Last month at ACI in Baltimore I attended an interesting session about range hoods. It was chock-full of useful information and very well presented (often a hit-or-miss proposition at many conferences).
I was planning on waiting until the slides were available online, but I’m anxious to share this information. I will update this post when I have a link available.
Why we need range hoods
I thought I knew what range hoods do, and I had the basics down, but it was interesting to find out all the nasty stuff that ranges, particularly gas ranges, create when in use. There’s moisture from whatever we are cooking, as well as from the fuel as it burns. I get that carbon monoxide (CO) is produced as the burners start. What I didn’t know about was the nitrogen dioxide (NO2), ultrafine and fine particles, and most surprisingly, formaldehyde, a byproduct of heated oil.
Clogged holes on greasy burners change the air/fuel mixture, leading to incomplete combustion and more CO. Then we get to the oven. More CO is produced as burners cycle on an off in gas ovens. Any food residue on the oven walls turns into tiny particles when heated that are released into the air and breathed in by a home’s occupants. The hotter the oven, the more particles are released. The very high heat of self-cleaning ovens creates even more of these nasty particles.
Using a well-functioning range hood, vented to the outside, helps keep some these nasty things out of our lungs. And staying out of the house when your oven is on self-clean mode is a good move.
The air in homes with gas stoves don’t meet EPA standards for outdoor air quality (yes, I said outdoor) much of the time. About 40% of these homes exceed standards for NO2, 20% for formaldehyde, and 5% for CO. Apparently the EPA thinks it is OK to have poor indoor air quality inside a house, but not outside.
Well-functioning and vented to the outside
OK, so now that we are terrified to even turn on our stoves, what should we do? We still need to eat.
Well, start by using a range hood. The study that this presentation was based on evaluated a variety of range hoods both in homes and in a lab where they could adjust heights and depths, and switch out different hood types. The conclusion was that most of them don’t work very well, and the ones that do work are so big that they are probably backdrafting something in the house. (For more information on this problem, see Makeup Air for Range Hoods.)
The things that make a range hood work are a large capture area – the bigger and lower the better – and a minimum of 200 cfm of air flow. Unfortunately, most hoods aren’t deep enough to cover the front of the cooktop and most of them are placed higher than they should be so people don’t bang their heads on them. (I’ve done it plenty of times.) And we haven’t even talked about those recirculating hoods that are just noisemakers.
Behavior has its place
One interesting, but rather obvious, solution is to use the back burners, as that is where the range hoods draw the best. Also, it is a good practice to start the hood before you turn on the burners to exhaust CO.
High-volume hoods should be used at their lowest settings, and if they don’t have a makeup air system (and how many do?) in a tight house, opening a window helps avoid backdrafting.
Like many things we learn about how homes work, range hoods seem to not work properly more often than not. If we can convince homeowners to buy (and manufacturers to offer) large, low-volume range hoods, install them low enough to work effectively, and convince homeowners to operate them properly, then we might make some progress on improving the air quality in our kitchens and homes.
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keeping grease out of the HRV
My husband and I are building a Passivhaus and were not planning to install a range hood until our HRV vendor (Zehnder) recommended we get a recirculating model. The reason is to reduce the amount of grease and smoke that reaches the HRV. The range hood will have a replaceable carbon filter as well as a washable aluminum mesh filter.
Does this approach make sense to you, or is it still a hopeless exercise?
Curious to see the slides.
Curious to see the slides. And in the meantime, I'll just wait for these comments to degenerate until Martin advises everyone to make soup.
Andrea - I don't profess to be a Passive House expert, so I won't make any specific suggestions, lest I bring down the wrath of those who are experienced in the program. As is the case with many things, how you use your house will have the biggest effect on how it operates. If you don't do a lot of heavy duty cooking, particularly frying, then the recirculating hood combined with good whole house ventilation should work fine. If you decide to start frying turkeys indoors, or go into the catering business, you may need to rethink your kitchen ventilation. Then again, maybe we should go back to the old days when kitchens were in separate buildings.
It already fell
AJ - I'm not all that wound up about range hoods not working, although the charts and data on the nasty toxic chemicals and CO indoors was a bit frightening. I don't have a range hood in my little hovel, although I will install one if I ever build my new house. While the presentation I saw mostly addressed how poorly range hoods work at drawing the nasty stuff out of the house, I think the bigger problem, particularly in high end homes, are the oversized hoods that dray 900 - 1200 CFM. When they are installed in a tight house, better keep your pets out of the way when that sucker is running or you may lose them, not to mention having your ears pop.
