You generally don’t want to go higher than 30”. Every bit you go higher will make it less effective. Most hoods recommend 24”-30” as the range, whereas most cooking appliances specify 30”.

We were thinking 36 inches as we have taller cooks in the kitchen at times and we don't want anyone bumping their head. All it says in the Dacor manual is the minimum distance from cooktop to hood is 30 inches.

The other question I have is how to position it over the cooktop. Can we push it back a bit so it lines up with the front of the burners vs the front edge of the induction cooktop? Again, I am concerned about headroom. We only have 8 foot ceilings and the chimney and hood are large (given it is an island hood), so we would only be able to go so high without needing to make modifications to the chimney part of the hood.

The island is 51 inches by 10 feet or so. We think the big hood will look better centered over the island, versus the dotted lines that represent the initial thinking on hood location. We are having to change the location of the hood which is going to mean cutting in to drywall unfortunately.

Here is a diagram. In additoin to the fan, everything but the door/drawer fronts has been installed. We can't wait to do cooking show videos with this kitchen.

Shannon_WI Is the recommendation of 30-36 distance between fan and cooktop applicable for island fan as well?

The reason the hood specifies 24”-30” is its specs are concerned with capture and venting, not fire safety. The reasons a cooking appliance says 30” is for fire safety, not ventilation. But that only splits to combustible surfaces. Noncombustible hoods can follow the best venting practices and be lower.

I suggest you have your tallest person stand at the edge of a counter, and bend over. You’ll notice the head moves in an arc, downward. The head is never higher than it is vertically. A hood that is lower over a surface and within your field of vision is safer than one up and out of your field of vision. I actually cooked with a good at 24” for several years. Never bumped my head, because I could see it. That’s still 6” higher off he cooking surface than an OTR.

Either way, people adapt and learn. This guy was 6’ 6“ and actually stood inside the hood at some points, but had to duck to get his head around the perimeter. He never bumped his head. He had developed muscle memory.

Im not advocating for 24“. Just be very careful about placement, and base that in science and experiments.

First, the hood capture aperture should be centered over the locus of the boundaries of likely pan sizes on the hobs. Symmetry considerations in meeting aesthetic taste, however, may demand some adjustment of this.

Second, higher mounting means that the hood should have a larger aperture because the cooking plumes expand as they rise. A particular hood may be large enough for 36 inches, or it might not. Generally, if the capture aperture overlaps a 10-degree from vertical boundary expansion from the pan boundaries invoked above, capture will be reasonably complete. In any case, decent sight lines into pans is important, and for island installations, sight lines across the island may be desirable. One then decides from these factors height and aperture size.

Containment success depends on the aperture entry air velocity vs. the plume velocity in most residential hoods. Air velocity times aperture area yields required volumetric flow rate. Induction cooking plumes are modestly lower in velocity than gas cooking plumes due to the lack of hot gas combustion products, so the flow rate per unit area can be modestly lower than for gas cooking.

We also need to figure out how far forward is optimal. We have two i-joists runs we can use. The perfect positioning is, unfortunately inbetween the i-joists. We initially ran the galvinized pipe in the closer i-joists to hang over the counter edge (not good, but we did not have the fan at the time on sight and mis-judged just how large it is). Now we are looking at taking down some drywall, drilling a sizeable hole in the i-joist web (location and hole size within guidelines) to use a 90 degree elbow around 2-3 feet before the end of the duct run. This will place the hood further back from the edge of the cooktop (the hood will nearly cover the boundaries of the burners, but not quite.) I know you are supposed to keep elbows and turns to a minimum. What will this extra 90 do to the functioning/noise/efficiency of the hood? The only other option is lots more finished ceiling removal so the run can go fully in the further back i-joist to limit the need for an extra 90 elbow. What to do???

JuneKnow: Thank you for your explanation, picture and advice. It does make sense if we can easily see the fan, there is less chance of someone hitting their head.

Kaseki: Thank you for your expert information. We will run some tests and measurements. I will use the 10-degree guideline to test the capture area. Maybe the issue we are having with having to choose between a hood too far forward or too far back might be more easily decided upon with those measurements.

We have an electric 30" range and a 30" hood above but we wrapped it so it's closer to 38" wide. It's not finished (DIYers here) but here's a picture for you. We have 8' ceilings. The hood is mounted 30" above the range and sticks out 21" from the wall. Never come close to hitting our heads on it.

Some factoids about my hood system. I use the largest Wolf Pro Island hood over a peninsula. With the short duct cover and my ceiling height at that position, the gap between hood and counter is 34.5 inches. Front-to-back, the hood is centered over the hobs. Peninsula positioning and attic reframing was planned to allow the duct to pass a major microlam beam used to replace a previous ceiling joist supporting wall. Alternately, the cooktop hobs are centered under the hood, given potential adjustment to the peninsula and the cutouts in the counter stone during the design process.

