How much vent hood CFM do I need?

jmaskovsJuly 10, 2012

For my kitchen renovation, my contractor is planning to install a 30" Whirlpool 400 CFM vent hood with a 6" duct. We are also planning to install a 30" RNB Bluestar range (which has 2 22,000 btu burners). Bluestar recommends a 600 CFM vent hood, but our contractor doesn't want to install one that large because it will require an air recovery system. Is the 400 CFM vent hood powerful enough? Opinions please!!!!

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GreenDesigns

If you only use one burner at a time or don't use your oven, then 400 CFM might work. If that's the case, then you probably wouldn't buy a Blue Star! In many places, it violates code to install an appliance without following the manufacturer's installation instructions----including ventilation.

Bottom line is, if you want a pro grade range, then you're looking at buying a pro grade vent. It's not optional.

    Bookmark   July 10, 2012 at 9:55PM
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SparklingWater

400 CFM's of vent flow for a 30" Blue Star four burner isn't enough. Tell your contractor 600 CFM's minimum as that is what Blue Star recommends and that's what you need arranged. MUA isn't that hard to add in and you'll need it especially if your house is "tight", i.e., limited inherent air leaks. Good luck.

    Bookmark   July 10, 2012 at 10:08PM
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ribs1

Sparkling,
What do you mean make up air isn't that hard to add?
Depending on where you live, code requires conditioned make up air. The cheapest make up air unit that I could find that would meet code here was $7,000 not including installation.

Make up air is a serious issue not to be taken lightly. The consequences of not properly addressing the issue are at best, not meeting code and at worst depressurizing your house and death due to CO poisoning.

    Bookmark   July 10, 2012 at 10:33PM
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SparklingWater

Sorry ribs1, you're caught in a situation many of us don't face and I surely empathize with the expense of conditioned Make Up Air (MUA) due to code.

I read the OP asking about Blue Star BTU and venting CFM's. MUA is increasingly considered and discussed when re-doing or building a kitchen, and should be discussed pre-purchase of a pro-range. MUA has been an active topic of interest here on GW as well as on greenbuildingsdesign.com in "Makeup Air for Range Hoods" (linked below) and on many HVAC forums. Good reading and sources for solutions for industrial conditioned MUA (which perhaps will help you somehow too ribs) are also found on http://www.greenheck.com.

MUA, passive or active, is available without excessive expense for most of us as we are not restricted, as yet, by specific code requirements. I'm firmly in the camp of matching one's cooktop/range purchase to one's budget for proper ventilation and MUA and more to avoid any risk of acute illness/death due to carbon monoxide poisoning from lack of replacement air.

Here is a link that might be useful: Makeup Air for Range Hoods

    Bookmark   July 11, 2012 at 12:01AM
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weissman

As I replied on the kitchen forum, YOU need to make the tradeoff between CFMs and the cost of makeup air. Depending on what you cook, 400 CFMs may be adequate. CFM recommendations by manufacturers make the assumption that you are running all the burners on maximum. I have a 600 CFM hood over a pro-style range and in 10 years I have hardly ever needed to put the hood on max - mostly low or med. As long as you're getting a real range hood and not an OTR, 400 CFMs may meet your needs.

    Bookmark   July 11, 2012 at 12:55AM
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weissman

Make sure your contractor is correct about the code requirement in your area. In many locales you don't need makeup air for 600 CFMs, but some of the colder climates have lower limits.

    Bookmark   July 11, 2012 at 12:57AM
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attofarad

I live in the mild SF Bay area, and can run into make up air requirements, if there are gravity gas appliances or fireplaces in the conditioned envelope. I got rid of my fireplace, put my water heater in a sealed (from house, not from crawlspace/attic) closet, and moved the furnace to the attic. Otherwise, I'd be limited to about 425CFM without MUA. So, check your local code. Here is mine:

1. Make up air
Buildings with gravity appliances such as water heaters, furnaces, gas fireplace inserts, etc, within the conditioned envelope must provide make up air if the combined exhaust rate of the two largest exhaust fans (including clothes dryers and kitchen hoods) exceeds 15 cfm/100 sq ft. Identify on the plans if appliances are direct vent or gravity and show the location on the plans. If this requirement applies, indicate that make up air is required.

    Bookmark   July 11, 2012 at 1:45AM
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kaseki

Two good messages there, attofarad. The first is that at least in one locale the MUA requirement is worded to accommodate both house size and configuration of combustion appliances. The second is that one escape from deliberately adding MUA can be to seal off those combustion appliances that have CO backdraft risk.

I think gravity appliance would exclude blower-fired oil burners, but at a somewhat higher negative pressure than for gravity appliance backdraft, these too can backdraft, so an even more refined requirement would list negative pressures vs. appliances. I have seen such a list somewhere. I'll report the link if I find it.

kas

    Bookmark   July 11, 2012 at 10:59AM
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