Even being ignorant of the chemistry, it is a no-brainer to use a hood to get smoke and at least some of the air-born gunk out of the house. Who wants to still smell last weeks fried whatevers? And now that we know about formaldehyde, crank that sucker up a tad.
Range hood heat loss
I recently had a infrared scan done which, not surprisingly, showed major heat loss from my range hood at the roof. Any suggestions on a suitable replacement of the duct work with a damper and insulation?
I certify homes for energy star and NAHB green. Most of the kitchen fans I see in high end homes are part of the Microwave. They are close to the stove top but when tested don't work as advertised. I've seen butterfly dampers that stick, fans installed backwards, and exterior dampers painted shut. And thats just the kitchen. Bath fans are often installed incorrectly with vents blocked for various reasons. Its a real circus out here but I love it!
Carol - you bring up a good point about heat loss. A fan vented out the roof has the potential to create a thermal chimney, allowing heat to escape from the house in cold weather. Even a fan with a good damper will leak some air through it. You might try insulating the duct in your attic and make sure that you have a damper at the back of the hood and it is operating properly.
The best solution, although most likely custom and expensive, is to have a mechanical damper in the exhaust duct that opens automatically whenever the fan is on, then closes when it is off. Anyone else have any interesting ideas?
Andrea et all, I can attest to the fact that make-up air works amazingly well. We just moved into our new ultra-tight (designed/built to PHIUS standards and awaiting final certification) and I have installed a Broan MD6T 6" damper that opens when the range hood is operating (over a Thermador LP Pro Series range). Even on the lowest of the 6 speeds (it's a 725 CFM Zephyr) you can feel the draw of fresh air coming from under the refrigerator and it's close to the range so we believe we're not drawing all the conditioned air out of the house. I installed the make-up duct under the refrigerator to dual-purpose it for added efficiency to the fridge and provide that necessary fresh air while keeping the grease out of the ERV. One thing we do not have is an ERV "return" in the kitchen which also helps prevent drawing any grease into the ERV system. The smells don't linger, the humidity is within tolerance (45% to 60% on average), and we're not having to do the valsalva (sp?) ear-clearing maneuver when I'm cooking. I have posted the make-up air 'schematic' on my blog at http://pittsboropassivhaus.blogspot.com but basically it's a 24v current-sensing switch that opens the damper when the range hood is on - not sanctioned by Broan since it's not their range hood but it works great! If you use an electric or induction cook-top then a venting range hood isn't as necessary but I agree with the others that smells lingering aren't necessarily always desired. Even when I cook things like Bacon or italian you can't smell them in the bedrooms or downstairs so I firmly believe the system is operating as designed and we're not introducing those "nasties" into our ERV.
Range hood IAQ, safety
Carl -- Thank you for bringing this up. Are you referring to the LBNL study by Brett Singer?
I believe that is the presentation I saw. ACI should be posting his presentation on their website soon. When it is up, I will link to it in this post.
A few years ago I moved to New York City from Vermont. After 30 years in an intense heating climate, I had no idea what I was in for in the big city during the summer. One of the biggest comfort issues was cooking in the summer. Cooking pasta without a range hood? Fuhgeddaboudit. Heating up all that water to radiate was unbearable. Baking? No way. We do cool with a through-wall unit, but it was easily over-powered in the kitchen area by anything more ambitious than an omelet. Since installing a range hood, however, we can actually use our kitchen. No one ever mentions it, but a vented range hood does seem to vent a considerable amount of heat and, if nothing else, makes cooking much more pleasant during the cooling season.
Just another idea re ventilation hoods.
When I was still working for a living, we had the pleasure of extinguishing a restaurant fire, which had
started in a ventilation hood in the kitchen, now granted it was an older building and the ducting may not have been up to code. however a stove fire resulted in the fire going up the hood, and catching the grease in the ventilation ducts,on fire, which melted and ran down inside the walls and spread the fire. It may be wise to use the shortest ducting to get outside and not into the attic space. Fires do happen!!
CFM Is Critical
Carl, I don't believe that you focused enough on power/air flow and the duct run. A vent hood's power/ability to move air needs to correlate with the power of the range or cooktop. The commonly recommended formula is 100 cfm for every 10,000 BTUs from the stove. This involves more than just the power of the vent hood's blowers. The duct run, if it is too long or has 90 degree turns can reduce the airflow/ draw power of the hood.
One other factor is mechanical capture (how recessed the capture area of the hood is--where the filters are). Many modern low-profile hoods have a harder time than those with a recessed cavity. I am having a 1,200 cfm hood custom made by Rangecraft. They can actually make a low profile hood with good mechanical capture.
