How many CFM for a range hood do I REALLY need?

Question

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.

Answer 1

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.

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

Answer 2

The three most important things in venting a kitchen is Ductwork, Ductwork, and Ductwork.

I'm beginning to find out the hard way after replacing several builders' quality range hoods with stainless steel units that are more "sealed" and easier to clean. But in the process of setting them up, I'm seeing that ALL under-cabinet range hoods from virtually ALL brands are made by ONE manufacturer from designs that are literally decades old.

These range hoods have an exhaust opening that is either 3.25" X 10" or 7" round, or a cross section of 32.5 Sq Inch and 38.4 Sq Inch respectively. The rectangular opening is used most of the time, and that is usually transitioned down to a 4" round Dryer-vent sized duct as there is no real code on this and 4" seem to be a minumum. Now a cross section of a 4" round vent is 12.56 Sq In, which is a loss of about 20 sq inch in the cross section, or 62%. From a 7" round vent, it's a 68% loss!!!

So no matter how high the CFM your new hood is capable of, if your home is typical, there is only so much air you can push through a much-reduced opening before you hit resistance (not counting kinks and bends in the ductwork), and the range hoods fans provide for that by allowing for the excess exhaust it can't push out to be "sequestered" back in the hood. As this sequestered smoke is coming through the filter, in theory the grease would condensate on to it as the exhaust had cooled down.

Looking at something else entirely, a fan on the back of your computer is capable of exhasting air at rates from 80 to 100 CFM, some for gamer machines at 200 CFM. Wha? And how much volume are we talking about? Say a kitchen is 10' x 10' x 8', so at 100 CFM, it would take 8 minutes to completely exhaust the air out of it. If you're cooking a steak on the stove top, you can create smoke at a much faster rate, maybe 3 Cubic Feet per SECOND or 180 CFM, and if the the range hood can't keep up with the pace due to it being underpowered or the ductwork greatly reducing its effectiveness, the smoke would spread quickly to 6 cubic feet per second or 360 CFM.

The math does not work out.

In a commercial restaurant environment with open cooking inside, when the exhaust is turned on, you will notice a significant drop in air pressure. This is not good for heating / cooling bills, but it's better than dying from the smoke.

A window fan is probably more effective in most kitchens.

Answer 3

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.

Answer 4

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.

Answer 5

If you are in Michigan, and you use a hood with the CFM over 400, you are required to use make up air at the rate that the hood is capable of. If the hood is capable of 1200 CFM but you say you are going to use low speed which is over 400, you have to provide the full 1200 CFM make up air. The average inexpensive hood is 250 to 400. Again the requirement is for OVER 400 CFM so you wouldn't need make up air on that. Only if it is over 400. That can be done in several ways. Check with your local Mechanical Inspector for their expert advice.

Answer 6

You mention that your home has an HRV. You should discuss the draw of your range hood with the mechanical engineer who sized the HRV for the technicians who installed it. A high-CFM range hood may overwhelm your HRV unit.

Answer 7

I don't know where you are located, geographically, but local building codes will determine a lot of your limitations. If you are just switching out a range hood you won't need a permit, but if it's part of a larger kitchen renovation then it would be covered under a building permit.

Lots of what has been said above is good information, but what will affect your ventilation, for your concerns, is how much dirty air gets into the hood. The thing that will affect that the most is how high above your cook top the hood is located; the higher the hood, the more cfm's you'll need. you don't want it so high that it doesn't collect any of the dirty air, and you don't want it so low that it impede's your ability to use the space. Again, local building codes will specify what that minimum height is allowed to be, where I am it is approximately 18" for non-combustible materials.

High volume range hoods might sound like a good idea, but something that most people will not consider is what that high exhaust rate is doing to the rest of your house. ACH (air changes per hour) is a measure of how often the volume of air in your house is exhausted to the outside and replaced with fresh air. Again local building codes will tell you what is allowed, but you need to have a minimum ACH, to provide fresh air to the house, and there is also a maximum.

You probably want to be right around 0.5 ACH (meaning half the volume of air in your house is exchanged every hour). A typical 1200 sq.ft. house can only have about a 200 cfm range hood before exceeding 0.5 ACH, after that your house starts to become too negatively pressurized. Negative pressure means that outside air will start to leak into the house without being conditioned, ie. cold air leaking in during winter. In short negative pressure is bad, so imagine what a 1200 cfm exhaust fan would do to your home. You would have to put in a make up air unit bringing in 1000 cfm to balance out the range hood.

Depending on your climate that make up air would also need to be tempered in winter, which means a big 220V heater, sucking power.

Again, I don't know where you live, and much of this is only required when a building permit is involved, but it is still best practice.

Answer 8

I am putting in a new hood on a 36 inch 6 burner range from Wolf. In the manufactures instructions it recommends a 900cfm hood. We probably won't be using all 6 burners at the same time either. We live in Montana and I don't want to exhaust a bunch of heated air in the winter either. However you have several solutions to this problem I guess. Go for the 900cfm or approximately 900 and run it on the lower settings. As said by someone else that probably would be quieter and discharge less of your conditioned air. Or go smaller say a 650cfm and probably be able to satisfy your needs. But here is the bad part, if you cannot keep up with the fried smell with the 650 it would be a bummer. I say go big or go home! If I have to run the thing on 900 to clean the air and loose some heated air from the house, so be it. Follow the hood manufactures duct sizing exactly not a HVAC guys. The guy who ruffed in our new house put in a 6" duct when we need at least a 10 inch. Gotta change that out now.