More Building Science
Energy efficiency is a gateway drug to building science. When you really start learning about it, you come to the inescapable conclusion that there’s so much more to making homes better than just saving energy. A house is a system, you know.
There’s moisture and comfort and indoor air quality and more. Today, let’s home in on the indoor air quality part, specifically as it relates to kitchens.
Indoor air quality in the home
I keep an eye on the work of some of the researchers who investigate indoor air quality: Dr. Brett Singer, Professor Jeffrey Siegel, Professor Shelly Miller, and Professor Richard Corsi, to name a few. One thing I’ve heard loud and clear from them and from the mechanical ventilation crowd (Dr. Joe Lstiburek et al.) is that the first step to good indoor quality is source control.
That can mean just not bringing in materials that are going to have a negative effect on the indoor air quality, like avoiding furniture made with materials that offgas urea formaldehyde. It can also mean attacking the source of indoor contaminants. A kitchen range hood does that by removing the harmful stuff that gets into your air whenever you use your range or oven.
At Building Science Summer Camp this year, Dr. Singer gave an update on indoor air quality in the home. Naturally, what happens in kitchens was a significant part of it. Here’s his slide on the contaminants that come from the cooktop.
As you can see, your indoor air quality is diminished not only by cooking but also by the burners themselves. Burning gas is the worst but those electric resistance burners put a lot of ultrafine particles in the air. The best way to cook if you want to minimize your impact on IAQ is to use an induction cooktop. Then you just have the stuff that comes from the act of cooking itself. As you can see above, that can be pretty bad.
In addition to those, I saw this tweet from Florida Solar Energy Center researcher Danny Parker:
Singer didn’t mention that on his slide, but if you check out the Wikipedia page on sulfur dioxide, it might help you remember to turn that range hood on every time you fry some up.
And that’s just what happens on the cooktop. If you have a gas oven, you could be dumping toxic levels of carbon monoxide into your air. I once tested a brand new gas oven and it spewed about 1,000 parts per million of CO at startup.
Clearly, the best solution for these poisonous and toxic contaminants is to get rid of them at the source. That means using the exhaust fan that you should have right there over the cooktop and oven.
And that leads us to the two main problems I promised in the title. They are:
1. Not using a kitchen exhaust fan
Most times people have the fan right in front of them when they’re at the stove cooking. Many of them fail to turn it on. Other times there’s not a fan installed. Yes, it happens. Here’s another slide from Singer’s presentation:
Only 13% of people in this study used the kitchen exhaust fan most of the time. Ten percent never used it and 21% didn’t even have an exhaust fan. Only about a third of the participants in the study used the exhaust fan half the time or more. The most common reason they gave for not using the exhaust fan was they thought it wasn’t necessary (48%). The next most common reason was noise (21%).
This is probably the easiest thing anyone can do to improve their indoor air quality. When you’re standing there in front of the stove, reach for the range hood switch before you turn on a burner or the oven.
2. An ineffective kitchen exhaust fan
Turning the kitchen exhaust fan on is a great first step in source control for better indoor air quality. But that doesn’t guarantee you’re getting rid of all the bad stuff generated on your cooktop or in the oven. Here are some of the reasons why it doesn’t all get sucked out of the house:
- The exhaust fan doesn’t vent to the outdoors – Yes, it’s true. Some are recirculating hoods that usually pull the air through a little filter and send it back into the room, often greasing your forehead on the air’s way back in. They’re standard in Passive House certified homes to prevent extra penetrations of the building enclosure, but in those cases, they still have kitchen exhaust via the ERV. (I still prefer direct exhaust ventilation from the hood myself.)
- The exhaust fan makes a lot of noise but doesn’t move much air – Bad ducting of kitchen exhaust reduces the air flow, just as it does with bathroom exhaust fans.
- The range hood has a low capture efficiency – Unfortunately, you can’t go out and comparison shop based on this metric. The researchers at Lawrence Berkeley National Laboratory, including Dr. Brett Singer, are spearheading this work. They’re not there yet but someday you’ll be able to sort range hoods by color, size, air flow in cubic feet per minute, and capture efficiency.
