This is based on information and conversations with people at CaptiveAire, Greenheck, Braun, Building Science Corporation and Lawrence Berkeley National Labs as well as posts on Green Building Advisor, AVS Forum, Houzz, HVACTalk and elsewhere.
Please add comments below or on the FAQ post on Houzz.
Part I is an overview and hopefully provides the most critical information that the majority of people need in a concise and readable format.
Part II will go in to greater technical detail for those interested.
A note on COVID: When we exhale, along with CO2 we also exhale any viruses present including COVID. Similar to its relationship to VOC’s and Carcinogens, CO2 is a proxy for the amount of pathogens (viruses in this case) that may be in the air. A doubling of CO2 levels also means a doubling of exhaled viruses and so a doubling of the risk of infection. The lower that CO2 levels are maintained the lower the risk of COVID transmission. Unlike CO2 however, pathogens such as COVID can also be filtered somewhat with effective paper/fiber filter media, ideally MERV-13 or higher. So for the best indoor safety with regards to COVID a combination of air exchange ventilation and good filter media is best. Note also that electronic air cleaners are believed to not be effective regardless of manufacturer claims.
Part I – Overview
EVERYONE who has a kitchen should read all of Part I (down to the heading Part II – Deeper Dive). Your builder, HVAC contractor, local building inspector and appliance sales folks very likely do not know or understand these issues. It is up to homeowners to take responsibility for their and their family’s health.
This will hopefully give you the tools to have beneficial and understandable conversations with your builder and others.
Poor indoor air quality in U.S. homes, aided by poor kitchen ventilation, is a key contributor to our lowest life expectancy of all developed countries and highest rates of chronic disease. Not only do we not live as long but we have more debilitated years, particularly towards the ends of our lives. So rather than 81 healthy years plus 1 disease impacted year we have 69 healthy years plus 8 disease impacted years. Range exhaust hoods are a critical functional part of your house, not just something decorative.
I’ll repeat – It is up to homeowners to take responsibility for their and their family’s health.
Important – The Need For Ventilation
One morning I returned home from breakfast and could still smell the less appealing stale bits of our prior night’s wonderful dinner. What I could not smell was the harmful VOC’s in the air.
We all know (or should) that gas combustion, such as with gas burners, ovens, furnaces, water heaters and other gas appliances produces a number of harmful VOC’s. The only gas appliance in our homes without extensive and often sealed exhaust is also one of the more powerful and the one closest in proximity to us on a regular basis – our kitchen range.
It’s not just gas though. Cooking itself, regardless of heat source being gas, induction or something else, also produces harmful VOC’s and carcinogens with CO, NO2 and H2S leading the way. Possibly worse is the amount of particulate matter produced (for more: The Emerging Risk of Poor IAQ on Cognitive Decline and Alzheimer’s Disease)
We are increasingly learning more, such as research indicating that NO2 and other stuff from cooking in poorly ventilated homes may be why COPD rates are not declining with smoking rates. Frying eggs on an induction hob produces surprisingly significant amounts of Sulfer Dioxide (SO2) and Hydrogen Sulfide (H2S) – you don’t want to breath too much of that stuff.
More than a few researchers have wondered out loud why we are still allowing houses to be built in 2021 that we know are causing health problems because of poor ventilation in general and poor kitchen ventilation in particular.
So contrary to what many induction manufactures would like us to believe, ventilation is critically important for induction as well as gas.
Perspective – Our Health
Let’s back up a bit for some perspective.
The U.S. has the lowest overall life expectancy of all developed countries. We also have the highest rates of preventable chronic disease. And yet we have the best healthcare system in the world and spend significantly more, often two to three times as much, on healthcare per capita as other developed countries.
The problem is garbage in garbage out. We are so unhealthy that our healthcare system cannot make up the difference.
The primary causes of our health problems and early deaths in the U.S. are:
Sedentariness – We don’t move enough. More specifically, we don’t walk or bicycle enough for transportation. There is a very high correlation between the amount that people in a country walk and bicycle for transportation and how healthy they are. Second to this is time spent out of doors – which nearly always involves walking, running or bicycling for recreation. Interestingly, there is zero correlation (actually reverse correlation) between a how many gyms or gym memberships a country has and health. I do weights/mobility workouts three days per week and enjoy doing so but that alone won’t keep me healthy and the healthiest people in the world don’t go to gyms.
