How much vent hood CFM do I need?
jmaskovs
11 years ago
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SparklingWater
11 years agoRelated Discussions
So how man CFM's do I really need?
Comments (10)ditto weissman. -- I'll modify this statement: "... I can tell you anything under 400cfm is a waste no matter what you do otherwise..." Here goes. There are millions of cheap propellor blade fans installed, that function, in a sort of way, removing most of the smokey greasy air that some kinds of cooking generates. They do work. They are rated at various CFM numbers all less than 400 cfm. In reality, these blower motors are very weak if the air in the duct meets up against resistance. So, a short duct through the wall = they work OK, while a long duct up through the attic to the roof = they work erratically or poorly, and sometimes well. Some people have never seen these "hoods". Cheap rental apartments have them. You can see them in cheap hardware stores. The rated CFM is just a lie printed publicly which nobody can refute. It was the cfm produced in a resistance-less tube in a laboratory. In reality, something innocuous can make the fan perform poorly. Many things. Even something as dumb as "wind outside" i.e. your regular everyday weather. --- The next thing up the scale of goodness is a squirrel cage blower. You can see them in cheap hardware stores too. Some are weak, some are strong. Some can push air very well no matter how much resistance the duct length causes. They are all "rated" for a CFM in the hundreds. 300 CFM is fine if you truly get 300 CFM and if your cooking style compensates when the hood needs a 1/2 minute to catch up. Blowers inside "OTR" microwaves have to push air through tight spaces that bend and turn before the air even gets out of the microwave and into the house duct. I wonder if the blower motor inside an OTR microwave was "rated" only prior to it being installed inside the microwave. I suspect this is the case. Then, the OTR microwave can be sold as if it had a Large CFM while it might never ever reach that CFM under ANY circumstances. Let's get the Consumer Reports police to look into this. Although it is true that air can move around anything, it is also completely false when you want SPEED. Obstacles and resistance put a maximum cap on speed (of air flow). -- The chart they publish with the fan blower motor shows you how much degradation in performance was observed during the lab test, when a form of air resistance was created. But this too can lie, in a way, even while it remains true and accurate. -- Your duct diameter or duct dimensions are big factor. When you post these dimensions you will get more specific advice. Hth. Here is a link that might be useful: help make a hood decision (kitchen forum)...See MoreDo I really need a 600 cfm range hood for an induction cooktop?
Comments (37)leel.....glad to get your feedback on the induction. Thanks! As for me, I certainly wouldn't mind having to buy a new pan or 2 for the new oven. :-) gsciencechick....thanks for the pots and pans suggestions and the confirmation about induction. I'm ready to be done with gas (although I love the way it cooks) because it's just so dirty to keep clean. muskokascp.....I HEAR you!!! Whatever happened to the days when we just went to the store and asked the opinion of the saleman or perhaps picked out a unit because of style or color? These days we almost have too much info, and it can become crippling, honestly. At some point I just need to make a decision. davidro1.....I love your 'final answer' comment. I WISH!!!! Although I certainly know a lot more than I knew before, I still have questions. But today at the appliance store helped clear up some things. (Meaning that they don't know anything either and I can basically do whatever I want). The inspector on our job (both HVAC and electrical) don't know anything about MUA and can't advise us what we should get. So I just told them I'd get something that wasn't too powerful, and they seemed fine with that. They told us to adhere to the recommendations in the manual that comes with the hood. I swear!!! It's not rocket science.....why is it that nobody knows about this technology?...See MoreHow much Hood CFM to get for Wolf's 6-Burner Rangetop?
Comments (10)The necessary information may be found by searching this forum. To summarize: Obtain a hood that overlaps the cooktop by enough to capture the rising and expanding effluent. Assume for this discussion a hood with a 42-inch wide by 27-inch deep aperture area. This is shy of 8 square feet. Desirable velocity at the baffles is 3 ft/s, leading to a need for 24 cu ft/s or around 1400 cu. ft./min. Experience shows that only half this is needed due to baffle behavior in directing the effluent flow close to the baffles. (Or to look at it another way, only half the baffle space is aperture.) This consideration yields a value of 700 cfm. To actually flow 700 cfm, the blower needs to do so at some pressure drop determined by the baffle loss, transition loss, duct loss, and MUA replacement air flow pressure loss. For likely conditions, a 1200 cfm rated blower would be desirable. At best, a VAH hood's rated actual flow (600 in Homechef59's message) only avoids part of the pressure loss. It is not equivalent to a 900 CFM blower with a baffled hood except when each is hanging in air without a house and ducting. While a 1200 cfm rated system is recommended in this example, stepping down to 900 rated cfm may be adequate depending on what plume velocity your cooking effluent actually achieves, and what area of aperture you end up with. Some effluent escape (capture but missed containment) or effluent missing the aperture (missed capture) can be expected at some level of flow and/or hood shrinkage. Note that this basis for CFM estimation does not depend on the number of burners that may be in use at one time, but on the pan temperature(s). Every plume rises and has to be captured and contained, and the other hood areas not directly impacted by a particular plume do not help capture and contain it. kas...See MoreBlue Star 36 inch range. Do I really need a 1200CFM hood?
Comments (6)There are many people here who can comment on the math. My HVAC person did that and installed MUA. Because of our cathedral ceiling and the hood being at the low point, the hood will have to have an internal blower. (Trust me, we tried.). The Wolf representative pointed out that you need enough CFMs to work if everything is at full power, but if you don’t need full power that much, the higher CFM fan allows you to run at a lower, quieter speed (variable speed blower) for a lot of everyday cooking for 2 people. They make a 900 CFM blower, which is what we went with for a 36” rangetop with a griddle (40”hoodliner in 43” cabinet)....See Moreribs1
11 years agoSparklingWater
11 years agoweissman
11 years agoweissman
11 years agoattofarad
11 years agokaseki
11 years ago
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