What you can do to improve your indoor air quality
Kitchens dump a lot of indoor air contaminants into our homes. You can mitigate that by always using your kitchen exhaust. And if you’re in that group that doesn’t have any exhaust fan in your kitchen, get one installed as soon as you can. To improve your capture efficiency, cook on the back burner as much as possible.
If you happen to building a new home or remodeling the kitchen in your existing home, make sure you put in a range hood, get one that completely covers all the burners, and make sure the duct is sized and installed properly and vents to the outdoors. Oh, and don’t install a microwave with a built-in exhaust fan over the cooktop. Their capture efficiency is poor.
Allison Bailes of Decatur, Georgia, is a speaker, writer, building science consultant, and the author of the Energy Vanguard Blog. You can follow him on Twitter at @EnergyVanguard.
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25 Comments
Thanks for the interesting article. My thinking around kitchen air quality has almost entirely revolved around smoke & odor removal, with sensors to detect harmful gases, but this makes a lot of sense.
For the first piece (forgetfulness), I wonder if there's opportunity to remove the human element of it by automating the process. The range could be connected to the home's HRV or hood to trigger them to come on whenever the stove/oven is used. For the hood this could be setup to always turn on at a low speed and for the HRV it could trigger a boost that would run for 10 or 15 minutes after the stove/oven turned off.
Electric resistance/induction stoves would be simplest, since you could connect the outlet to the trigger, but I imagine you could setup something similar with gas that was connected to the hood light. Imperfect, but would capture usage more often than not if people are used to using the light when they cook?
After much (perhaps too much) thought on the subject, I'm convinced the best way to automate a range hood is with an IR sensor pointed at the cook top/oven vent. If it see 120F+ it turns on, could also potentially increase speed with increased temperature. This requires no change in the install and works with any type of range.
"since you could connect the outlet to the trigger"
I'm having a hard time visualizing what you mean by this. Stoves are always plugged in (some are hardwired, but even those with plugs never get unplugged). The trigger to turn on the range hood or HRV would need to be tied in to the controls of the stove. Unless you are suggesting some sort of current sensing device that will turn on the ventilation when the stove draws above a particular current threshold. That is certainly possible, but I'm not sure it's simple. If your HRV had a provision for a boost mode, it would be fairly easy. A range hood with buttons on the front to set the fan level, that's a different story. You'd need to hack the electronics of the range hood so that it would respond to an external signal.
I think the primary strategy to address the laziness of people turning the fan on is education. I know I certainly wasn't aware that pretty much every cooking activity was of concern. I have always only used range hoods when I could somehow sense, usually by smell or sight, that a large amount of air pollution was occurring.
Hey Trevor, I think the trigger part would be the simplest. Actually getting the trigger to do the triggering is the part that's kept me from responding until now. I thought it would be relatively simple to connect a smart switch to at least the HRV, but after a few days of searching I'm realizing it's a more substantial undertaking to figure out.
For the trigger, here are the options I've come up with:
1. Use a SmartThings (or similar) temperature sensor to detect when the temperature rises above a certain point.
2. Use a 220v electricity monitor to detect when the stove draws electricity.
3. Use something like Sense Energy Monitor that connects to your main electric panel and attempts to determine each device's electricity signature. It can then be setup as a trigger when it detects particular devices are turned on or off. I have one of these and it works so-so at the moment.
For the switch:
1. I think the hood vent could be connected to the light switch fairly easily in mostly lower end hoods so that the vent turned on when the light turned on (likely the approach I'll take for the hood). This is unlikely to be a widely adopted scenario, though.
2. The HRV seems like a better candidate, since it at least can be turned on remotely using a switch. The problem I've found, however, is that connecting the HRV switch with other switches isn't so simple. I would think this would be a high demand situation (i.e. turn on HRV for 15 minutes when bathroom light turns on), but apparently it's difficult to find HRV specific switches that operate this way.
Either way, I'll keep at it. This is the kind of setup I'm interested in having in our house.
Let's no forget about the make-up air requirement for exhaust fans over 400 cfm. See 2015 IRC M1503.4 Makeup Air
Good reminder as to why I steer clients toward inexpensive 250 cfm flow maximum for range hoods; and these are still too much if a wood stove is introduced--even with dedicated make up air. Not a good combo.
Thanks Armando!