Food – We eat too much, we eat too much junk food, we eat too much food contaminated with harmful chemicals and we eat too many fad diets. Micheal Pollan put it best “Eat Food, Not Too Much, Mostly Plants”.
Forced Death – Typically called Accidental Deaths but a problem with that is that most are actually not accidental but easily preventable. Topping the list is the 40,000 people that drivers kill each year. Our highest in the developed world rates of road deaths is by design – road design. If we designed our roads to CROW standards then we’d have about 1/5 as many deaths overall, 1/11 as many bicycle deaths and 1/17 as many pedestrian deaths. We’d have 1/13 as many permanent disabilities that often lead to early death.
The Air We Breath – So, we get to air quality. It may be 4th on the list but is still critically important. There are two components to this; Outdoor Air Quality and Indoor Air Quality or IAQ. We hear a lot about outdoor air quality and it is a very serious health problem. But some research is beginning to indicate that Indoor Air Quality in the U.S. may be as much of a contributor to our high rates of preventible chronic disease and eventual death.
- One quick note. Something that appears over and over in preventative health research is bicycling for transportation. Bicycling provides critical moderate daily activity, reduces death by driver, and does not pollute our air. It also results in stronger communities and more human interaction which also improve health.
- OK, a second note. If you want to avoid the plethora of chronic diseases and disabilities that we are increasingly seeing in people over 40 then read Dan Buettner’s Blue Zones.
Indoor Air Quality / IAQ
“Badly constructed houses do for the healthy what badly constructed hospitals do for the sick. Once insure that the air in a house is stagnant, and sickness is certain to follow.”
– Florence Nightingale, 1859 Notes on Nursing
Our homes have become serious health problems. They have become increasingly sealed up and, particularly in the U.S. where few people open windows, increasingly closed off from the outside. The problem is not lack of oxygen though but too much bad stuff that is not getting exhausted. CO2 leads the way with many other things close behind including a variety of VOC’s, Carcinogens, Pathogens and Particulate Matter.
As far as we know high levels of CO2 are unlikely to lead to death but too high of CO2 levels do impact both mental and physical performance as well as sleep. CO2 levels above 550ppm begin to reduce cognitive performance and over 700 begins to lessen strategic thinking. At 1500 ppm strategic thinking ability has decreased to 50%. The higher the CO2 the greater the negative impact. CO2 is likely the most prevalent VOC in most homes.
CO2 is also an excellent indicator gas. It is constantly produced by our breathing and is easily measured. If CO2 levels are high then we know that we do not have sufficient ventilation and that other VOC’s, Carcinogens, Pathogens and PM may be high as well. Likewise, low CO2 levels (with humans present) are an indicator that we likely have sufficient ventilation and are likely removing other harmful stuff.
CO, NO2, CH2O, Benzene, Acrolein, Pathogens such as COVID and other elements are the common deadlier elements found in many homes. While less prevalent than CO2 they are far more harmful when present. NO2 for instance is produced in significant quantities when frying bacon. This is not likely a problem in an older leaky home, one with a lot of open windows or one with proper kitchen exhaust – but may be problem in a more sealed up home with windows closed and poor ventilation.
“A house built ’to code’ is the worst house allowed by law.”
– Joe Lstirbuck
Current standards for general whole house ventilation, exhausting stale CO2, VOC, Carcinogen, Pathogen and PM laden air, as well as kitchen ventilation are far behind known science. If the ventilation in your home is only ‘to code’ then it is likely not sufficient for good health. Keep in mind that U.S. code not too long ago allowed asbestos and lead based paint. Of 141 chemicals prohibited by EU regulations, only 3 are prohibited in the U.S.
To repeat – It is up to homeowners to take responsibility for their and their family’s health.
There are three key components that should nearly always be well above current U.S. codes; Air Exchange (typically with an HRV or ERV), Kitchen Exhaust (typically a range exhaust hood) and Make Up Air (bringing in fresh air to replace that exhausted by the kitchen hood and other exhausting appliances).
Here we’ll cover the two latter elements.
