Stovetop wall hood - CFM and MUA advice

renov2018

5 years ago

We are remodeling our kitchen and are replacing a downdraft island range with a separate 30" Kitchenaid induction cooktop on a counter (not island) and single oven on adjacent wall. We want a chimney style wall hood over the cooktop. Our kitchen is open to the family room, but the actual kitchen portion is an uneven U with island in the middle and the kitchen is completely open to the adjacent family room. The dimensions of the kitchen portion 7 ft X 15.6 ft (width of kitchen) X 18.4 ft. The cooktop will be on the bottom of the U and the single wall oven will be on the 7 ft wall. The family room extends another 22 ft in length beyond the 18.4 ft side of the U that is part of the ktitchen. The ceiling height in the kitchen area is 8 ft (higher in the family room portion) Using the formula I've seen online, and multiplying by 15.6 (width) by 18.4 (longer wall of kitchen) by 8 (ceiling height) suggests we need at hood of about 600 CFM. However, I have the following questions about estimating CFM.

1)The duct will go into the ceiling and will almost immediately have a 90 degree angle to travel 5.5 ft to the outside of the house. I know that having this sharp angle requires a higher CFM, but how much higher should it be?

2) I have read that induction ranges require a lower CFM than gas, but how much lower?

3) We have a very tight house. We had an energy audit when we first moved in and they were concerned about CO2 return. When we cooked with our island downdraft fan on a higher setting and had a fire in the fireplace (31.5 ft away) smoke would begin to be sucked into the house. Therefore, according to what I've read we should have an MUA installed as well. Is my judgement correct on that?

Sorry that my first post on Houzz is so long, but I am so very grateful for Houzz. It has helped us so much in the process of designing the kitchen.

Linda

Featured Answer

kaseki

5 years ago

OK, in here are some requirements. Note that hood flow rate (CFM) is not determined by room size, but by cooking plume upwards velocity and hood entry aperture area. It is not determined by duct restrictions, although the blower's required rated CFM will be affected.

Hood entry aperture: The hood needs to overlap the rising and expanding cooking effluent plume that originates from the hot pan base, and will also (with gas cooktops) include the combustion product plume surrounding the pan. Ideally, the hood has to overlap by enough to at least enclose a zone 10 degrees outward from vertical relative to the locus of areas of the pan bases that might be used. Note that this calls for area increases as the hood entry aperture height increases. For wall mounted hoods, this overlap is not required at the back wall, and due to the "semi tunnel" effect of counter, cabinets and back wall, it is usually sufficient to have a hood 6-inches wider than the cooktop and extending at least three inches outward (toward the cook) from the cooktop edge (not the pan edges).

Flow velocity: The velocity at the gaps in the baffles needs to be high enough to entrain the plume, and the velocity in the capture volume below the baffles has to be high enough to cause plume reflected from metal surfaces to continue upward to the baffles. An apparently sufficient flow velocity is 90 ft/min at the aperture. Induction cooktop plumes do not have gas combustion products, and have been measured to be somewhat slower in rising, so perhaps 60 ft/min may be adequate. (This rate may or may not be sufficient for heavy searing or wok cooking.) 90 ft/min implies 90 CFM per square foot of hood entry aperture.

Blower requirement: The blower has to achieve 90 CFM/sq. ft. x square feet at the pressure loss resulting from the path: baffles -> ducts -> exit cap -> MUA system -> baffles. These pressure losses can, in principle, be measured, but for expediency sake, I recommend choosing a blower rated at 1.5 times the product of the hood aperture flow velocity to be achieved times the area of the hood entry aperture.

Duct diameter: Duct diameter should support at full blower flow rate a duct velocity of 1000 to 2000 ft/min, with a preference to the lower value if some of the ducting will be cold enough to allow impingement condensation of grease vapor particles.

MUA: Yes Virginia, there must be MUA, else the blower pulls no air or the house implodes (not likely with a hood blower). There is always some MUA if any air moves, but it can easily be inadequate. If there is inadequate MUA, the house pressure falls relative to the outside, and the pressure loss causes the blower to move less CFM per the blower's fan curve. Air will leak into the house via undesirable paths, including back-drafting combustion appliances, thereby potentially introducing carbon monoxide into the house.

