I spent time I didn't have this AM looking for a way to contact the site team to point them to my questions, and was unable to find any method of contact relevant to forums. I suppose I'd have to buy a sofa or something to get a POC. :)

  • Dormelles

    I believe the additional thread you want is this one: https://www.gardenweb.com/discussions/2766920/overview-of-vent-hoods

    I hear you on the sofa to get any attention. Too bad the direction H seems to be going. :(

  • kaseki

    Very likely. Thanks for the link. Rereading it I see why I thought we should have a more exhaustive treatment.

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