I wrote the following for our interior designer who did not understand why we want a large deep hood. I thought others might find it useful. This is based on a number of conversations with people at CaptiveAire, Greenheck, and Braun as well as posts here from @Kaseki and others.

Please add comments and I will update this as I have time.

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This morning I returned home from breakfast and could still smell the less appealing bits of last night’s dinner.

We currently have a Vent-A-Hood with a proper make-up-air system that has overall proven quite inadequate and as I’ve recently learned is actually not a good hood option despite what sales folks proclaim. They are poor at capturing and exhausting effluent, are not quieter than other hoods, noisier than some, and are a major PITA to clean. The one thing I can say for VAH is that it does seem to do a good job of removing grease from the air stream before exhausting it - if only it collected and exhausted more of the effluent.

Standard residential options from Wolf, Best (Braun), ModernAire, Zephyr and others are only marginally better than VAH. This is perhaps due primarily to a common flaw of all of them in having near zero containment area along with a light bar that reduces the open aperture.

The Problem - Effluent

Hot effluent from cooking (odors, grease, steam, heat and by-products of gas combustion) rise from the cooking area. As it rises it spreads out and has quite varying velocity with erratic bursts, sometimes in excess of 4,000 CFM.

For us our primary concern is our health and getting rid of the by-products of gas combustion. Second is reducing cooking odors as much as possible for ourselves and when we have guests. Third is reducing the heat in our kitchen when doing a lot of cooking and Fourth is reducing grease build-up on kitchen surfaces.

The critical bits to capturing and exhausting this effluent then are:

1 - Hood Aperture. The overall opening or aperture at the bottom of the hood must be large enough to capture the effluent as it rises and spreads out. Generally, for non-commercial kitchens, the hood should extend 3-12” beyond the cooking surface on each open side. So, for a 48” x 24” range we’d ideally want at least a 60” wide by 36” deep hood opening. This will still leave some effluent uncaptured but is much better than what is typically installed and beyond this we’re encountering very marginal gains.

CFM cannot make up for too small a hood opening. It takes about 7 times as much airflow (CFM) just to make a 27” deep hood perform like a 30” hood. So, rather than 600 CFM you’d need about 4200 CFM. That is not practical.

The consumer appliance industry focuses almost exclusively on CFM. CFM alone is actually a quite poor indicator of hood performance — but does make for good marketing.

It is also critical to understand that we are talking about the opening to the containment area, not the overall hood size. A 27” deep hood with a 4” light bar along the front edge will actually only have about a 23” opening.

2 - Containment Area. This is where most consumer hoods fail. Effluent from cooking does not rise evenly but in spits and bursts. These can overwhelm the exhaust system and when this happens the odors and grease roll out from under the hood and spread throughout the kitchen. Hoods need a large open volume of area to contain these bursts until the exhaust system can exhaust it.

For example, you have a 1200 CFM Modernaire hood and are pan frying a steak. As it cooks the effluent velocity is varying between perhaps 200 and 2,000 CFM. Any effluent over the approximate 800 CFM that the hood can immediately exhaust will not have anywhere to go and so will roll out from under the hood and spread throughout the room. When you turn the steak over and it hits the pan you get a quick 3,000 CFM burst of greasy smelly effluent that hugely overwhelms the hood’s exhaust capabilities. With the exhaust blower overwhelmed and no containment area the odors, grease, heat and steam spread throughout the kitchen (and house). And then largely remain there until all of the air in the house has been exchanged a few times.

This is why commercial hoods are always so big and empty.

3 - Exhaust (CFM). With a properly designed hood the exhaust system need only keep up with the average effluent produced as the volume of the hood itself contains the bursts until they can be exhausted. A properly designed hood can actually use fewer CFM’s so long as the velocity is enough for the filter baffles to remove grease.

To be continued...

Jack Kitchen - Range Hoods

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