Good post - often people on this forum post things saying that since I'm getting induction not gas I don't need a really powerful hood. Several of us try to tell them that the issue is mainly what you cook, not which fuel you use for cooking it. Thanks for your post.

Chac mool,

Glad to hear you're happy with your new hood.

Great post! It's nice to have the voice of experience.

If you scroll down on the How Induction Works page on The Induction Site, they give a formula for determining the amount of pull you need on your hood.

A truism I learned here is also that a more powerful hood on a lower setting will work better than a less powerful hood on its highest setting, but Trailrunner or someone would have to explain the physics.

In my own experience, I'm finding that my way overpowered hood works great on its lowest setting for a lot of ordinary tasks. I don't have to crank it up unless I'm frying, making stock, or have four or five pots going all at once.

Good post - often people on this forum post things saying that since I'm getting induction not gas I don't need a really powerful hood. Several of us try to tell them that the issue is mainly what you cook, not which fuel you use for cooking it. Thanks for your post.

When "WE" say that we mean you don't need the 1200-1600 cfm that is highly desirable for models like the Bluestar RNB or Capital Culinarian.

Induction would be more in that 450-800 cfm range depending on cooking styles and circumstances. And it is really not what you cook but how you cook it.

Every induction fan tells how me efficient their cooktops are relative to gas. This is true to a point and that excess inefficient energy that turns into heat rising above the cooking surface. Plus gas combustion byproducts. Those are extra loads that need to ventilated.

Combustion byproducts must be exhausted from the house interior; grease and odor cooking products should be exhausted from the house interior. From those simple starting points, the detailed requirements can be difficult to evaluate: First, because there are a lot of somewhat conflicting "rules of thumb," and second because what is being cooked and how it is being cooked varies with the household.

The fundamentals are these:
o The effluent, whether from induction, electric coil, gas, or coal cooking expands as it rises, requiring the hood to overhang the cooking area in order to capture most of the effluent.
o The effluent has an upwelling velocity mainly due to warm air buoyancy that must be matched within the hood in order to contain it. Otherwise it "bounces" out by reflecting off of the hood interior parts.

Effluent expansion is in the range of 20 to 25 degrees half angle. The higher the hood is mounted above the cooking surface the larger the hood has to be to capture the expanding cone of gas and cooking products.

Upwelling velocity is around 3 ft (1m) per second. Large hoods require more cfm than smaller hoods in order to have enough air velocity over the entire aperture of the hood.

Refinements on these principles include details such as: Baffle filters will not capture grease if the velocity is too low for centrifugal effects to occur. Hood interior angles can aid containment for a given air flow rate. The fluid flow principles at play are not that different than those for light reflection; one cannot indefinitely condense the rising air without some reflecting back out of the hood.

If you wok cook on induction, whether with flat woks on a cooktop or with a specialized induction wok hob, there will be a lot of upwelling vaporized oil and steam at a minimum, and grease added at a maximum. Conditioning a wok will be an extreme case. The hood cfm required for this task is significantly higher than for making hard boiled eggs. Broiling on a gas grill will require yet higher flow rates.

So pick a large enough hood to capture. Choose a hood, blower, and duct system with enough flow for containment. If you intend extreme cooking activity, it would be wise to look up more information about what the effluent parameters are for various cooking surfaces.

As always, maximum performance comes at a price in money, and in aesthetics, and likely in reduced cabinet space, space for ducts, etc., so it may be cost effective for some to save the worst effluent cooking for an exterior grill, or for a restaurant.

kas

Good review. Thank you!

This is a big thing: ".... if you do opt for a weaker (blower/ fan/ motor/ in your) hood, be certain this hood provides a decent catchment area over your stove, ..."

Fluid flow :
: a batch of heated air rises fast; steam rises fast, and faster than heated air (because each H2O molecule is far lighter than O2, N2 and CO2 molecules)
: steamy air, grease-particle air, smoke or whatever, it all needs to be captured.
: once it occupies the volume of the catchment basin, it overflows. Same as a fishtank, pot, bathtub, swimming pool, etc, except everything is turned upside down.
: lighter air HAS to rise. Gravity. This isn't "defying" gravity.
: air is not compressible. (Unless in a closed system and force is applied)
: lighter air, carrying smoke and microscopic grease specks, will overflow the catchment volume and spread throughout the next bigger volume (kitchen or house). -- unless your blower is pulling enough air to match the new air rising.

Lessons learned:
1/ wok slow. Add new material bit by bit. Wokking slowly will avoid overflowing your catchment volume in your hood.
2/ a hood designed to look good without any catchment volume at all, is a hood designed primarily to look good: it needs to pull a huge flow of air to be able to catch a portion of the steamy greasy air that will slide past the flat surface and continue upwards to the kitchen ceiling and house.
3/ manufacturers and buyers don't think much about how they will bring new air inside the house. We must face the physical fact that every molecule sucked out of the house needs to be replaced. New air. Air is not compressible or expandable (unless pressurized).

To me, principles at play are not close to those for light reflection. Fluid flow reminds me of drain plumbing because draining water means moving (displacing) an equal volume of air as the water moves downhill, and then bringing an equal volume of air from the other side to replace the volume left empty as the water moves farther downhill. "Venting in drains" is the hardest thing to understand, for most homeowners and renovators. DWV.

After a few more years of screaming about the need for a systematic approach to providing new air to replace the air lost out the exhaust duct, we may see more solutions being packaged and promoted. How about a tube column of forced fresh air rising from a groove encircling the cooktop? It makes a curtain going from the countertop groove to the exhaust catchment basin. This, in an island cooktop, would contain the air rising from the wok, if the groove circled the entire cooktop, 360 degrees all around it. An air curtain which is going straight to the exhaust does not have to be conditioned air. Its humidity and temperature can be neglected, since this air is being sent to the catchment and exhausted forthwith.

hth

davidro1:

My attempt to relate air capture to light reflection was intended to show similarity between the inability to concentrate light at the tip of a pyramid by illuminating from the base and the inability of a pyramidal hood to concentrate effluent to a too-small aperture at the top. The air will spill back out just as light will reflect out. You are welcome to reject the similarity on the grounds of different principles involved.

Conservation of momentum applies to the rising air and effluent, and Schlieren photography shows that the spillage from a hood with inadequate flow occurs after the uprising flow strikes a surface. Under some circumstances it is possible for it to lose its buoyancy and settle back down, but that is an unlikely primary dynamic for the hood capture and containment case.

Gasses are always compressible, although I don't think arguing this point has anything to do with the validity of your observations. I will note that both the density variations in the rising effluent and the density variations natural to the atmosphere (atmospheric turbulence causing atmospheric scintillation) are due to small temperature and pressure variations.

I agree with your lessons learned, although Lesson 1, while it solves the inadequate hood issue, tends to defeat the purpose of wok cooking, where quick searing of the contents is the goal.

Commercial systems have tried various schemes for adding make-up air around the cooking unit. Some are more successful than others, as measured by Schlieren photography. It is critical for success that the MUA not disturb the rising effluent in a way that causes it to expand or become more turbulent and miss being captured. Hood measurement labs typically provide their MUA via perforated diffusers in a wall some distance from the cooking unit/hood setup being measured.

The best I could do given the limitations of my house was to introduce the MUA via a diffuser at the end of a hall about 30 feet from the hood. The hall should straighten the flow somewhat and cut the velocity as the air flows toward the hood. This system is not completed yet, so I cannot report any observed deficiencies.

kas