House pressure should be kept very close to ambient outside, or else combustion appliances will require their own MUA. If they do have their own MUA, then from a safety point of view, some pressure loss can be tolerated in the MUA system -- that is, it can be passive with some modest filtering. Otherwise, a "blown" MUA system (one with its own blower) is needed for higher CFM ventilation systems, and the control for this can be complicated in some circumstances. Even one partly open screened window (not sufficient for where code requires a specific MUA approach) can lead to enough pressure loss for back-drafting.

In cold climates, the MUA system will want to have an air heater -- this can require up to 100k BTU/h.

I recommend that all combustion appliances either reside in a closed room with its own MUA, all have ducted MUA, or a HVAC specialist be engaged to deal with the needs of the kitchen ventilation. You can read about residential MUA systems at Fantech's site, or for the overall commercial approach (good for background information) in the first dozen or so pages of Greenheck's publication:

http://www.greenheck.com/media/pdf/otherinfo/KVSApplDesign_catalog.pdf

Note that commercial kitchen systems are often easier to tune because the hoods run at constant speed so the MUA can also. Large office buildings without a kitchen issue also require MUA, but this can be passively controlled by allowing the building to have slight positive pressure.

I also recommend reading the many hood threads on this forum dating back to at least 2008 to gain more perspective.

kas

Sort by:Oldest

Comments (12)

