advice on vent hoods

lori_inthenw_gwApril 18, 2012

I'm thinking of a chimney-style, stainless steel version attached to a wall. This would be over a 36" wide induction cooktop.

Should I be looking at 42" wide hoods to allow the 3" per side overhang? (new construction so no constraints on the width at this point)

How do I know how many cfm I need? Is it any different because of using induction vs gas?

I have read the threads here related to this topic, but I'm still not clear on how I should start narrowing my search. This will be for a new and "tight" house, and I know there is some sort of energy recovering air handling thing that's part of the heating system if that matters...

I don't even know what my budget should be for this. I'm looking at Bosch and Miele cooktops and Electrolux ovens at the moment, if that helps with perspective, but not looking at the tippy-top of the line.

Any insights appreciated!

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What part of the country?
What type of cooking?
I don't fry anything and Wolf induction going in next week.
We live "up north", so have a 400 CFM rule, so putting in a 350 over the 36 inch cooktop.

    Bookmark   April 18, 2012 at 6:11PM
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Pacific NW, pretty stringent codes. No frying here, either other than a stir-fry once in a while. I wasn't sure if the requirements for an induction top were different than for gas.

    Bookmark   April 18, 2012 at 9:32PM
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The overhang is needed to capture the rising effluent because the effluent expands as it rises.

An induction cooktop avoids having hot gas combustion products (mainly water and carbon dioxide) add to the cooking effluent, but the primary need for good ventilation (capture and containment) is keeping cooking food effluent, in particular grease, off the ceiling, walls, furniture, draperies, and rugs. Many will also want odor removal. The CFM difference between cooking and gas, all else being equal, will be slight.

For models of the required CFM, please research it on this site (among many sources) first in order to save, at least me, from having to type the same information every few days.

What goes out is replaced by outside air. The purpose of deliberate make-up air (MUA) is to perform this replacement while keeping the house pressure from going more than slightly negative. This is important if there are combustion appliances, such as furnaces, hot water heaters, and dryers in use because a small negative house pressure can cause them to back-draft, bringing carbon monoxide into the house. Those with fireplaces will also appreciate having the smoke go up the chimney instead of into the room.


    Bookmark   April 19, 2012 at 8:37AM
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Thanks, kaseki. It makes sense that the width requirement would be the same. The point about the differences in CFM requirements between gas and induction was an important part of my question, so that helps. The reason I wondered was because I have seen so many comments along the lines of "well, if you're gonna have a gas stove with X,000 BTU capability, you will need a stove with at least Y00 CFM..." Or something like that. So it wasn't clear to me that the CFM requirement would be the same for induction.

It does make sense that your style of cooking is an important aspect of the question. (We grill outdoors all year, don't fry or generate a lot of smoke in the kitchen itself.)

But when you say "models of the required CFM" that gets to the heart of my question. Where do I find that? Is it specific to a cooktop or specific to a building code or what? I can do the research on models and narrow it down if I have a place to start.

And I do understand the principle behind make up air, and I know our HVAC system has a component that heats the incoming air because of this being a new and tight house that contains a wood stove. Is that ERV or HRV component the same thing as MUA or not? It seems like it would be, but I have new acronyms to learn in every part of this project...

Thank you for taking the time to respond. I tried to make my question short and specific so as not to require Ventilation 101 and extra typing, but I guess I was only partially successful at that!

I have some "showroom time" set aside this weekend, so maybe I will find someone more helpful on this topic than the last guy I talked to in person. I'm not out to select a model just yet, but I would like to narrow the field a little more and get a number (cost) to plug into the spreadsheet.

    Bookmark   April 19, 2012 at 10:19AM
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Will there be cabinets flanking this hood or will there be a gap between the hood and the cabinets?

    Bookmark   April 19, 2012 at 3:54PM
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The cabinet design is not final, but I think probably a gap. Are there practical pros and cons or is it an aesthetic choice?

    Bookmark   April 19, 2012 at 4:10PM
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By models, I meant commentary on rules (of thumb or scientific) that address the relationship between the cooktop activity and the cfm.

Besides the heroic task of reading all the threads with "cfm" in them that the local search engine, or google directed to "" can find, there are several other sources. One that I came upon a few minutes ago is a digest of previous Greenheck papers on the subject.


