More BS* + Beer
After setting the tone for the need to get kitchen ventilation right—it’s an indoor air quality and human health issue—our panelists, including architect Kyle Macht, builder Ben Bogie, and This Old House home tech expert Ross Trethewey, took a deep dive into kitchen ventilation in the most recent episode of BS* + Beer.
The trio of experts discussed the effectiveness of different types of kitchen vent fans, proper CFM sizing as it relates to the building codes and the size of the cooking appliances, makeup air, tight-house solutions, energy penalties, ducting strategies, remote fans, and more.
If you’ve ever doubted the need for kitchen ventilation, watch this episode. If you’re unsure of how to choose a range hood fan, watch this episode. If you’re concerned about bringing cold or humid makeup air into your home, watch this episode. Unless you know everything there is to know about kitchen ventilation, watch this episode. You will be sure to learn something from this dynamite team of professionals.
Enjoy the show!
The next show is Thursday, August 20, from 6 to 7:30 pm: The intersection of preservation and performance. Join us next week as we harness an ongoing debate about the role and significance of preservation vs. potentially more climate-friendly and socially-equitable housing options and bring perspective to the conversation with two top-notch panelists. Brent Hull will join us and present on what we can learn from old houses and Laura Case from the Southface Institute will share case studies from their EarthCraft Sustainable Preservation program. See Brent and Laura’s bios below.
Use this link to register for The BS* + Beer Show.
Brent Hull, owner and president of Hull Millwork and Hull homes, is a nationally recognized authority on historic design, architecturally-correct moldings, and millwork. Brent trained in the art of museum-quality preservation at the prestigious North Bennett Street School in Boston. Brent is also the author of three books including “Traditional American Rooms” – celebrating style, craftsmanship and historic woodwork, and “Historic Millwork” – a guide to restoring and recreating doors, windows and moldings from the late 19th Century to the early 20th Century. His most recent book is “Building a Timeless House in an Instant Age” written to help builders and homeowners build authentic and beautiful houses. He is the recipient of five John Staub Awards for classical architecture in craftsmanship and historic restoration. Brent has been a builder, remodeler, and millwork owner for over 25 years. His company has completed 100s of projects and has vast experience in the field. He is a lover of historic millwork and historic design.
As the Director of Technical Services, Laura Case provides leadership and technical expertise for programs that support Southface’s mission to drive our communities towards a regenerative economy, responsible resource use, and social equity through a healthy built environment for all. Programs and service offerings that Laura manages include organizational sustainability planning, building assessments, consulting, and certifications on the commercial, residential, and community levels. Certification programs including LEED, EarthCraft, National Green Building Standard, Enterprise Green Communities, and Energy Star. Laura has decades of experience in commercial construction project management, campus planning, and developing design standards for clients including the State of Georgia and Emory University. For fun, Laura renovates her house, climbs mountains, paddles, and swims.
The BS* + Beer Book Club
Because the hosts of the BS* + Beer Show all love to read, we thought we would celebrate the authors in our industry by adding a book club to the show every few months. We’ll announce the book, give you a few months to get it and read it, if you haven’t already, and conclude with a BS* + Beer Show episode where we will invite the author to join us, present, and take questions.
The first book we’ll read is “The New Carbon Architecture” by Bruce King. Bruce will join us on the show on September 24.
Here’s a bit from Bruce’s publisher:
A tour de force by the leaders in the field, The New Carbon Architecture will fire the imagination of architects, engineers, builders, policy makers, and everyone else captivated by the possibility of architecture to heal the climate and produce safer, healthier, and more beautiful buildings.
I hope you will join us on September 24th as well.
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33 Comments
We know that airflow doesn't exclusively follow the neat arrows people draw - there is always some mixing. So even if a perfectly balanced powered makeup air fan is delivering close to the range, how much additional heating/cooling/dehumidification capacity is needed to keep the kitchen comfortable while cooking?
For that matter, given the wide range in heat/humidity sources, how comfortable is the kitchen even at 0 CFM?
How are users expected to balance the variable speed powered exhaust with the powered makeup air (to avoid house pressurization/depressurization)?
I haven't yet watched this episode but IMO this concept has been thoroughly studied. I've seen studies which used "smoke" to measure the performance of different sizes hoods and/or placement of the makeup air ducts. Iirc you can find them at BSC website.