colbran
5 years ago
This will be short and not at all technical for you, but I have a gas stove and added up all the CFM business based on if I had the turbo burner on.....I didn't want to take any chances not having enough vent to pull things out. Sadly, bought the wrong size Vent-A-Hood with a Magic Lung that was supposed to fit under the cabinet but didn't, and now I have a brand new one sitting here I can't return to the tune of $1400. Whatever you do, be careful with ordering online that your specs are right. I shopped vents hard...went to look at them in person, shopped them on the internet, checked over reviews, etc. until I was blue in the face. The one thing I wasn't willing to sacrifice, was CFM. I don't care how much the vent cost was, I think it will be well worth it. Just my low key non-technical version for you....good luck!
renov2018 thanked colbran
Related Discussions
hood questions 101--not cfm or mua
Q
Comments (10)
The OP wrote: "Kaseki--So, perhaps the appropriate thing to say is that mesh filters are not as effective as baffles but can work if kept clean? Noise is a concern of mine. I don't want something that sounds like a jet on a runway (exaggerating a bit here). Perhaps baffles are the ideal compromise. " I thought what I said was they can be equal if clean, without explicitly noting that clean is a condition that may be brief. For the same flow rate, I would expect mesh filters to generate a bit more hiss. Fan blade turbulence noise will depend on the fan and other factors, but whatever acoustic level the tip noise is, I would expect little perceived difference to the cook between baffles and mesh filters, everything else being equal. "I know VAH markets itself as much more efficient--e.g. their 300 CFM will actually perform the same as someone else's 450CFM. Are people here in agreement with that statement? I guess if that were true, I might be willing to put up with a bit of extra noise and cleaning hassle. " This has been commented on here in many threads, including a recent one by me. It is unlikely the motors are more efficient (air moved per hour per kw-hr used). Grease collection fraction over the particle spectrum has never been published, to my knowledge, so comparative collection efficiency is unknown. The claim of being equivalent to a 50% greater cfm standard configuration is, in my view, likely to be a comparison of "apples to oranges." Normally, fans are specified as having a particular flow rate at zero static pressure, and as the pressure difference across them increases, the flow decreases. The relationship is typically plotted in a "fan curve." Respectable fan manufacturers will publish the curves for their fans. Broan has fan curves on their web site. For other examples, see Rotron and Fantech (published as tables). What I believe VAH are saying is that in their fan housing, their fan moves X cfm, but if it were in the open as other fans are rated it would move 1.5X cfm. Unfortunately, that is not the whole story. There are other pressure losses in a system, such as duct friction loss, turning loss, transition loss, filter loss, and, sometimes the largest, the lack of adequate make-up air loss (the house pressure goes negative). So, without a fan curve one cannot determine how much lower the cfm will be for a VAH in situ. Not that the situation is all that much better for the fans with fan curves, because the hood and baffle losses have to be guessed at for most hoods (commercial being an exception). And house pressure has to be estimated depending on what MUA system, if any, will be in use. As a gross rule of thumb, I assume that baffle hood systems normally configured without more than 0.03 inches of MUA loss will meet the VAH factor; that is, the actual cfm will be 2/3 the zero static pressure cfm rating of the fan. (Note that more negative house pressure will backdraft some combustion appliances and should be avoided.) kas
...See More
myths about cfm's and mua
Q
Comments (17)
A make-up air system for the range hood isn't going to run constantly like a normal HVAC system that heats/cools/filters your internal air throughout the day. It also cannot be compared to a commercial MUA unit for a vent hood over a range in a restaurant because they cook from the time they open to the time they close. So a residential MUA system will have much less of an impact on internal comfort levels than what most people are envisioning. Most HVAC systems are designed to run with slightly negative air pressure to begin with. That's why things tend to blow into your house rather than out. Ask yourself, "how often do I turn on my range hood compared to opening the back door/garage door/front door of my house?". Every time you open a door to your home you are letting in a large amount of unconditioned freezing cold or hot and humid air. And when enough outside air is mixed with the internal air to affect the temperature, your HVAC system will activate and begin exchanging it for conditioned air. The same process will occur with a passive make-up air system. Everyone seems to be afraid of the air outside their homes, but who ever says, "I'm gonna step out for some dirty hot and humid air"...no one. Everyone always says, "I'm going outside to get some fresh air." It really is as simple as allowing your house to "breathe". And like foodonastump said, no house is sealed 100% airtight so your home is going to find a way to bring in outside air. You are just giving it an extra set of lungs by adding a make-up air system during operation of a high CFM range hood to avoid creating enough negative air pressure to stall or backdraft combustion vents. foodonastump, the 2009/2012 IRC M1503.4 code provision reads "Exhaust hood systems capable of exhausting in excess of 400 cubic feet per minute (0.19 m3/s) shall be provided with makeup air at a rate approximately equal to the exhaust air rate". So the relationship between the exhaust opening and fresh air intake need to be symmetrically proportional at minimum. And the building inspectors will default to the commercial code which reads that make-up air needs to be supplied at 80% of the exhaust rate. And because there are residential range hoods that can exhaust 600 CFM through a 6" or an 8" round duct (several manufacturers exhaust up to 1000 CFM through an 8" duct while most use a 10"), the question becomes even more confusing. And based on the fact that any exhaust vent in your home, combustion or not, is fitted with some type of backdraft damper/flapper, the reasonable path of least resistance for fresh air intake would still be an open and clear 6" duct, even at a 600 CFM exhaust rate. davidrol1, An air curtain is a great application for commercial settings but is just too uncomfortable for a residential homeowner. Hope this helped a little. Nate
...See More
New Hood meets frustrating new city code for MUA/CFM
Q
Comments (16)
What you can do to achieve safety may not be congruent with what the Code requires you to do because someone who wrote the code wasn't thinking of the originating requirement, but of a derived requirement. The functional requirements are (a) avoid back-drafting combustion appliances, and (b), keep the house pressure close enough to the exterior pressure to allow the ventilation hood blower to pull the needed airflow (and also not pull dust and whatever through cracks in the walls). In cold climates, there is an additional requirement to condition the air so one doesn't freeze, although I think requiring that is over-reach by nannies; most people would tailor the length of their high flow rate needed cooking to a short period given the rapid chilling of the household. If you have to go whole-hog, MUA-wise, then there will be a set of cost elements as hvtech42 noted. If active MUA is needed, and it will be if you want to filter the intake air in the presence of combustion appliances, then you might want to check out Fantech, which has a set of such systems for various flow rates. I concur on the utility of investigating the exact wording of the Code requirement. Some locales take notice of whether back-drafting is a risk or not in their Code requirements.
...See More
Range hood, Duct, and MUA advice needed for condo
Q
Comments (1)
Bump
...See More
zneret
5 years ago
Take out the cabinet, dude!
colbran
5 years ago
Appreciate that zneret, but it was already sticking out far enough and didn't want it to go farther than that.
zneret
5 years ago
You probably explained it already but why would the hood be sticking out too much? You could point me to the thread.
colbran
5 years ago
last modified: 5 years ago
I don't necessarily have that particular item pointed out in my thread, but if you want to take a look "I could use some backsplash help!" and sort through pictures and posts, feel free. The entire bank of cabinets was up already and then the vent put underneath the cabinet front, and of course, it wouldn't fit in there....by 2.5 inches. It is a hidden hood and I apparently ordered the wrong one back last fall. Sadly, with my long remodel, it wasn't installed until just recently (way past a 30 day return policy). I personally think I had the right one google searched, but when I got into their website, somehow one letter was added to the search and I ended up with the wrong one. I'm pretty thorough and don't make costly mistakes like that. But, I'm the one who ordered it, so clearly it happened. Too bad there isn't a website for brand new items that can't be returned due to stupid internet policies....
weedmeister
5 years ago
1. perhaps a bit more, but not too much.