I use a roof-mounted Wolf/Broan 1500 CFM rated blower that actually achieves around 1000 CFM with passive (at present) heated MUA. Hood system ducting from hood up comprises 10-inch duct, a roughly 60 degree bend upon reaching the attic interior, followed by a Fantech silencer, and another roughly 60 degree bend to the roof slope with duct lengths I've forgotten.

In addition to the silencer, my ducts are mostly wrapped with dampening material.

This system is sufficient to cover a 36-inch induction cooktop and a nearby Cooktek induction wok hob mounted in separate but joined cabinets.

The effect of a 90-degree bend on pressure loss varies with flow rate and duct size; use an on-line calculator to find pressure loss. Use pressure loss to determine air flow loss by consulting your blower's fan curve. Repeat recursive analysis. (Excel solve could likely find the answer if all the functions were known. Unfortunately, we may have to guess about baffle and hood pressure loss, and further, as the hood flow rate changes, the house pressure changes, and the MUA flow changes.) The easiest approach is to over design.

Thank you itsourcasa for that image and info. It looks great! So if the hood sticks out 21 inches from the wall, I gather the cooktop sticks out a bit further than the hood. That seems to be a good way to ensure no head bumping. Please correct me if I am wrong and if the hood is fully over the cooktop?

You should not be so concerned with head bumping. After a couple of times at the cooktop with the hood, the brain remembers and knows where the hood is. There are millions of tall people in the world with hoods over their cooktop and are not bumping their heads. Even people not that tall, say 5’6” or 5’7”, have the hood right at head-bumping height and are fine.

ontariomom: One can always frame out a path for a duct. The biggest issue is getting the necessary beams carrying the joist loads into the attic without removing a lot of ceiling. Then again, you might do as my Dear Wife did: Upon seeing our uncovered kitchen ceiling joists, she decided to add a construction week by raising the ceiling by about 18 inches in the center of the kitchen (not where the hood would be). Aesthetically, this turned out to be a positive change.

Kaseki. Thank you for sharing the details of your design. Currently we have used all 8 inch galvinized pipe with only 2 45 elbows to get it to the outside of the home. We did not upgrade to include a silencer, although we looked into it as per some of your past posts.

We are going to have to make some sacrifices unfortunatetly. We can't change the design of the i-joists, the positioning of the island, the fan etc at this point. So, we have to decide if it is acceptable (albeit not optimal) to push the fan back so it hovers over most of the hobs (more centered on the island depth (51 inches). The other choices is for it to hang over the counter top and that seems less good (in this scenario we would have to hang if very high so it was not a nuisance). We are trying to reach the engineering department for the i-joists on the remote chance that there is any way to cut into the i-joist and then support them another way (I don't see them having a good solution that won't compromise the integrety of our home or be to code).

Thanks Shannon for the reassurance. I think our bigger concern now is whether to have the hood hung too far over the counter edge or pushed back and not completely covering the hobs. The location of the i-joists is preventing the most ideal placement of the hood.

@ontariomom yes the range sticks out 29" to the front handle and hood is 21". We researched a lot before installing the hood since we were DIYing it. It's perfect.

Perfect. I think that an island hood has less wiggle room than a wall mount one (as the wall helps contain the steam etc from cooking. Still, I hope we can set it back some like you did.

Just found out we can cut into the i-joists if we header the section above via cross members. To be sure we do it right, we have to contact our structural engineer and get him to do the design. So now we have two choices as the choice of having the fan hang over the counter top is off the table.

1) Cut into the i-joists and use cross members to header that section (essentially frame off an opening in the i-joists sort of like it is designed for a staircase or an elevator). This requires:

a) Getting structural drawings from the engineer

b) Cutting a good sized square out of the drywall and then cutting down one i-joist in the location needed for the hood).

c) Reapairing the i-joist break with cross members to head off the opening.

d) using a 30 or 45 degree elbow to re-route the ducting to a better location that we choose to position the fan (e.g. fully over the cooktop or slightly pushed back).

OR

2) Cut a larger hole in the i-joist webbing (not the rim) but not larger than allowable, use a 90 degree elbow to re-route the galvinized pipe about 4 feet from the fan location to be in the joist behind the one it is in currently. This options if the far cheaper one.

a) Cutting a good sized square out of the drywall and drilling a 8.25 inch hole in the i-joist web.

b) Need to use another 90 degree elbow which will make the fan noisier and less efffective

c) Deal with having the fan hover over the back 2/3s of the cooktop and some of the island. So, in otherwords, we won't get the exact location and might miss capturing some of the steam, etc. especially when using front burners.

What would you do? Option 1 or 2?

I wouldn't comment on the joist mods without seeing all the details, but I would tend to sister joists and add headers rather than adding a large hole to one or more joists, even if they are the engineered type. But my house has no joists of that type, mine are either solid wood, microlam beams, or trusses, so I don't have any experience with the plywood "I" beams.

