Help! Intended height from the cooktop to the hood?

3 years ago

Hi everyone,

We are redoing our kitchen and worked with a kitchen designer to design the new kitchen. It was completely gutted and we're starting from scratch. The cabinets are currently being installed by our contractor. We're putting in a 400 CFM hood, 30" wide, above the range (this model https://www.amazon.com/gp/product/B07236MH49/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1), instead of an over the range venting microwave.

Our contractor told us that the above the hood cabinet that was ordered is meant to be above a microwave, since if a hood is installed, it would be at 36 inches above the cooktop which is too high to be functional in his opinion. We called our kitchen designer who told us that the correct cabinet was ordered and the design of hood heights have been increasing over time and we wouldn't want a hood that is too low for practical purposes. We conveyed this back to our contractor who said that he's happy to install the hood at the height in the design but we should also check the specs of the hood to see what the designed height over a cooktop should be. The specs of the hood say the maximum height above the cooktop should be 24 inches.

What should we do? Should we install the hood we have at 36 inches? Should we return this hood and look for a hood with a higher maximum height above the cooktop (we haven't looked for alternative hoods yet so don't know what others hoods maximum heights are, but we are looking to stay at 400 CFM max so we dont need to install a makeup air system). Or should we pay several hundred dollars more and order a new bigger cabinet above the hood?

Here's a picture where you can see the above the hood cabinet installed:

Thank you for your input!

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Comments (7)

tozmo1
3 years ago
Likely this is permitted work so you'll have to pass an inspection. In my community, inspectors are very picky about vent hood heights. They pulled out my vent hood spec manual and measured when I did my reno. I would follow the vent hood recommendations. They are there for your safety. Your contractor sounds more knowledgeable than your kitchen designer. I'm surprised he/she did not know that you have to follow the specs of appliances.

Install the vent hood at the correct height. Perhaps the contractor can build some kind of panel to cover the venting (I assume that's what that hole is for in the wall) and somehow make it look like it is supposed to be that way with trim or a valance effect.

The vent hood at the link you provided is 250 CFM not 400. Did you get the wrong hood? Getting a 400 CFM (or what ever is determined by your stove) might solve the problem.
User
3 years ago
last modified: 3 years ago
Competing by specs always are decided by the most restrictive. In this case, a range requires a greater clearance from the cooking surface than the hood allows. The hood is an inadequate choice for the range. You need something with higher CFM, larger duct, and way more than 250CFM. And Make Up Air. Those types of hoods are mounted 30-36” above the cooking surface. Which is what was designed to be done.