Jon R, good questions. We covered some of them in our conversation. Bill Robinson, in New Orleans, LA, had asked about dehumidification of incoming air and I don't believe the responses were conclusive. My take is that most of the air coming in should be the air that is getting exhausted, at least if the makeup air is correctly positioned, which Ben Bogie showed how to do in detail. But I think the gist was that you also want a dehumidifier and whole-house ventilation. It's not like you're cooking all day in most cases. Good point about variable speed. We talked about it a bit, but it may have been more in the chat box which of course does not show on the recordings. It seems like there could be demand for easy to use, more-advanced controls for makeup air.
John Clark, we went a full two hours and barely covered the basics, or maybe one step beyond the basics. Several audience members commented that they didn't know that discussing kitchen ventilation would be so riviting, keeping interest for not just one, but two full hours! There has been a lot of study on this topic but it seems that the information is not well understood or distributed.
IMO the biggest issue is that ducted range hoods only came back into fashion about 10 yrs ago after a 20+ year hiatus. People forgot how to design homes for them. I know I've crowed about it before and it's one of my pet peeves but I have yet seen a new home with a properly sized range hood combined with uncompromising ducting. Proper sizing along with well designed ducting can alleviate the frequency where max CFM's are required.
Part of the issue is with regards to kitchen placement. Obviously the most efficient design requires that the range sit on an exterior wall or have a relatively unobstructed path to run duct vertically through the roof or out an exterior wall.
Common range hood design failures.
#1 Range hood is only as wide as the range itself.
-Money saver for builders because they don't have to order custom cabinets
#2 Under powered fan.
-Money saver. Small fan contributes to poor performance when combined with an undersized capture area.
#3 Range mounted on an interior wall with rectangular stud bay rigid duct.
- Space and light are a premium. Builders won't for example install a range hood on an exterior wall. They won't use round ducting going vertically because it costs more money (stainless cover) and sacrifices storage space because there would be no cabinets above the range.
#4 Poor duct design.
- Builders use the shallowest floor joists they can get away with which compromises the lay out of HVAC ducting, wiring, etc. Range hood ducting is almost an after thought based upon the zig-zag runs I've seen. I've seen zig-zag runs which were held together with poorly applied aluminium tape. How air tight do you think that is? I'd be afraid to run the fan fearing that grease/moisture would get get deposited through the leaks. Truly pathetic.
#5 Only a minor quibble, but exterior OR in-line blower motors can go a long way towards reducing the maintenance of a range hood. Most range hoods have the fan mounted at the hood so they're going to be limited in sized and can be difficult to maintain. Blower fans mounted on the exterior of the house result in quieter operation and from what I've been told require less maintenance.
What models have the fans mounted on the exterior of the house? Is this crazy expensive compare to a normal range hood?
I didn't find anything from BSC about the effects of exhaust fans (or waste heat/humidity) on residential kitchen comfort - surprising given the amount of money being put into kitchens. It's clear that pulling 400 CFM from the rest of the house can add a very significant 30,000 BTU/hr to heating load - with unknown distribution, so even up-sizing the heating system won't fix it (unless you have per room zoning). Similar distribution issues with cooling and dehumidification.
Given the lack of quantitative data, perhaps best practice is to a) always use powered makeup air delivered near the pollution source and b) install an extra mini-split (preferably Daikin Quaternity) in the kitchen. Anything else may be "plan on being uncomfortable".
Jon,
One of my big takeaways from the session was the idea of introducing the makeup air in the general area of the range. The ideal would be to create a laminar flow hood where all of the makeup air picks up the cooking fumes and exits through the hood, just like the little arrows say it should. In this ideal condition, there is close to zero impact on heating/cooling/dehumidification. A couple of approaches were shown that come pretty close to this ideal.
Even if you don't get 100% capture, you can still do pretty good if you're careful. If you only capture 70%, the 30kbtu impact goes down below 10k, and it's only for the hour or so that you're cooking. That's not going to have much effect on your overall energy balance. With the negative pressure zone created by a properly sized hood pulling 400+cfm, I'd expect that you can do far better than 70% capture efficiency. OTOH, if you inject the makeup air into the HVAC returns, then you've got 100% mixing and a big impact on energy.
Great session, and certainly provides some food for thought.....