IMPORTANT – Air Quality Monitoring: If you want to monitor the air quality in your home (and you should) the better options are; IQAir AVP, Awair Element, and a few others. IQAir is my favorite for most people and is easy to use. AVOID Foobot along with any others that do not have an actual NDIR CO2 sensor. Note that the uRAD A3 has recently been found to have some significant errors in CO2 measurements so I no longer recommend any uRAD products.
Dose (And a little dirt is good for us)
It’s critical in these discussions to separate the bad from that not-really-so-bad. A lot of things that are harmful to us are actually only harmful in high doses. The CO2, SO2, H2S and NO2 from frying some eggs and bacon twice per year are likely not too harmful to us, but if we do that every morning then it’s a different story. Keeping some perspective is a good thing, especially with the sky-is-falling fear mongering about so many things in our world today. My goal is to focus on what really is bad and not worry about the rest.
Likewise, some bad things are good. It’s healthy for our immune systems to be exposed to certain things. People who run tours say that Americans are known for getting sick easier than Europeans because we aren’t exposed to things in our food and by being out of doors that build up our tolerance (while on the flip side our food supply has a lot of chemicals and other bits that are harmful to us).
A Bad Hood
In our prior home, where I smelled the stale odors of meals past but not the harmful VOCs dangling about, we had a Vent-A-Hood that we were told was the best and quietest exhaust hood we could get. We had good ducting with one long 90° bend and an oversized wall cap for lower static pressure. We also had a proper make-up air system that brought in as much air as our range hood exhausted.
Overall this ‘best system’ proved inadequate for proper exhaust and it was not at all quiet. With some research I learned that it was actually not a good option despite what sales folks universally proclaimed. VAH are poor at capturing and exhausting effluent, are not quieter than other hoods, noisier than many, 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 too small of capture aperture made worse by the inclusion of a light bar. More on this in a bit.
U.S. consumer range exhaust hoods are loud and irritating – unnecessarily loud and irritating. This is bad in two ways. First is that noise itself, besides being irritating, is bad for our mental and physical health. Second is that the #1 reason that people give for not using the exhaust hoods in their kitchen is… noise.
How loud? While Europe has protocols in place for properly measuring exhaust hood noise and requires manufacturers to post this information, that is not the case in the U.S. When manufacturers do post information in the U.S. it is using their own measuring methods in a way that makes their hood as marketable as possible.
We consumers are on our own, so we are collecting information on exhaust hood noise ourselves. Your participation would be appreciated:
It is possible to have a system that is both very effective and very quiet. The keys, in order of approximate effectiveness are; proper containment volume, placing the fan/blower distant from the hood, Inclusion of a silencer in the duct, proper isolation of blower and other components from the structure, proper make-up-air, baffle design and duct design. If you pay attention to these then you can have a system that performs better and is quieter. A little bit below, more in Part II.
Here are some noise measurements of some installed exhaust hoods by absolute noise level. It’s important to keep in mind that dB is a log scale and that a 3 dB increase is a doubling in perceived loudness. So, for instance, the difference in 51 dB(a) for the Accurex and 60 for the Modernaire is a very noticeable and big difference.
The chart above isn’t totally fair since some hoods are more effective than others. CFM is kind of a poor indicator of effectiveness but it’s the best we have so here is the noise per rated CFM.
Rated CFM is kind of problematic as well though since most hoods actual CFM performance varies from the rated amount based on duct design, fan curve and other elements. When actual installations are measured, extremely few systems do better than 50% of rated and many are below 30%. One exception that I know of is the Accurex system that operates at 980 CFM so 70% of it’s rated CFM. The Modernaire and Wolf systems with their remote blowers (stronger fan curves than typical consumer hoods) likely do better than 50% as well.
Actual effluent removal performance, our primary goal with our exhaust hoods, is a bit more complicated. We know that high CFM’s cannot make up for lack of proper aperture or containment volume. The Accurex will likely perform better for effluent removal at 400 CFM (which is totally silent*) than any of the others at 1200 CFM. SO, be careful about relying on CFM’s for performance or manufacturers ratings of how noisy their system will be. More on this in Part II.
FWIW, the EU recommends that exposure to noise levels above 53dB(a) be limited to no more than 4 minutes.
Here’s a very very quick version of the chart in Sones (converted from dB(a)). Note that while dB or dB(a) is an objective measure, Sones is subjective. More to come.