2. A bit lower. I would do 300-400cfm lower, but I'd never do less than 600cfm.

3. Yes. If you're sucking smoke down the chimney, you need MUA. I'd probably do a pressure activated (barometric) damper. Whether you need heated air depends on location. There are inline electric heaters available, thermostatically controlled, that will raise the temperature of freezing air.
renov2018 thanked weedmeister
renov2018
Original Author
5 years ago
Thank you so much, kaseki,colbran, and weedmeister for your very helpful responses. Kaseki, the education you provided was phenomenal, and I really appreciate the time you took to explain the science behind the decisions. I had read the forums fairly extensively, but you have added new depth to the discussions. It is interesting - came home to read these responses after a 2 hour meeting with our contractor. One member of the team is urging me to get a hood that is 450 CFM and is dubious about the need for MUA. They also indicated that I didn't need the 36" hood for the 30" cooktop. (This is a very good company - considered to be one the best in town (I won't say where) and I know we don't have a commercial kitchen. On the other hand, we do use high heat to sear meats and to stir-fry. The person doing the construction diagrams did indicate that the information I presented and the questions I was asking were in fact consistent with safety standards (why you want a wider hood, MUA concerns etc). . It is an induction cooktop, which does reduce combustibles compared to our current gas stove, but I still want to make sure that we remove odors and smoke adequately. Because we live in a cold climate, we would need to address heating the air from MUA. From reading other forums, I get the sense that if I am running the a lower CFM hood at its highest setting, noise is likely to be greater than if I am running a more powerful unit at a lower setting. (Don't know if that's correct or just someone's opinion) I'll continue to read the sources you suggested to see if that is true. So far I can't find any dB information on spec sheets. Again, thanks to all of you for your responses.
renov2018
Original Author
5 years ago
Never mind about the dB. I see that some of the spec sheets do list sones. (However some have no noise information.)
tatts
5 years ago
A 450 CFM hood needs to suck in enough air to fill a large-ish powder room every minute (8'x7'x8'). Every. Minute. Unless you live in the draftiest house ever built, you'll undoubtedly need an MUA unit.
kaseki
5 years ago
deptchair wrote:

" From reading other forums, I get the sense that if I am running the a
lower CFM hood at its highest setting, noise is likely to be greater
than if I am running a more powerful unit at a lower setting. (Don't
know if that's correct or just someone's opinion)"

This is true because the dominant noise generated by ventilation hood blowers at high speed (as well as by in-line and exterior mounted blowers) is from blade tip turbulence (and blade edge turbulence too, I imagine, depending on blade design). Hence, moving 300 actual CFM from a small screaming-mimi (read aerospace) fan will be much louder than from a modest hood blower, which in turn will be louder than from an even larger slower fan. In the residential limit, Casablanca type ceiling fans can move very high flow rates nearly silently.

Blade tip turbulence noise can be attenuated when in-line and external blowers are used by adding a silencer between the blower and the hood, assuming adequate duct length and diameter is available. See Fantech's offerings for examples. When a silencer is used, hiss from the hood's baffles tends to be next in line for noise, and while this is audible, it is usually sufficiently unobtrusive for cooks to be able to carry on a conversation.
Jakvis
5 years ago
CFM vs. MUA explained in 10 words and why they need to be balanced.

You can only blow out what you can suck in

:-)

Stovetop wall hood - CFM and MUA advice

kaseki

Comments (12)

colbran

Related Discussions

zneret

colbran

zneret

colbran

weedmeister

renov2018
Original Author

renov2018
Original Author

tatts

kaseki

Jakvis

COMPANY

BUSINESS SERVICES

GET HELP

CONNECT WITH US

Stovetop wall hood - CFM and MUA advice

Comments (12)

Related Discussions

renov2018Original Author

renov2018Original Author

COMPANY

BUSINESS SERVICES

GET HELP

CONNECT WITH US

renov2018
Original Author

renov2018
Original Author