I haven't gotten a chance to watch the episode either. But for reference, here's Joe's column where he talks about big hoods, kitchen ventilation, and make-up air:
BSI-070: First Deal with the Manure and Then Don't Suck
https://www.buildingscience.com/documents/insights/bsi-070-first-deal-with-the-manure
An excellent document covering "how to supply make up air" comes from the California Energy Commission's study around 2003. They have a bunch of Schlieren images showing the heat plume and capture efficiency. The takeaway: the heat off the range *wants* to rise into the hood. Don't "try to help it" by pushing air around to make an 'air curtain' or something--you'll probably screw it up and reduce the actual capture efficiency. Supplying air away from the range hood, or at your feet, are both good options.
Design Guide Improving Commercial Kitchen Ventilation System Performance
https://www.15000inc.com/wp/wp-content/uploads/Improving-Kitchen-Hood-Design.pdf
BSI-070 is quite helpful, although I think it's overly optimistic about make up air following drawn arrows. See the conveniently sharp upward arrow bend in fig 2 (vs continuing into the room). Maybe with very low throw or carefully aimed floor registers directing air up the sides/front of the range at low velocity. As drawn, I'd expect supply air (especially cold) to continue across the kitchen floor.
I suspect that the 60-70% MUA spec may need an automatic (ie, flow sensing) system to be accurately met with a range of exhaust speeds.
2nd reference is also great. Note all the cautions about the % of MUA (60% max, sometimes far less) and causing spillage. Which I guess leaves us with powerful zoned kitchen heating/cooling/dehumidification systems to maintain kitchen comfort.
Ya, I don't remember any which were concerned with comfort (see below). They were primarily concerned with performance.
https://www.buildingscience.com/documents/insights/bsi-070-first-deal-with-the-manure
Personally I don't think comfort is an issue worth spending much time on due to the intermittent use. Not that I'm the be all end all, but I have yet to see a house with a range hood which has is large enough and/or mounted at the correct height. Ducting of the hoods have been horrendous.
In any case I'll try to find the study which used smoke to test the size of the hood along with the placement of make up air. Basically their results mirrored that of what you see in the BSC article (make up air at floor level underneath the hood). That set up provides the shortest path with the least amount of mixing of outdoor with indoor air.
> I don't think comfort is an issue worth spending much time on ...
I think there is high variation among owners. Some cook for hours every day and others haven't touched the stove for weeks.
Similar for other home comfort issues. Some want "tolerable" and others want "luxury". There isn't a single right answer.
I believe Joe Lstirburek's conclusion was "if you can't stand the heat, get out of the kitchen" haha
Haha. Classic Joe. One thing we talked about on the show was outdoor kitchens (aka "dirty" or "summer" kitchens.)
And we come full circle. It used to be that summer kitchens were used because you didn't want the heat of a wood stove inside the house in the summertime. Now we've got induction ranges, but the heat and pollution is still too much for our very tight houses. How do those of us in the north country use a winter kitchen?
Yesterday's summer kitchens are now rented basement apartments ! ;)
John, the original summer kitchen at my house is now our dining room, but point taken.
Peter, even here in Maine, it's warm enough to cook outside at least half the year. That means half of the indoor pollutants. Of course outdoor cooking often involves charcoal or gas, so you're trading indoor pollutants for greenhouse gas emissions. But we gotta eat, and raw food diets have limited appeal.
Thermador apparently used to make ovens that vented directly to outside, via 4" duct, which seemed like a good idea to me (both for summertime baking needs, and IAQ).
this, I agree with. But it is always nice for a breeze :)
I didn't watch the whole episode yet but wonder why not connecting the make up air right after the hrv fresh air output? This way, you could add a filter box, a preheater and even a dehumidifier to temper all the air coming into the house and not just the make up air. Wouldn't it be a good design?
Someone said during the video that connecting the make up air duct to the hvac return was not an optimal strategy. I was wondering why. Is it because the ducting is usually not designed to handle that extra ventilation load?
Thanks!
Alex, interesting idea, but the volume of air needed is an order of magnitude higher than balanced ventilation systems provide.
[Someone said during the video that connecting the make up air duct to the hvac return was not an optimal strategy. I was wondering why. Is it because the ducting is usually not designed to handle that extra ventilation load?]
-The ventilation load isn't all that great. The problem with connecting into the HVAC duct is that the air itself isn't being filtered and humidity isn't being addressed. This approach is a variation of supply-only ventilation (When used as a whole-house ventilation strategy the damper will open when the blower motor for the HVAC system turns on) which is the cheapest but sub-optimal choice in itself.