Loudness is also not the only element of noise. How high or low of frequencies are present as well as fluctuations in loudness and frequencies play important roles as well. Something that many people have noticed with remote blowers separated from the hood by a silencer is that the noise produced is not as irritating or unpleasant as other options even at the same loudness. So not only are the Accurex, Modernaire and Wolf systems above quieter per rated CFM but also more pleasant as all three include a remote blower and silencer.
Clean baffle filters are also typically quieter than grease laden so keeping filters clean can help with noise.
SO, to cut to the chase. The quietest (and at the same time best performance) system will be a commercial system from Accurex, CaptiveAire, Hoodmart or similar with a proper sized hood with proper containment and proper baffles + well designed duct + duct silencer + a remote inline or external blower (that’s designed and balance for quieter and more efficient operation). Second best is the same system but with a consumer hood such as Wolf or Modernaire.
* Because it is so quiet the Accurex system’s LOW setting was modified to 600 CFM (actual measured), the minimum at which it could be heard, to give assurance that it was actually on and working.
The Core Problem – Effluent
Hot effluent from cooking (odors, grease, steam, heat and VOC & PM by-products of cooking and gas combustion) rise from the cooking area. As it rises it spreads out a bit and has highly varying velocity with erratic bursts, sometimes in excess of 4,000 CFM. We don’t want this stuff in our home.
For our new home then our primary concern is our health and getting rid of the harmful by-products of cooking and gas combustion. Second is reducing cooking odors as much as possible for ourselves and guests. Third is reducing grease build-up on kitchen surfaces. And we want it to be as silent as possible.
High CFM’s Are Not Needed! To be effective a hood must Capture the effluent, Contain it until it can be exhausted, and Exhaust it to outside. It’s important here to keep in mind the CFM’s (exhaust) are only 1/3 of the equation. A properly designed and sized (capture area and containment volume) hood running at 300 CFM’s will usually perform much better than an improperly sized hood running at 1200 CFM’s. DO NOT get hung up on CFM’s.
So the critical bits then are:
1 – Capture Area. 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. Rising hot effluent is a very strong force and cannot easily be bent or ‘sucked in’ to a hood so it’s important that the opening itself be large enough to capture it. Anything not captured will become a problem in the house.
Ideally 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 a 62” wide by 36” deep hood opening (which is the minimum you’ll see in commercial kitchens). 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. Reality though says that we’re likely going to have something less than this. Every inch counts though so a 27” deep (front to back) open capture aperture is much better than 23” deep and a 23” better than a 22”. An 18” deep hood might capture most of the effluent from the back burners but little from the front burners so we want to stay away from anything that shallow and ineffective.
It is critical to keep in mind that we are talking about the clear opening (to the containment area) itself, not the overall hood size. A hood marketed as 27” deep but with a light bar in the front may actually have only a 21-22” deep opening as the structure of the hood is 1-2” and then the light bar reduces it by another 4”. That is why you never see a light bar in commercial hoods – they want the capture area to be as open as possible to collect as much effluent as possible.
2 – Containment Volume. This is where consumer hoods most often fail. Containment volume is just a bunch of empty space inside the hood. But it’s very critical empty space.
Effluent from cooking does not rise evenly but rather in bursts. These can quickly overwhelm the exhaust system CFM 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 hood and are pan frying a steak. As it cooks the effluent velocity is varying between perhaps 100 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 VOC and PM laden effluent that hugely overwhelms the hood’s exhaust capabilities. With the exhaust blower overwhelmed and no containment area the odors, grease, heat, steam, VOCs and PM that were captured then roll out from under the hood and spread throughout the kitchen (and house).
And then hang around until all of the air in the house has been exchanged which might take several days. Until it’s exhausted from the house everyone inside is breathing it. Over time, typically many years, this stuff builds up in our bodies and becomes the foundation for numerous health problems such as COPD, dementia, heart disease, cancer and others.
This is why commercial hoods are always a big empty box. That empty area is critical – it’s the containment volume, it contains the effluent until the exhaust system can remove it. Typically this volume should be 18x the capture area so a 60” x 30” hood will then be 60*30*18 = 32,400 square inches. Since the baffles take up some of this volume, most commercial hoods are actually 24” high inside to account for this.