Here is a link that might be useful: Greenheck guide

    Bookmark   April 19, 2012 at 9:09PM
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Interesting Greenheck link. Seems that the effectiveness of baffle filters removing grease is pretty disappointing at just over 30%. Let's hope that my duct silencer isn't very good at removing grease...maybe I'll install it on a slant and add a drain plug. OTOH, I don't really cook anything with grease (annual hood mesh filter cleaning was overkill), no meat, never fry, very little oil, so maybe not a big concern.

    Bookmark   April 19, 2012 at 9:28PM
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Your handle should have clued me in.
Love the NW - especially Mt Rainier!

Our codes are pretty strict also - anything over 400 needs make up air.

We are putting in a Broan Evolution which runs at 350.

I am not sure about chimney style - but do some surfing.

I found some of the smaller units only put on of the 2 fans in the unit.

I felt Broan had lower sones and seemed to market to the lower CFMs and wanted to keep under the radar.

We are putting in a 5 burner Wolf next Friday - I can't wait! Ovens installed and operational - but haven't tried them yet...

    Bookmark   April 19, 2012 at 9:36PM
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Kas: oh, that kind of "model." Yes, I know what it means, work with water models almost daily, but in the context of appliances, the term threw me off in a different direction. I looked at the Greenheck info, but am not savvy enough on the topic to figure out how to scale down from the commercial kitchens it describes. More homework to follow.

a2gemini: Yes, I could see Mt Rainier out my window earlier before it started raining this afternoon! We had trees between us and that view for the 25 years we've lived here, then a few months ago, one of the trees fell on a neighbor's care and the other 2 were cut down!

    Bookmark   April 19, 2012 at 10:27PM
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"Until recently, residential codes had little to say about providing makeup air for range-hood fans. But the 2009 version of the International Residential Code (IRC) will, at long last, address the issue.

Here�s the new IRC provision, which is found in section M1503.4: "Exhaust hood systems capable of exhausting in excess of 400 cfm shall be provided with makeup air at a rate approximately equal to the exhaust air rate. Such makeup air systems shall be equipped with a means of closure and shall be automatically controlled to start and operate simultaneously with the exhaust system."

(the author goes on to say just get yourself a modest 30 inch range and don't bother with all those complications!)

Here is a link that might be useful: green building advisor advice on MUA

    Bookmark   April 19, 2012 at 10:42PM
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We are also in the NW and have decided to go with a Broan unit that has very reasonable passive MUA unit that integrates with it. We don't anticipate that we will use all 600 CFM of power often (though it is what our range manufacturer recommends) but figure that this will keep us code compliant. Not sure if they have a chimney style hood that integrates with the MUA unit.

    Bookmark   April 19, 2012 at 10:43PM
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at least when I use the AJMadison database and plug in all the specs, it gets narrowed down pretty quickly. I was using 300-400 cfm as a starting place, which seems fine for now. One was a Braun, another was Best.

(I still don't get whether or not that energy recovering ventilation device in the mechanical room is related or totally separate from make up air for vent hoods, but maybe someone in the HVAC forum can tell me. Meanwhile, I will just plug in a number to my cost spreadsheet for now.)

    Bookmark   April 20, 2012 at 12:19AM
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In San Jose, CA, the MUA requirement is based on whether there are gravity devices (non-direct vent gas furnace, gas water heater) within the conditioned envelope. If not, none required. If so, then if the capacity of the two largest exhaust fans (e.g. kitchen hood, clothes drier) combined exceeds 15CFM per 100 sq ft of home area, make up air is required. For example, a house of 2400 square feet could only have 360CFM without make up air.

    Bookmark   April 20, 2012 at 1:31PM
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Flanking cabinets that touch the vent can act as a 'channel' for the rising plume. Hence, a wider hood is not required. No cabinets or a gap means no channeling, hence a wider hood.

    Bookmark   April 20, 2012 at 3:46PM
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Thanks, weedmeister. Sounds like you are saying that with the gap, the hood could just be the same width as the cooktop? I will keep that in mind as we move forward. Right now I'm assuming that 400 cfm would be adequate. This would be for a 36 inch induction top. There are no gas appliances and the boiler and hot water are also electric, there is a heat recover system as part of the HVAC. If something about my assumption is screamingly wrong, I hope someone will clue me in!

    Bookmark   April 20, 2012 at 5:45PM
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