[I didn't watch the whole episode yet but wonder why not connecting the make up air right after the hrv fresh air output? This way, you could add a filter box, a preheater and even a dehumidifier to temper all the air coming into the house and not just the make up air. Wouldn't it be a good design?]
- The ducting for a HRV/ERV I suspect isn't sized for such use and you'll also generate a venturi which will increase the speed of the air passing through the HRV/ERV which decreases efficiency. Of course this air will not be filtered.
Alex,
I address your question at least partially in my response #8 above. If you introduce makeup air very close to the cooking area, then it is primarily that makeup air that is drawn out through the exhaust equipment. The more of the makeup air that is drawn right back out, the less of an impact it has on indoor heat and humidity loads. If you could get to 100% makeup air removal, and 0% indoor air removal, then the makeup air would have zero effect on indoor loads.
If, however, you add the makeup air into either the HVAC or HRV duct systems, then it mixes with the indoor air and you've got to condition it. You either condition it before or after it enters the living space, but either way, you are paying the energy cost of addressing the heat and humidity loads of the makeup air.
> makeup air very close to ... primarily that makeup air that is drawn out through the exhaust equipment
Apparently there is more to it than just "very close to". You have to carefully control direction and lower velocity so you don't cause mixing or spillage. And even then:
"As with other local MUA strategies, the quantity of air introduced through the backwall supply should be no more than 60% of the hood’s exhaust flow."
So only 60% capture of makeup air (not to be confused with capture of pollutants). Leaving 40+% to be conditioned.
Jon,
Carefully controlling direction and lower velocity are both part of the design of laminar flow benches, as used in handling hazardous chemicals. These, by necessity capture 100% of contaminants and have very close to 100% makeup and exhaust air matching. But in this case, the hood is designed prior to, or at least in parallel with the process being contained. This was one of the points in the talk. We are currently taking a cafeteria style approach to kitchen venting, with the emphasis on "style." If a stove and hood were designed and marketed as a system, complete with makeup and exhaust air, it would not be difficult to achieve close to 100% capture efficiency. Of course cost might be an issue, but considering that people spend more for their kitchen equipment than I generally spend on a car, I don't think that's the limiting factor. Style, maybe. I think it's more of a lack of interest and knowledge.
FWIW, commercial kitchen hoods get pretty close to 100% contaminant capture, even without being designed as a package integrated with the cooking equipment. They do this by introducing a nearly laminar airflow curtain pouring down from the outside (kitchen) edge of the hood. This curtain has a relatively high flowrate and velocity. It keeps the smoke from rolling out of the hood, entrains it in the downward flow, and then both get drawn into the hood. Ross Trethewey showed a very cool residential example that they built as a "hood within a hood." It was just a short clip, but it showed excellent capture efficiency, and I'll bet that close to 0% of the MUA leaked into the house. I suspect that you still design the space to run at a slight negative pressure when the stove is running (MUA<Exhaust), but even so, the net effect on space conditioning is probably almost negligible.
@Alexi
[What models have the fans mounted on the exterior of the house? Is this crazy expensive compare to a normal range hood?]
There are suppliers of range hoods which allow consumers ala-carte choice with regards to the type and size of the blower motor. IIRC Proline used to offer various motor locations (https://www.prolinerangehoods.com/faq)
I haven't watched the video, but can anyone tell me if the advice differs from previous GBA advice where a ~300cfm hood, vented to the outside, larger than the cooking surface was deemed sufficient, without significant depressurization problems? My house will be tight so if there's a shift towards make up air now is the time for me to sort it.
Alex, I would say the conversation was generally in agreement with that. But there were a lot of fine points to consider.
I had the impression that make-up air is still preferable, at least if it can be provided near the range (like behind or around or under it). In that way you improve capture and lose less conditioned air because it's mostly makeup air that gets exhausted. Perhaps if you can't provide it right where it's needed then there's not much benefit to it for lower CFMs. Now what I don't recall is if there was a discussion on the minimum CFMs for good capture. I've read elsewhere that in woodworking you should have 1000 CFM in order to capture the PM2.5 (not sure that this is widely accepted, though). Seems like the same would apply to cooking, but then again I suppose cooking doesn't spray particulate matter with the same force as woodworking.