Below is an Accurex hood with proper aperture and containment. The clear open aperture is 28”x56”. The open interior is 24” high at the front to allow for proper containment volume. The lights are placed in the top of the hood, primarily so that they don’t interfere with effluent capture but this also results in better work light for cooking as the way the light reflects off of the surfaces in the hood produces a more even light and eliminates shadows. As well, the bottom of the surround is 6’5” above the floor so is above head height for most people which makes cooking more comfortable (and can be higher). This system is also much quieter than any consumer system.
CFM cannot make up for too small a capture area or too little containment volume. It takes about 7 times as much airflow (CFM) to make a 23” deep hood with limited containment perform like a 28” hood with proper containment. 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 poor indicator of hood performance — but does make for good marketing.
Consumer hoods are energy inefficient. Proper containment volume (and capture area) are more efficient and allow for greater effluent removal with lower CFM’s. Lower CFM’s uses less energy for the blower itself but perhaps a bigger element is that this is exhausting less already conditioned air and so bringing in less outside air that then needs to be conditioned. If I can get the same effluent removal for 300 CFM as other hoods at 900 CFM – that’s a lot less air that needs to be heated, cooled, or dehumidified. An Accurex hood will cost $240 – $380 per year less than a consumer hood (assuming 10 hrs per week of use) in MN.
Editorial Comment: It is really rather stupid that consumer hoods miss on this. Creating a hood with proper containment volume is not expensive – it’s largely empty space. On many hoods simply extending the sides down just 6-12” would be a huge benefit and would both improve performance and allow for lower (and so quieter) CFM’s. My guess is that this is the result of consumer hoods being designed by marketing people instead of engineers or others who understand this.
3 – Exhaust (CFM). With a properly designed hood the actual exhaust system need only keep up with the average effluent produced as the containment volume of the hood itself contains the bursts until they can be exhausted. A properly designed hood can actually use fewer CFM’s which is quieter and less energy. One caveat is that the airflow velocity must be high enough for the filter baffles to remove grease.
4 – Make Up Air (MUA). The air that the hood is exhausting needs to be replaced. For hoods rated below about 400 CFM this air can usually be replaced through natural leaks in the house, open windows or better, a ducted damper from outside (note that to be fully effective a passive MUA duct must be about 3x the cross-sectional area of the duct for the hood so if you have a 6” duct for the hood then you need about a 10-11” duct for passive MUA). Above 400-600 CFM requires a mechanical make up air system with a blower that matches the blower for the kitchen exhaust. Some environments will also need a heater or furnace and possibly a dehumidifier to condition this air.
Make Up Air is as much a part of the exhaust system as the hood itself and should not be overlooked. Lack of proper MUA will reduce the performance of your exhaust system, increase its noise and create health risks.
Not installing proper make up air carries numerous health risks including death.
- The highest risk is backdrafting of other gas appliances such as a furnace or water heater that are not sealed combustion.
- Significant air pulled through leaks in wall systems not only reduces indoor air quality because whatever is in the walls gets pulled in to the house but moisture pulled in to wall cavities from outside can result in mold in the walls which can be a very significant health risk for many people.
- Poor hood performance from improper MUA will mean less VOCs exhausted and more spread throughout the house.
Where the air is introduced in to the house is also important. Ideally you want at least some of it to flow across the kitchen towards the hood so that it pulls extraneous effluent in to the hood. Introducing it in to the duct system of the house can work well or quite awful with its effectiveness dependent on a number of factors. More below in Part II.
Some Current Solutions
For a kitchen that is more show than functional 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 / Backdraft / Pop-up Backdraft. As mentioned above it is just about impossible to overcome the force of hot rising effluent. Downdraft is nearly useless and will only exhaust perhaps 0 and 5% of effluent and VOCs. A back drafting system that rises up behind can work slightly better though it will remove only about 5-10% of effluent.
B – Recirculating. Potentially a good solution though none currently on the market actually work well and all should be avoided. To work properly they need a system that effectively filters the air to remove grease, moisture, odors, VOC’s, Carcinogens and PM — the purpose of an exhaust hood. The filter system to do this requires about a 3’x5’x5’ space, costs about $9,000, uses a lot of energy and requires weekly maintenance.
