More Q&A Spotlight
DCContrarian, a frequent contributor to GBA’s Q&A forum, now has a question of his own: How much air should a kitchen exhaust fan move?
“So how many cubic feet per minute should the fan that vents a cooktop be?” he asks in this recent post. “I’ve done an exhaustive search of the GBA archives, and there are lots of articles for sizing the makeup air to the vent fan, but nothing I’ve been able to find that helps you decide how big the fan itself should be.”
Most of what he’s been able to find online “lacks any sort of analytic rigor,” he continues. The rules of thumb he has found—1 cfm for every 10 Btus of output for a gas range, or 10 cfm for every inch of width for an electric range—suggest the capacity of the range hood should be about 300 cfm.
But what about a number that’s science-based? That’s the issue for this Q&A Spotlight.
The IRC addresses makeup air, not fan capacity
As Armando Cobo points out, a section of the International Residential Code (IRC) requires makeup air be provided for exhaust fans rated at more than 400 cfm. Each exhaust fan of that size must be “mechanically or passively” provided with makeup air at about the same rate that indoor air is being expelled.
“That says nothing about how big the exahust system should be,” DCContrarian says, “just the makeup air.” Plus, that code provision would not apply to an all-electric house—just to those with a fuel-burning appliance.
Where does that leave us? Picking a number out of the blue and letting GBA readers lob rotten tomatoes at it, suggests Malcolm Taylor.
“I’m pretty sure you just pick a size and we reply it’s either too…
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25 Comments
So, anyone got some recs for off the shelf lower CFM (300-400) range hoods with good form factor for capture?
Rebecca,
One of the main criterion I'd use is how noisy the fan is. Most range hoods aren't used because people dislike having them on. A poorly performing range hood that is routinely turned on during cooking is still more effective than a better one that sits idle.
You can get external fans so the noise is outside. I’m getting one by fantech, which also has a hood liner to go with it.
Carson,
Outside or in another space? I thought the ones Fantech supplied were inline fans?
Outside. Maybe not the prettiest, but let it bother my neighbors;). https://shop.fantech.net/en-US/rvf--8xl--ec--ext--centrifugal--fan/p95294
Carson,
Perfect! Quiet trumps aesthetics on fans anyway. I don't mind how it looks though. I may retrofit one on our range hood. We use it but grit our teeth the whole time.
Looking at those, maybe that's the way to go with bathroom fans too?
Sounds logical right? Why don’t you see more external fans in residential? Cost? I’m not actually sure how much quieter it will be, the only ones I’ve seen online also used silencers which I’m too cheap for. I’m also debating wether to figure out a venting for broiling in the wall oven. Maybe that will just be an open a window scenario.
Carson,
I never considered them because I didn't know they existed.
I really dislike the way bathroom exhaust fans are installed. Their housing is hard to air-seal and the connections are not accessible once the drywall is on, making replacement difficult to do well. So I have started mounting the whole thing - fan and housing - in an airtight plywood box, where all the connections are from inside.
But I'm reconsidering that approach since your post. What downsides can you think of? I guess you have to add an inline damper?
(Response to #17) Malcolm, that's an excellent idea!
I’ll try to do a post after my build with what worked and what didn’t in my somewhat unorthodox build. Downsides I can think of: 1. yes requires backdraft damper. 2. Cost, at least for a rangehood it’s cheaper to get the hood insert with a fan instead of a passive hood liner, external fan, and backdraft damper. Installation is likely more expensive as well. 3. Installation and maintenance may be more difficult depending on where on the exterior it’s mounted. 4. Increased noise and wall wart on exterior. I’m not sure aesthetically if it would be any worse than a regular vent though. 5. Temperature range of fan operation. I hope cold is fine, but I saw it shuts off at 104F. Fine where I am, but an issue for a place like Texas.
Carson,
If I switch mine out I'm going to use the existing hood and just remove the motor. Looking at the selection in the big box stores, there are a few quite nice ones that are probably cheap because of their poor controls and motors. If you only wanted the carcass they might be worth using rather than a dedicated hood from Fantech.
I was wondering, what if one lives in a cold climate (i.e. Toronto Canada). Would the external fan led to rapid degradation from grease deposition in the winter?