If you limit greasy cooking to the burners/hobs best overlapped by the hood -- wherever it ends up being located -- some escape of steam from cooking on the other row of burners/hobs will be of little consequence. Just don't let the hood be so misaligned that you get snickers from guests.

@kaseki I really appreciate your expertise. Our unit is a 600 cfm single blower unit. The width of the hood is 36 inches and the depth is around 33 inches deep. We are okay building code wise to mount it higher than 30 inches, so we are planning to mount it 35-36 inches off cooktop. The cooktop is an electric/induction/4 hob unit.

What is the main disadvantage to adding an extra 90 degree elbow around 4 or so feet before end of the fan ducting to move the fan termination back as we can't deal with it where it is? Will the extra elbow lead to more noise and less functioning for the fan? If more noise, what material did you use to insulate the pipe. If it is very noisy we can add a silencer later to the attic. If we can deal with having the fan pushed back from the front edge of the cooktop, the extra noise and loss of fan efficiency, I think this is by far the simplest fix for us (and that fix also means finished drywall removal. However, it does not require cutting a hole and re-framing the i-joist with cross bracing.

Dominant noise is fan blade tip turbulence [exception: oversized blowers when run at partial power]. Duct noise past the fan will probably be hard to discern. Bends in ducts are equivalent to adding a lot of duct length. With a 90-degree bend, the momentum vector of the air flow is somewhat inefficiently changed 90 degrees (duct is not frictionless) so there is more turbulence than with straight duct of the same length. This adds noise and pressure loss.

Your hood aperture is likely somewhere around 4.5 sq. ft. in area. For induction, not less than 60 ft/min is needed for containment of whatever is captured by the hood aperture. For searing and wok cooking, this value could rise to 90 ft/min. These values require that the actual flow rate be at least 270 CFM, with 405 CFM a desirable goal. Baffle pressure loss, inadequate MUA pressure loss, hood transition pressure loss, and duct pressure loss are the factors dragging down a blower's rated CFM to its actual CFM. Lacking analysis of everything relevant, a factor of 1.5 might be typical of the effects of pressure loss. This factor leads to a required blower rating of 400 to 600 CFM. Since you are there, I would suggest you can get away with adding a 90-degree bend in the path. 8-inch duct at these flow rates should be relatively low pressure loss.

400 CFM in 8-inch duct is about 1150 ft/min which is on the desirable low side of the 1000 - 2000 ft/min rule for minimizing grease particle impingement and/or condensation. In cold climates, warming the duct before ventilating grease aerosols is good practice.

Thank you ever so much, @kaseki! Two more questions:

What did you mean by "In cold climates, warming the duct before ventilating grease aerosols is good practice". We are in cold climate (Canada).

What material did you use to wrap your ducting for noise reduction (I believe you said you wrapped a portion of your duct pipe?)

I meant running the hood for a minute or so before generating a lot of grease. The "or so" applies if one's duct is exposed to a particularly drafty attic. Briefly running a hood before use can also get the air flow moving in the house and stabilize any active MUA system in use.

I used "Q-Pads." These are sold by automotive parts stores supplying the body shop trade. You preferably want the version that is intended to stick on vertical surfaces and not just on car floors. As backup, I used long Ty-Rap® type retainers wrapped around the pad and duct. Very long ones are usually available from electrical distributors and some hardware stores.

The purpose of these pads is to reduce vibration from an external blower propagated back down the duct skin itself, as distinguished from noise propagated through the duct air, best suppressed using a Fantech silencer.

Regarding head-bumping...We're a tall family (5'10" to 6'7") and have no issues bumping our heads on the hood -- it's installed 30" above our cooktop). Even my 6'7" son doesn't bump his head. You "learn" or have a "feeling" for what's in front of you (survival mechanism!). Even if you do bump, it will only happen a couple of times b/f you no longer to.

If you're really concerned, then I suggest you get one that doesn't have sharp edges. We got a Vent-A-Hood with an "emerald lip" that softens the sharp edges. (https://www.ventahood.com/index.php/products/premier-magic-lung/ml-wall-mounted?id=360&model=SEPH18)

Fuel-type isn't as important a factor in a hood as cooking is. If you do any high-heat, smoky (e.g., browning meat, sauteing, etc.), then I would go with stronger rather than weaker. If you just boil water, then probably 600 cfms or so are fine.

Thank you Buehl for sharing your experience with a family of tall kitchen users. I am happy to hear you have not had any head bumping incidents. Is your fan pushed back a bit from the front face of your cooktop/range? We are planning to push ours back a few inches, but still have it over the hobs. We actually need to push it back anyway due to i-joist restrictions on duct locations. We already have our hood (it is 600 cfms and is a large iDacor sland hood). It can't go back and has been custom painted a silvery/light blue colour. It does not have sharp edges. It will be infinitely better than the builder special fan we had before the addition/renovation project.