C – Flush / In-Ceiling. These can provide better effluent removal than Downdraft of Recirculating but generally not by much. For these to be effective they must be quite large in order to capture the effluent that spreads out as it rises. With a 8.5’ high ceiling an in-ceiling hood would need to be approx 5’x8’ for a 36” range.
D – Over-The-Range-Microwave and Vent-A-Hood. Will capture and exhaust about 20-50% of effluent. However, these are quite noisy and more difficult to clean than standard consumer hoods.
E – Wolf, Zephyr, Bluestar, Best/Braun, ModernAire. These consumer hoods suffer from poor design, primarily lack of containment volume but also inadequate capture area or capture area blocked by a light bar. They will capture and exhaust about 10-50% of effluent. The remaining 50-90% of steam, odors, grease, VOC’s and PM 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 in a couple of ways. First is in using cold smoke. Hot bursty effluent that results from actual cooking reacts much differently and will not bend in to a hood the way the cold smoke does. This is also a constant volume which is much easier to deal with than the bursty effluent of actual cooking.
G – Accurex, Hoodmart & CaptiveAire. These are commercial hoods. They have sufficient open capture aperture, containment volume, filtration, and exhaust to properly handle the effluent.
Below is a sectional drawing of a CaptiveAire hood (black) compared to a typical consumer hood (blue). Note that about 4-6” of the aperture of the consumer hood is taken up by a light bar so even though its a ’27” deep hood’ the actual usable aperture is only about 21” deep. The red line shows where the baffle-type grease filter is in a typical consumer hood and gives and indication of the difference in containment volume. There’s a reason that every commercial hood has that gob of empty containment volume – because it works.
Solutions – The Gap
Solution F above is not a mistake. There is currently a huge gap between the minimally effective consumer hoods (D & E) and commercial hoods such as Accurex (F).
What is especially perplexing and disappointing about this is that the primary thing missing is… a bunch of inexpensive empty space.
Hoods in D & E, 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 harmful effluent or do not mind lingering odors. And these are certainly better than a downdraft, recirculating or in-ceiling.
We had a Vent-A-Hood over our all gas Wolf range for about 15 years in our prior house and while we wanted something that works much better and quieter for our new house, this was seemingly OK. One question we do have is what damage may have been done to our health over that 15 years from VOC’s and PM not exhausted.
A good solution for many people (F) is somewhere in the gap — a hood that has a sufficient and fully open capture aperture of 27” to 30” deep, a proper volume of containment area, effective and fairly easy to clean filters, and a multi-speed inline fan. Hopefully someone will begin offering something like this.
Also, in most cases anything is better than nothing. An Over-The-Range-Microwave with a clean filter and that’s used every time you cook is far more effective than a lessor hood or a better hood that’s never used. Any effluent removed is good.
A few miscellaneous bits to keep in mind. If you have a relatively competent general contractor and HVAC contractor then you don’t need to fully understand this but it might help to read through it so that you are familiar with the terms and challenges.
CFM – Cubic Feet per Minute. The volume of air that is being moved (exhausted). The CFM ratings of many exhausting device are based on what is called free air flow. In actual use these hoods/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.
A hood advertised as ‘600 CFM’ may actually only produce 450 CFM once exhaust ductwork is installed. If there is a proper 600 CFM powered make-up-air (MUA) then we’re good. However, if there is insufficient MUA, especially in a more tightly sealed house, and so greater static pressure on the supply side, then we could be below 100-200 CFM actual.
Static Pressure – The resistance to airflow in a system’s components and duct work. Impediments to airflow in the hood and duct system such as a 90° elbow create static pressure and makes it more difficult for air to move freely. Generally, the greater the static pressure the lower the CFM’s. So you generally want to keep static pressure as low as possible.
Some things that create static pressure include the hood itself, baffles in the hood, hood collar or transition (typically round), duct, bends and wall or roof cap. A system without proper MUA will also incur static pressure pulling air in to the house and this can sometimes be very significant.
Note that static pressure varies based on air velocity.
For reference, one of the people who did a blower door test for us and also measured the performance of our range hood and a couple of bathroom exhaust fans said that he has never seen a range hood perform better than 50% of its rating and many only about 10%. That’s static pressure at work.