Steve,
It might be more prone to, although all commercial restaurant fans are roof mounted and don't seem to have any problems that way.
Steve, a good question but what would cause this? Are you thinking the exhaust would instantly turn to ice? I would hope the exhaust would be warm enough to not freeze up the fan but don't really know. Might be worth contacting fantech and asking, but I didn't see a minimum outdoor temperature.
Yeah will ask Fantech. In makeup air systems, one reasons I've read why they don't try to do heat recovery is due to the grease precipitation with temperature drops, but perhaps it's more about grease precipitation on the heat recovery mechanism vs on the fan/duct system.
I had a similar concern about the silencer component and grease precipitation leading to narrowing on the high surface area, but I'd imagine they wouldn't sell this system without testing this out...
The part of the question that still isn't addressed (at least not in the text--maybe it is in one of the cited sources?) is where in relation to the stovetop does the makeup air come from? Can you get adequate capture when it's mostly makeup air being exhausted?
Paul,
That's an interesting question. The two parts the discussion that dealt with that are fairly general:
" ...others say that the makeup inlet should be far away because the cooking by-products will diffuse and you want to be venting the whole house"
But DC goes on to conclude that hood capture is what's most important:
“...the strategy isn’t to suck the contaminants out of the air, it’s to catch them in the hood, and then empty the hood frequently.”
If DC is right, the location of the makeup air shouldn't matter.
I don't understand this sentence: "When it’s 36 in. over the cooking surface, the hood should overhang 3 in. on each side; at 42 in., the overlap should be 6 in. on each side." Does this mean that if the hood is 36" above the range, it should overlap the left and right sides by 3", so a 36" range would have a 42" hood? What does "the overlap should be 6 in. on each side" mean?
Never mind, I just reread the sentence and understand what it's saying. Thanks anyway!
Jenna,
Yes you have it right. A hood 36" above the cooking surface should be 3" wider on each side. One that is 42" should be 6" wider on each side than the cook-top. This is fleshed out in the link provided in the article above: https://bamasotan.us/range-exhaust-hood-faq/
For what it's worth, there is a code requirement for kitchen ventilation in the IRC.
Section 1505.4.4 specifies minimum 100 cfm intermittent or 20 cfm continuous.
I know of two good resources on this issue.
The first is LBNL who have done extensive studies on range hood effectiveness. You can google and find videos of the investigators talking about it, a long-ish paper is here:
https://www.aivc.org/sites/default/files/lbnl-5545e.pdf
and a slide show with a fair amount of the same info is here: https://indoor.lbl.gov/sites/all/files/lbnl-6547e.pdf
The second resource is the estimable ROCIS organization who have a general page here: https://rocis.org/kitchen-range-hoods
and a guide to selecting hoods here: https://rocis.org/sites/default/files/user-files/ROCIS-Ducted-Range-Recommendations-Dec-2019-Update.pdf
On page 6 is a list of recommendations, including at least 250-300 CFM with a high Capture Efficiency (CE).
They keep pushing out the date, but ASTM E3087-17 is supposed to be implemented any time now, which creates a standard for range hood capture efficiency. I can't remember the organization doing the studies, but there is a big database of hoods. Currently that field is blank for all the hoods, and frankly the Sones ratings are totally inconsistent as well. So while it would be great, I wonder how long it will really be before we get good capture efficiency ratings.
In the meantime someone referred me to Victory Range Hoods out of Canada. They seem to have very good products at good prices, and will put smaller fans in any of their hoods. That's a big benefit in my mind, because most decent hoods have huge cfm numbers. What I especially like is instead of those crappy mesh filters they have the baffle filters in many of their hoods. Most brands you have to spend thousands to get those.
I did extensive research when selecting a range hood and ended up getting a GE range hood insert with 610 cfm max rating. Generally running a larger capacity hood at lower speed is much quieter than running a smaller capacity hood at its maximum speed. Also, the higher capacity hoods have larger size ducting for lower static pressure. Many can be limited with a board jumper to 290 or 390 cfm to comply with code for no make-up air, which is what I did. However, some Best models comply with code by sticking a piece of metal in the fan exit port to block half the flow, which seems insane!
Scott is the page correct?
"The Wolf Design Guide is a case in point with ventilation suggestions starting on p.158"
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