Thinking a bit out of the box
Thinking a bit out of the box to industrial vent hood design - why are we pulling the air up through the chef's breathing zone (if that tasty fried smell and particulates are bad for him) and not back toward the rear of the stove or down? If the stove is on an outside wall, you could vent it straight out at a height just above the stove top rather than up - shortest, most direct run and least amount of air to move.
David - I agree with you that contorted duct runs cut down on the total air flow, but with larger hoods that is rarely a problem. Even thought this post is about hoods that don't adequately draw, in many homes oversized hoods are a bigger problem. That 1200 CFM hood is pulling about 3 tons of HVAC out of your house, unless you have a makeup air system to replace it. Very few people need that 100 CFM per 10K btu. The average cook just needs to get the excess moisture and smells out of the kitchen, not air condition the neighborhood.
Gas vs All Electric
Its always amazed me that the required ventilation for new construction is the same whether the house has gas range, oven, furnace and dhw or an all electric house. Most people who believe they can only cook with a gas range are pleasently surprised when they try an electric induction cooktop.
As we build tighter and tighter homes, gas appliances worry me.
Re: heat loss
I like Carl's suggestion of a mechanical damper in the exhaust duct and would like to know if there are any rated for this use, not just for make-up air.
The best alternative from building supplies is a single disk-flap exhaust cover inside of a poly apron to minimize infiltration. The butterfly dampers made by our local sheet metal shop use roughly-installed foam tape gaskets as a stop for the metal dampers, they don't seal that well, so one would have to reposition the tape if choosing that route.
The rectangular dampers for range hoods are very primitive, as are the louvered exterior covers without apron. To minimize the draftiness and leaks I re-taped the single duct-joint, and made a pair of manual internal dampers using a yogurt-lid tacked to a 1 x 1 prop. Their diameter is a perfect fit when wedged between hood wall and the openings for squirrel-cage fans, if a little floppy. It's a bit of trouble to take in & out and only stops massive infiltration from our old hood on windy days, not at the outside of the brick wall. A cheap end-of-pipe solution, and a cumbersome stop-gap, much to my wife's annoyance. I'm hoping this new 4" vent cap with apron installed on a good angle will be a factor-four improvement so I can toss the interior custom dampers.
Mount range hood closer to the burners? Fugitabowdit. I tore out the range hood one place I lived because I was always hitting my forehead on it. You couldn't even lift a fork full of spaghetti out of the pasta pot without hitting the damn thing.
I have often thought that a fire suppression system should be a standard part of all range hoods, given the number of fires that occur after someone forgets they put something on the stove.
To stop heat loss during cold periods, I cover the 8 X 9.5 inch mesh filter with plastic wrap or foil.
Make-up air for the kitchen vent comes through the bathroom vent and vice-a-versa, not that this house is that air tight.
Sometimes its best to keep
Sometimes its best to keep ones mouth shut....
Automated Range Hood
Why haven't any manufacturers come out with range hoods that automatically turn on when the oven or cooktop is turned on or vice versa? We typically measure more CO spillage at the oven than any other appliance in the home. Our clients are always amazed when we show them high CO production in their kitchens and that they are responsible for venting their oven and cooktop even when you steam or or odors are not sensed. Any thoughts on how we get past homeowner behavior and create a real solution for the masses.
AJ - FYI, barbecue is a noun, not a verb. Just sayin...
Thanks for excellent summary Carl, was pissed I could not make it to ACI, will look forward to your link and hopefully more ACI reports. Am thankful that people like yourself are willing to sit through all the meetings and then share the relavant information.
10 good reasons why range hoods DO work
Excellent points about makeup air and capture effectiveness. Not what I expected from an article called "Why Range Hoods DON'T Work"... they do if they're properly sized & installed.
I found this article a little while back, searching for a range hood for my summer home. Useful stuff for anyone trying to learn more about hoods.
Re: Automated Range Hood
@ Wes Harding: you know, I had the same question, and while calling up different range hood companies, I finally got an answer that made total sense. Apparently, there are so few hoods with heat sensors on the market, because heat sensors are not reliable, in terms of corresponding to what's cooking. For example, heating a pot of water for pasta will produce a lot of heat, triggering a sensor - but it's "empty" heat, there's no smell, so all we're doing is wasting energy (and running up the A/C bill...). Meanwhile, simmering garlic or a spicy sauce (on low heat) might make the kitchen stink, but the heat sensor will not fire up because there's not enough heat.