IMO, the issue is not to steer clients to 250cfm exhaust fans, but to steer clients to buy ranges that require an exhaust fan of less than 400cfm, like induction cooktops, electric or small gas ranges, or wood stoves for that matter.
Installing a 250cfm exhaust fan to a gas range that requires 900cfm, not only does not meet code, but is a bad idea on the health side of issues.
That's a good way to go, Dylan. In the same presentation I mentioned in the article, Singer also said that you can remove all the bad stuff you need to remove with 200 cfm...as long as you have a well designed hood.
Hi Alison,
Sorry for the very belated response, but --Good to know we're on the right track! We've tested several of these types of range hoods once installed, and found that with good installation techniques, we're getting a little better than 220 cfm out of a 250 cfm hood. Plenty of flow for the important tasks. Thanks for responding!
I'm not very knowledgeable about kitchen vent ducts, but I'm under the impression that you need to use care in assembling and routing ductwork to avoid greasy deposits that may build up or may leak out of a poorly designed vertical joint. Is this true? If so, then it seems to me that we wouldn't want to run kitchen exhaust air through the HRV, that it would be better to directly vent this air to the outside even if it means some heat loss. So which is it -- vent kitchen air directly out, or OK to pass through HRV?
Along the same line, should high humidity bathroom air while taking a shower be directly ducted out, or can it also pass through the HRV? I especially wonder about this if the HRV is drawing in air that is significantly below freezing that might freeze the the moisture in the outbound air. I've read several GBA articles on ventilation but I'm not sure about the answers to these questions. Any answers ? (and thanks in advance)
User 7073758,
First of all, can you tell us your name? (I'm Martin.)
1. The rule with vertical seams in galvanized ductwork is simple: The male end of the duct faces down, and the female end faces up.
2. You can't duct a range hood to an HRV. You can, however, install a wall-mounted grille or a ceiling mounted grille in a kitchen, several feet away from the stove, and duct that grille to the exhaust side of an HRV. You won't get a significant amount of grease in the air if you do it that way.
Some manufacturers recommend a grease filter, even with the grille at the specified distance. It's probably just a way of covering themselves from claims. I have a hard time imagining grease floating up to the ceiling while also traversing several feet horizontally.
Hi Martin, I'm Dennis (the commenter formerly known as User 7073758). Yep, I learned the vertical duct seam rule with woodstoves and "creosote."
We're drawing up plans for a tight "super-insulated" house to be located in Lancaster Co, PA. We have been intending that a whole house ventilation system would exhaust to the outside the air drawn from bathrooms and the kitchen and fresh air would be supplied to bedrooms and such. So my question was whether the whole house ventilation system can serve the function of the range hood and bathroom fans. You confirmed my suspicions regarding that the greasy kitchen air would not go to the HRV. So I assume the most common practice is to duct the range hood directly to the outside, and tagging onto the tail end of an HRV is one way to accomplish the same thing.
So how about bathrooms? We're on the southernmost edge of climate Zone 5 and the question is whether a bathroom needs a dedicated fan ducted outside to exhaust high moisture air...or... is the HRV capable of handling this scenario in the winter without freezing up? I've read that some HRVs come with heaters to prevent freezing, but is this the typical approach, or is there a better way?
Thanks
Dennis,
I have written an entire article on the topic you are asking about. Here is the link: "Does a Home with an HRV Also Need Bath Fans?"
Thank you Martin for the article link. Sometimes doing a search doesn't turn up the right stuff, or if it does, it's listed on the 47th page of hits. So a direct link is quite handy.
Thanks for update and the names of the researchers doing the work. I've heard contrary claims around kitchen exhaust recommendations historically - this piece helps ground some of what we may 'know.'
Most of my work revolves around multifamily dwellings. Individual kitchen exhausts are more or less a nonstarter. Costs for such a system are high. Finding room for all that ductwork and penetrations is challenging.
We typically do a centralized HRV or ERV that serves the entire building. We'll locate an exhaust grille about 8' away from the cooking area to exhaust the kitchen (far enough away to not pull much grease into the ventilation ductwork). Over the range we'll install a recirc hood.
