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I like to cook. I would like to put in a fairly powerful range (nothing huge, though; I don't need six burners and a griddle), and will need a hood that can carry smoke, heat, steam, grease, etc. from cooking out of the house. The trend seems to be huge hoods with, e.g., 1200 CFM, and a lot of people say you "don't need that", but then I do know that a lot of people don't actually COOK in their kitchen. I'm not convinced that I have any need for 1200 CFM, but I'm also not convinced that something like 350 CFM will be sufficient. Assume I'll have a 30" Wolf all-gas range (not sold on Wolf, but most of what I'm looking at is in that ballpark in terms of BTUs, so it's a good comparison), and be using two or three burners on the stove five or six nights a week, the oven three or four nights a week, and cooking things that may produce large volumes of steam and/or smoke and grease vapor (boiling/simmering large pots of water, stir fries, roasting meat, pan-searing steak) at least half of that. I don't want all that going out into my house, and I certainly don't want it clogging up the filters/core of an HRV, so I need a hood that can exhaust it. How do I size hood CFM appropriately for both the amount of gas that will be combusted, and the amount/scale of my cooking? I don't want absurd overkill, but I don't want my smoke detectors going off when I roast a chicken.

NOTE: I've split the second question I asked out into this question.

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The better the fan the less likely you are to set off the fire alarm. You have to keep the filters clean. –  user270637 May 7 '14 at 20:15
Something else to consider is noise. Fans running at 100% are louder than at 50%. If you need to vent 500cfm, you might be better off with a larger unit but running it lower which will be quieter. –  Steve Jan 8 at 20:55

3 Answers 3

up vote 10 down vote accepted

When calculating the minimum size of a range hood, there are three things you should consider. The size of the cooking surface, the amount of heat produced by the cooking surface, and the volume of the kitchen.

If the range hood is attached to a wall, you should have 100 cubic feet per minute(cfm) per linear foot. So if you have a 30" wide range, you should have a hood rated at 250 cfm ((30/12)*100 =250). If the hood is over an island, you'll use 150 cfm/linear foot. In this case that same 30" cook top, would require 375 cfm ((30/12)*150 = 375).

Next we'll determine the minimum capacity based on British thermal units(BTU)/hour, by dividing the BTU/hour by 100. For example, if we had a cooktop that produced 40,000 BTUs, we would need 400 cfm. If you are using an electric range (measured in watts), simply multiply watts by 3.41214163 to determine BTU/hr.

The final calculation, will be based on the size of the kitchen. The air in the kitchen should be cycled 15 times per hour, so our formula will be ft³/4. If we have a 10ft x 10ft x 8ft kitchen, (10 X 10 X 8)/4 = 200 cfm.

We'll then choose the largest from these three calculations, and that will be the minimum size hood we need. If you are doing more cooking than the average person, or just want a little more air movement. You can always get a larger hood, this is just the minimum size you should consider.

International Residential Code (IRC), says the minimum intermittent exhaust rate for a kitchen is 100 cfm, while the minimum continuous exhaust rate is 25 cfm.

M1507.4 Local exhaust rates. Local exhaust systems shall be designed to have the capacity to exhaust the minimum air flow rate determined in accordance with Table M1507.4.

Table M1507.4

So you'll want to make sure the hood is at least capable of achieving these flow rates.

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Great info. It would be attached to a wall, and a 30" range in a not-huge kitchen, so I think the BTUs are the key. So for four 16k BTU burners, 640 CFM would be minimum. I can't seem to find any info on how many BTU the oven in one of these things can burn, but then again I don't think I'll ever have all four burners on high and the oven at 550. What about make-up air? Would say an 800 CFM hood be OK if I typically run it on low, and open a window if I am stir frying or blackening a fish? –  Adam Jaskiewicz Jan 4 '12 at 18:49

You also have to take into account the ducting run and turns, which derate the air flow. I have a 400cfm exhaust fan in my overhead uwave, and it works fine on the lowest of the 5 settings.I have 6 inch ducting that runs about 15 feet with one 90 degree turn. I also clean the ducting regularly by boiling vinegar on the stove for a few minutes per month, and then running the fan for a while to dry the duct out completely.

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I just finished updating my kitchen and I think you have to really be careful of going to far in either direction with sizing a range hood. Most places that don't have a fancy range hood are probably lucky to have 100cfm hood. I looked at all of the information I could and what I found was that while the minimums are something to pay attention to, you have to be realistic at how you would use one of these uber ranges. Sure, it can put out 64,000 BTU, but would anyone ever do that? Most cooking will be closer to the 1000 BTU than the 16,000 BTU range. Even when cooking hot and fast, these power burners will outrun your cooking. I have kept mine high for sustained periods for boiling water, deep frying and using my wok. Other than that, the burners tend to be on 3 or lower, not 11. I installed a 370cfm and I only kick it to high if I have done something wrong. I am glad that I no longer have a whimpy hood as I can keep the rest of the house from smelling like dinner.

If you still want to go huge, you have to (especially with gas heat and hot water) provide makeup air. This is a safety concern as you can suck combustion byproducts into your house when you crank up your hood. This will add to the cost of the installation of the hood. You also have to look at the loss of heated/cooled air at this point from an energy efficiency standpoint too. A heat exchanger seems like the solution here, but you can't really use the outgoing air from the hood to heat the incoming air as the smoke particles, moisture and grease will foul the exchanger in the long run.

So, if you have a 60 inch range and use it to its fullest, then by all means 1000 cfms are needed. A 30? No, just get a good quality range hood in the 300cfm range.

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