Moved into 1996 home w/kitchen island with 30" gas cooktop (w/sealed burners) that had a failed downdraft at the back. After replacing with a highly rated Best downdraft (600CFM), we find that the downdraft, even though it rises high at ~18", causes the gas flames to extinguish (and the cooktop has no autoshutoff!!). We can only use the lowest of 4 fan settings if a burner is in the lowest setting, and can never use the fan above 2nd setting. Really wanted to address IAQ but this isn't working! Guess its just another disadvantage of downdrafts - any suggestions?
After having similar experiences on several projects, I no longer recommend downdrafts. Hoods are best located above the cooking surface, even if the aesthetics don't appeal to you. (I also try to design ranges to be against walls, preferably exterior walls, though it's not always possible.)
1) "ERV is balanced ventilation, so it can't help correct the pressure imbalance caused by with kitchen exhaust"
Why not? I get that this is what the ERV *aims* for, what it's motorized to do, but it isn't all-powerful & it isn't a sealed vacuum pump (which might explode before you managed to force air through it when not running, or in the wrong direction). The pressure will "try" to equalize through whatever openings it can find. Doesn't the ERV function as a de facto vent?
I imagine you could test how air flows through an ERV under a pressure imbalance, empirically.
The original intention of "You HAVE to have makeup air" as I understand it was to avoid de-pressurizing the house and sucking deadly combustion exhaust into breathing air, but we're moving to no-combustion or sealed-combustion appliances so this part of it may no longer apply. Are short-term negative pressure conditions "okay" insofar as backdrafting is avoided through other means and enough pressure relief is provided to maintain a significant flow-rate at the hood?
2) We have techniques to silence duct noise, but they tend to involve a slightly more powerful fan, and then winding paths through excessive lengths of flex duct. If the grease droplet factor absolutely requires smooth metal ducting downwind of the hood... In a tight house could you perhaps make an unpowered hood attached to huge ducting w/ motorized damper, and run it all from makeup air pressure supplied from a noise-suppressed box? (presuming that the answer to question 1 is "no" for some reason, and every vent damper in the house is set up to allow positive pressurization).
3) I like the hydronic preheat idea that Kyle talks about, and it seems like there are a lot of different variations on that which you could implement. Since you have a fixed airflow, like hydronic you'd need to instrument that radiator's input and output water temperatures and modulate water flow to prevent freezing & prevent overheating the house. It intersects somewhat with the hydronic 'room temperature thermal buffer' idea I've been tossing around for a while, but after running the math comes nowhere close to singularly justifying one.
4) What are these carbon filters... doing... precisely? What little I understand about activated carbon for volatile chemicals is that it's much more effective in the very short term (hours/days) than in the long term.
What I understand about homeowners is that if it's not obvious that neglecting a maintenance process will cause immediate harm, that maintenance process will be neglected indefinitely by a large fraction of them (eg: "nobody's cleaned out this HVAC radiator in 30 years"), and very few will assume responsibility for high-frequency maintenance. One appropriate response to this with air filters is to design for very long maintenance cycles by using extreme amounts of surface areas. Another is the Warning Buzzer. I had a Warning Buzzer beeping every five in my family's basement for 20 years before something or other finally ran out of battery; We never figured out what it was coming from and decided to just live with it.
5) Emily suggests a QR code to a house plan. Unfortunately... this is quite unlikely to last 20 years, much less 50. There's no good infrastructure for digital document archival without a continuous influx of money, and even with that, paid services shut down all the time. We don't have the kind of corporate or software stability in the world of late capitalism necessary to come close to the stability of climate-controlled sheets of paper.
Burninate, I'll respond to your first item with an anecdote. On a house that tested below 0.2 ACH50, we included a powerful vent hood and could not talk the client out of a venting dryer. We included a 4" diameter passive makeup air vent. To keep the makeup air register near both the laundry and kitchen, the makeup air duct was over 25' long. It was not enough to keep the house from fairly severely depressurizing when either vent fan was running. (We also had a Zehnder HRV.)
While I'm sure the passive makeup air would keep the house from literally imploding, it was not enough to counteract 125-600cfm of exhaust. Add the fan and core of an ERV/HRV and I don't see how they could provide subtle pressure relief.
Run the numbers and you find that passive vents typically don't flow enough air unless the pressure is excessive. Use active make-up air with active exhaust.
In theory, a HRV or ERV could self balance such that interior pressure equals exterior pressure. I'm not aware of any that do this.
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