Besides, when you're in front of the stove, the hood controls are right there - just reach up and turn it on. Adding extra complexity and cost to the range hood doesn't make much sense, you're not gaining any functionality.
So, it's not reliable, adds to the cost of the hood, and largely unnecessary. That's the explanation I got, it makes sense to me, so I'm passing it on.
How to just say no to CO
Edited... Inductive ranges as some others have advocated sound like a great alternative to many issues raised well by Carl to do with gas ranges and vent hoods.
edit, just not on topic...
edit, just not on topic...
Not so Relevant.... Post....
Green Building Advice- Kitchen Range Venting
Green Building Advice
1- Build homes more airtight than the twentieth century ACH .6-3 is a good range, depends on your personal wants and needs and ultimately your budget.
2- A standard range that is electric and has a good carbon filter will work but if you want to go all out then go larger vented outside and install makeup air like outlined above. Huge commercial style kitchens and cooking styles you are on your own, as such is just not green.
3- For those like myself and many, choose to barbecue outside year round and forget frying anything substantial inside. That takes care of needing to vent totally. And barbecuing year round is a blast. Most that do love standing in a snowstorm at the outside grill getting a steak or whatever ready for a winter meal. And if more winters are like last we were just about snowless here anyway. Recapping, make your offgassing an outdoor ventless non issue.
And yes induction is neat for those that want to spend for it, much better idea over gas if all the gas issues raised above alarm you. Go induction then green leaning ladies and gents.
That is my positive summation of this blog and thread. I hope GBA also starts to push out more conclusive green information too.
edited... to relevant content
Clearance to combustibles
Go ahead and install the range hood as close to the burners as comfortable, heed Bob Rinehul's caution. However one must be aware of the manufacturers installation instructions regarding minimum distance from the hood to the burner and the distance to maintain from combustibles if you clad the hood with a combustible material.
Automated Range Hood
Wes Harding, I also has a similar question in my mind. There should be an innovative range hoods that automatically turn on when the oven or cooktop is turned on. - Herman
thank you for this great article and wonderful host of comments. i have a question that has yet to be covered. i do very little cooking in my house (i have a 26 ft vaulted ceiling - kitchen in great room, 26x35ft) and the cooking i do is usually some eggs barely cooked and runny. however, i have been helping a family and allowing them to stay with me and my family in the house. they eat bacon twice a day and after a week the house is beginning to smell like bacon (even though i clean the sh*t out of the kitchen after they cook). i wondered if there was any type of air purifier or air cleanser, perhaps something people use when smokers are around, that i could purchase to take care of this temporary use of the kitchen? what would you recommend? i have already asked them to cook with aluminum foil over the skillet and they will only be here for 8 more weeks, but i do not want to have bacon residue all over the house.
thank you for your time and help
Response to Jonathan
Regarding your bacon cooking guests - first, if you have a range hood on, have it running when the cooking starts and for about 20 minutes following. Run it on as high a setting as possible, opening a window to provide makeup air and avoid any backdrafting of water heaters or furnaces. Have them use the back burners to cook. If you don't have a range hood that vents to the exterior, open a nearby window and stick a box fan that pulls air outside while they are cooking. Or just ask them to not cook bacon for a while.
My grandparents neighbours have a range vent which comes out by their patio. The smell is becoming a problem to my grandparents who have to move inside when next door are cooking. Can anyone recommend any ways of minimising this smell so they can enjoy their outdoor space? Thank you
Response to Robyn Berry
The best place to post your question is on GBA's question-and-answer page. Here is the link: Q&A page.
If you post your question there, it will be seen by more readers, and you are more likely to get a response.
Response to Robyn
Unfortunately there isn't much they can do if the neighbors aren't interested in relocating their exhaust. They might consider buying a big fan and pointing it towards the neighbor's range exhaust outlet to push the offensive smells back their way. Thats the best I can offer, although others might have some good ideas.
Carbon monoxide is mostly a problem when burners are first turned on. The metal is still cold resulting in incomplete combustion. Even a properly functioning stove will emit 20x elevated CO when turned on. After it's been on a while, CO should drop to very safe levels. CO emitted from food residue in the oven is negligible. A faulty burner can produce 100x the CO of a properly working one, without being obvious to the homeowner.
Bottom line: the vent is more important when first lighting a burner/oven. After a minute when everything is warmed up, it's safer to turn the fan low or off. If you notice you're getting headaches from cooking, check for a CO problem. You can even ask the gas company to inspect the stove with a CO detector.
It's interesting that an article about indoor air pollution did not compare the emissions cooking with electric instead of gas. The advantages of gas disappear with induction electric.
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