Fan Curve – This goes along with CFM and Static Pressure. 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. The greater the static pressure the lower the CFM’s. The chart below for the Braun 335 blower tells us that with 0.15” of static pressure the fan will operate at 1200 CFM but at 1” of static pressure it’s closer to 700 CFM and at 1.65” about 200 CFM.
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. Remember, each 90° bend is the equivalent of about 9-11’ of duct. A 45° bend is the equivalent of about 4’ of duct. 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 also reduce overall noise as it acts as a baffle between an 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.
An inline duct fan with a duct silencer between the fan and hood is generally the best solution for quietness and based on my research this is likely correct. It is what we installed in our new home and it performs well and is very quiet.
A fan in the hood itself will be the loudest and in my opinion should be avoided.
Much noise is due to vibrations traveling from the fan and duct in to the structure of the house. To reduce this the fan, duct and hood should be mounted using perforated strap and isolation dampers with as little direct contact to the structure as possible.
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 located between the hood and inline or external blower will often help to decrease noise from the blower. These are available from Fantech. Make sure it is rated for use in a range hood exhaust system.
Filters – Mesh filters are generally not recommended. Baffle filters are a better option.
A system consists of several components that can be purchased separately or as part of a system. These components are; Hood, ducting, silencer, inline or external exhaust blower, controls, MUA blower (almost always inline), MUA heater (inline), dehumidifier (inline). Note that the MUA system will generally be the same regardless of the hood + exhaust blower selected.
Best – A commercial system from Accurex, Captiveaire, Hoodmart or similar will provide both the best effluent removal performance and the least noise. It will also be more energy efficient, provide better lighting and be higher and out of the way. This would consist of a hood w/ baffle filters + proper sized and installed duct + a Fantech duct silencer + an inline duct or external commercial blower + proper mechanically powered MUA from Electro Industries or similar. In our case we control everything with a Control4 system.
Expected costs: Hood $1400-$3000, silencer $300-400, blower $1000-1400, MUA (blower + electric heat combined unit) $1400-2800, ducting & installation $1500-2000.
A commercial blower will be quieter and perform better than a blower from Fantech but it would usually work to pair a commercial hood with a Fantech or similar inline or external blower to save some money. Make sure that the blower is rated for use with a range exhaust system and that it will provide enough airflow across the baffles to insure grease removal.
Below is our Accurex hood with a wood surround. The bottom is 6’4” above the floor so is comfortably overhead for all but one brother-in-law. Commercial hoods are typically 36” deep so Accurex custom made a 30” deep hood for us.
Better – Same system as ‘Good’ but a hood with proper containment volume. Unfortunately I am not aware of anything currently available.
Good – A consumer grade hood (with baffle filters but no internal blower) from Wolf, Modernaire or similar + properly sized and installed duct + a Fantech duct silencer + an inline or external blower + proper mechanically powered MUA. The hood should be 6-12” wider than the range it covers and have as deep an open capture area as possible.
Due to lack of containment volume and capture area this system, regardless of CFM’s, will not be able to achieve the effluent removal performance of a commercial system but might come close and should do a better job than typical consumer systems. It will also be louder than a commercial system (due to higher CFM’s and greater fan noise from the blower) but much quieter than most consumer systems.
An alternative to increase effluent removal performance might be to add some containment area below a consumer hood liner.
Some good resources for further exploration:
Part II – Deeper Dive
Further technical discussion for those interested.
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?
How many CFM’s do you need? Possibly not as much as you think. Something that I found interesting is the commercial hoods often run lower CFM’s than consumer hoods.
90 CFM / Sq Ft of capture area is a good starting reference point and is roughly what commercial hoods use. However, it’s important to note that this is what works with proper containment volume. As a couple of engineers pointed out to me, if you have less containment volume then you need a lot more CFM’s for similar effectiveness.
Make Up Air (MUA) System.
This is 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 at exhausting effluent, how quiet it operates 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 – 300-1400 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, and then bring in the amount of fresh outside air needed by all exhausting appliances combined. 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.
Heat – In colder climates it may be necessary to preheat the air. This can be accomplished with an electric duct heater, electric heater in the MUA unit, hydronic from a boiler, or a gas furnace.