I realize this is an imperfect solution. What I don't know is how imperfect it is. Is this a reasonable strategy that balances our goals of good IAQ with economy? Or are we subjecting future residents to dangerous contaminants unwittingly? I've had some loose conversations with other professionals about doing some rigorous IAQ testing of these types of buildings, but that hasn't happened yet.
On demand boosting of the local ventilation would be handy. Right now I don't think the codes are flexible enough to allow for on demand ventilation in dwellings.
This is somewhat a response to comment #3. My own testing of a hand full of projects has led me to believe that range hood make up air does very little, especially as it is currently installed. This being a piece of 8 or 10 inch flex duct, usually at least 10 to 12 feet in length, to the return trunk for the forced air furnace. I have not collected this data all together to look at, but a case I just looked up is as follows:
House blows 1.9 ACH50 at 849 CFM50
Range hood moves about 450 CFM
I think the make-up duct was 10" flex 14 ft long (and this is typical of what I see)
House depressurization with make-up sealed is 20.7 pa
with make-up open is 17.7 pa
with make-up open and air handler on is 14.2 pa
It does something, but if this is supposed to keep house pressures reasonable it seems to be falling short of the mark. Even when the pressures aren't this extreme I haven't seen one make enough of a difference to seem worth it to me.
I'm curious if LBNL, or anyone else for that matter, has data or more educated opinions on this than I do.
We install the make-up air supply directly into the kitchen, usually in the ceiling, since most of our homes are on slab, but also through the toe kick of a kitchen cabinet or kitchen floor for a home with a basement, and always near the range. The objective is to supply the air with the least interference possible for the exhaust fan. The air is supplied with out the air filter, air handler and duct interference, which maybe why your readings do not support a true air replacement readings. Also, all our ducts are metal, no flex anywhere in our houses.
Thanks for the response Armando. I'd be interested to know how much pressure is relieved the way you install it? My point is that, if we're not requiring powered make-up air for hoods this big, are we just wasting everyone's time. The code should read more like:
If the hood depressurizes the house more than XX pa, make-up air must be installed to bring it beneath that same threshold. Now that we're blower door testing most new construction this situation is reasonably predictable.
I just found and read Martin's article on this subject, and some links in the comments, which made it clear to me that this is pretty well understood, residential manufacturers and contractors just need to deliver. We need smaller flows with better capture or we need these big range hoods to work right.
https://www.greenbuildingadvisor.com/article/how-to-provide-makeup-air-for-range-hoods
I'm curious to know if anyone has experience with downdraft ventilation either integrated into the cooktop and flush with the cooking surface or the pop-up type that project up out of the surface 10"ish? Do either of these provide a successful capture rate? I have heard it described that they are more successful than a hood as they are closer to the source, which sounds plausible for shallow pots, but I'm not sure if this is more sales hype than reality?
Yes, I have, and they are worthless. As you said, they work better on a shallow pans to about 2-3 inches, but anything above that, its not good. They are very narrow and hard to clean as well.
Thanks Armando. Do you mind sharing what brand you experienced? It seems maybe the JennAir brand w/ a 310 cfm fan might be better than some of the others but not sure.
We had a electric downdraft JenAir, but it was awhile back. I've seen many downdrafts and talk to many appliance manufacturers and retailers in my days, and after all the BS is said, reluctantly they admit that exhaust fans on downdrafts are really not the best.
IMO, there is nothing better than an induction cooktop with a good under-cabinet, wall mount or island drop-down hood vented to the exterior.
I ran some low voltage wire from my ERV to my range hood. Have yet to find a current sensing relay that will trigger boost mode for duration that hood fan is on. Slightly lazy on this to be honest, and looking at the wiring diagram and the wiring itself, its rather tricky. I cannot (or do not want to) detect current to the appliance, because we often will leave the LED range lights on, even when we are not cooking. The fan has 3 speeds, so i would need to sense current after the speed switch, but before the fan motor.
Has anyone yet found a 36" hood with good capture efficiency and <300-400 cfm? Every one I find that isn't flat (ie that has a pocket to collect the air) is 900 cfm and up. It seems the only choice is to build one yourself with separate blowers and inserts, but then it's not clear how to make the pieces work together. But at least you could make your capture efficiency deep yourself by building it down past the insert.
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