The design is fine. It’s the hood that is a bad selection. It’s completely inadequate. Or re-select the range for an old coil top electric which is all that should be matched to a low ability hood like that.
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I'd go farther, although magic lungs may do magic flow patterns. The flow velocity from a hood aperture, assuming relatively uniform flow into it, is plotted in the ASHRAE HVAC Handbook (page 30.4) for a hood of dimension W by 3W. The flow velocity drops very fast as one moves down below the hood. At the center W inches under the hood, it is only 20% of that at the aperture. Most hoods, even at 30-inches above the countertop, are smaller than 30 inches deep, so the flow rate at the pans is more likely to be in the 10 to 15% range. So, if you move the hood up a bit from, say 1.25W to 1.6W, the velocity will drop from 15 to 10%, and you have lost 33% of the velocity. Fortunately, having high hood air velocity at the pan surface is NOT how we capture the cooking effluent. The hood air velocity could be zero there. The effluent rises on its own up to the hood at velocities a residential hood would not generate, and the hood will capture it if the hood overlaps the expanding cone of effluent. If the hood is undersized, then it will capture a lot of it, with the amount lost dependent on undersizing and, somewhat, to the air velocity at the hood edges relative to that of the effluent (which is typically around 3 ft/s). To repeat, effluent is not sucked up from the cooktop because it is hot and rises up on its own. If one needed that level of performance, e.g., for immediately capturing the odor of fresh cold cilantro, the actual air flow would have to be of the order of 6000 cfm for a six square foot aperture hood two feet above the surface. Fortunately I like cilantro, so the actual 1000 cfm I likely can achieve doesn't need to be boosted to an industrial level. Using cardboard cutouts or drawing an elevation view of the cooktop and cook, determine the height that avoids injury and allows good sight-lines into the rear cooking zone. Solve the capture problem with a large enough hood. If necessary, space it out and put a gap filler at the back. Let the more modest residential flow rate solve what it is supposed to solve -- containment -- what has to happen to keep the effluent from bouncing back out of the hood. Whatever effluent misses the hood will eventually be removed as the air is replaced at the hood flow rate, although some grease and water condensation onto surfaces can occur before it is removed. kas
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I suspect that the 30-inch minimum is driven by a safety requirement imposed by some Government agency or NFPA. The design of the hood may have been done outside Kitchenaid and the detailed specifications that drove the design may not have traveled with the hood, so to speak. The fact that this kind of information is usually either missing or generic or vague was what motivated me to research the subject so I could determine what would probably work for my kitchen. What I found was what I asserted above -- the performance in terms of effectiveness (itself not defined) is subject to so many variables that it is not possible to take a strong position on how high is too high. (This is not to say that manufacturer's shouldn't provide nominal guidance. Some do.) That is why I think the potential purchaser has to do a little sketching to evaluate how much of what burner/pan combination will be captured for the geometries relevant to his potential configuration and cooking intent. In other words, it is easy to specify a height range such as 30 to 36 inches, but there is no guarantee that the hood will be effective at 30 or ineffective at 36.1 without the entire configuration taken into account, along with the purchaser's view of how much "effectiveness" is sufficient. kas
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Mine is recessed into the ceiling and is basically just a hole that goes into a chimney like exhaust in the roof. It's about 66" from the top of the stove. I had a similar one in Culver City in a second floor apartment, and doves would sit on the chimney and coo. It sounded like they were inside my kitchen, and it took me the longest time to figure out where they actually were. I haven't felt the need for a hood, but if I burn something (which is rare), I have to get a floor standing fan and have it blow the smoke out the window or out the door in the adjoining laundry room. When I lived in Austin, I had a hole in the kitchen wall behind the stove, but at least it had a fan in it. What I have now has no fan. I'm not sure what it would be like to have a hood, since I can't remember having one. I have a picture of my kitchen in Austin somewhere, but I don't know exactly where. I'll try to look for it. Lars
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There are two operational requirements for ventilation hoods from which the detailed requirements derive. o Removal of combustion products from cooktops (usually from gas combustion, as wood and coal fired cooktops are rare on this forum) o Removal of grease vapor and odors The first requirement, if applicable, is mandatory. The second requirement is somewhat optional as determined by the homeowner's preferences, but most don't want to depend on a window fan to remove grease vapor. Lack of immediate capture and removal will cause the kitchen to slowly gain a coating of grease. In order to meet either requirement, the hood must accomplish two functions: capture and containment. Capture relates to having the uprising plume of effluent flow into the hood without missing it; containment relates to keeping it there and exhausting it without any leaking back out. Effective capture depends on the hood being large enough to extend beyond the rising and expanding plume unless the airflow is very high; hence, higher mounted hoods need to also be wider and longer hoods. Containment depends on internal depth of the hood, air flow rate, and other details of the hood design. As a rough approximation, assume that rising effluent expands from a pan boundary at a 7-degree half angle. This means that if the hood extends from the back wall significantly less than the cooktop does, one may wish to fry only on the rear burners and use the front burners for boiling water. Otherwise, odor and grease may excape the hood. In that case odor will be cleared from the house slowly as the fan exhaust replaces the household air volume a few times. This may require hours to accomplish. My choice is having the hood above my head, both to minimize bumps and improve sight lines. As a consequence I have to make it larger with higher flow rate. There is a potential hidden cost, though, of high flow rates. Depending on how much air one needs, undesirability of opening windows at any time of the year, and local political demands, one may have to install a makeup air system. All the above assumes that significant cooking activity is intended. If the kitchen is mainly for show, then the hood becomes less important. I have seen photographs of elegant houses with island mounted gas cooktops for which no vent hood was apparent. Catered delivery would seem to be the cooking method of choice in such cases. kas
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tozmo1
3 years ago
Here's a very good hood that is 400 cfm that can be installed up to 32". https://www.amazon.com/Zephyr-AK1200B-Cabinet-Breeze-Stainless/dp/B0746T4DLZ/ref=sr_1_8?crid=3I5USF030PTCJ&dchild=1&keywords=zephyr+under+cabinet+range+hood+30+inch&qid=1605018810&s=appliances&sprefix=zephyr%2Cappliances%2C171&sr=1-8

It's pricier than the one you have but it could work with a four inch filler between cabinet and hood.

What does your stove require?

@User I just have to ask, why comment if your aren't offering advice or genuine thought provoking questions? He has a dilemma and is asking for help. Is that what you offered?
Isaac Katz
Original Author
3 years ago
How about this hood: https://www.amazon.com/Broan-BCDJ130SS-Glacier-Stainless-30-Inch/dp/B071FC62SQ

It's 400 cfm but in the specs it says it's 30 in max from the cooktop. Can we put this one at 36 in above the cooktop and still be mostly effective?
Isaac Katz
Original Author
3 years ago
@tozmo1, is the hood you recommended better than this one?: https://www.amazon.com/Broan-BCDJ130SS-Glacier-Stainless-30-Inch/dp/B071FC62SQ
tozmo1
3 years ago
Hi Isaac, That looks like a good hood. I'm sure you'll find it works as well as the one recommended. The difference between the two is that the Zephyr can be mounted two inches higher than the Broan.

If you mount the Broan 36 inches above the cooktop, you'll get some effectiveness but will you pass an inspection? If a there is a building permit for this work, then an inspector will be coming to check it. The inspector will pull the manuals for the appliances and then measure to see if things have been installed to specs. If this work has not been permitted, then your only concern is how effectively it will work with how you cook. I don't do much frying so I rarely use my hood so in my case it doesn't matter. The only other thing you may want to check on is insurance. If you ever have a fire, how much checking will be done to see if the work has been installed according to specs? I don't know about that at all.

I totally understand how frustrating this is especially at the end of what looks like a really nice renovation. Is the concern that you'll have to pay now for a larger cabinet if you install the hood at the specified height? if so, I'd lean on the kitchen designer to eat the cost of the cabinet you can't use. That person definitely should have been very attuned to lining up specs of appliances with cabinets. You pay for advice from pros for a reason. They should know what they are doing or defray a cost when they give bad advice.

The only thing I think you shouldn't do is to install the hood that specs at 24", at 36". That won't be effective at all.

Hang in there, these obstacles are bound to happen in a renovation. Once it's all done, you'll love it. I'm all about cutting corners as long as those corners don't have an impact on things down the road.

Help! Intended height from the cooktop to the hood?

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tozmo1

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Help! Intended height from the cooktop to the hood?

Comments (7)

tozmo1

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tozmo1

Isaac KatzOriginal Author

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