The loss of capture will depend on how much the hood overlaps the hot surfaces generating the expanding effluent. Minimal overlap and the hood needs to be up close and personal. Large overlap and the hood can be high and, for conditions without cross drafts, still capture and contain most of the rising effluent. The large hoods used in commercial kitchens are normally set seven feet off of the floor (about four feet above the cooking surfaces) and are effective because of their large aperture sizes and proportionally large flow rates.

kas

The gas company seems to be familiar with a common local building code spec minimum distance for normal range hoods which is often 30" from the cooking surface (for example the inside surface of a thin frying pan supported on a grate) up to the grease filters or other given point.
Shown on the VAH website in the case of a least one Vent A Hood model installation, there is a 24" clear space followed by a 6" metal hood section which equals 30", of course. You will most likely need to obtain some reference from VAH or the retailer that shows this two section measurement meets compliance with the building code to the satisfaction of the gas company.

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I "just" installed a vah today. as far as I know 24" minimum, 30" standard, 36" max height.

I have an island hood and vah recommends 30" and makes the hood heights to correspond with that depth. For example, my hood is 30" tall and is setup for 8' ceilings. this puts it at ~30" off the cooktop.

Your model may be slightly different but go with whatever vah recommends

24" or 27" is to low, that's where people mount over the range Microwaves, losing 10% per inch is rubbish IMO.

Standard for every manufacture in the US is 30" - 36", sure closer is better but 24" will look wrong and be very uncomfortable when cooking and painful when you hit it on a daily basis, unless of course your hood is 20" or less in depth above a pro style range.

Vent-a-Hood might need it so close because the "MAGIC LUNG" wont work any higher above the cooking surface ?????

Heh, Trevor. Are you questioning the existence of magic. :)

kas

Kas..... I was a believer in "Magic" until I was 8 years old, from 9 - 12 years old I had doubts, 13 - 52 I see it as misdirection and good marketing....lol

I wouldn't call 10% rubbish. the gasses expand as they rise. it may be more about capture area rather than the efficiency or technology of the hood.

30" is the number to shoot for.

IMO... The statement "a hood will lose 10% efficiency per inch raised is rubbish.

Think about it if you mount this particular hood at 30" (Industry Standard) rather than 24" you will lose 60% of its efficiency, the numbers don't add up.

As said in my first post maybe this hood wont work above a certain height because its not a good hood or the wrong depth in relation to the range.

At the very least, the loss in capture with height (the only efficiency I can see being relevant) will be non-linear, so 10% can only apply at some particular height.

It is possible that someone looking at an aperture air velocity versus distance plot such as may be found in the ASHRAE Handbook: HVAC Applications picked some point relative to the aperture where a 10% relative change per inch for a particular hood geometry is true. Unfortunately for even this potential truth, the main principle of hood capture is not based on the blower induced air velocity at some displaced point, but in capture by overlapping the rising effluent and then ensuring it gets past the baffles or mesh filters and is contained. Peripheral air velocity only helps make the effective hood aperture slightly larger, because with a hot surface, the greasy effluent upward velocity can be 3 feet per second, while the blower induced air velocity may not be this large at any point under the hood or within its aperture before the baffles. So essentially, under the baffles, the blower weakly helps direct the effluent, but overlap dominates. At and above the baffles, the blower provides the impetus to air momentum change that gets the stuff past the baffles and removed.

kas

I'll share my real life hood experience for the last year.
The players: Bluestar 36RNB with griddle, Prizer custom pyramid/chimney style 27"x36" island hood, with baffle filters. It is mounted 36" above the grates, and coupled with a 1000 cfm (three speed) remote blower less than five feet away on the roof, 8" ridged metal duct with two 45* bends.
Cooking habits: Sort of short order diner style, cooking two or three meals a day. I use the griddle daily, sear steaks on a cast iron skillet, stir fry in a wok at least once or twice a week, fry tortillas in oil, and even deep fry battered fish now and then.
Observations: On low speed the blower works fine for 90% of cooking, in fact I don't even turn it on for simple saucepan steam or eggs. I use the medium or high speed setting (can't really tell much difference, other than more noise on high) when searing, pan frying, or stir frying, and occasionally, I do see a little smoke escape outside the capture area on the sides. (high fat foods such as bacon or burgers are the biggest culprits) but his only happens from the burners, not the griddle which is in the middle. I look at it this way, the food I'm searing or frying usually smells so good that a little smoke escape isn't going to bother me or my guests;) and after a year of heavy use, I have not seen any grease or gunk on surrounding areas, ceiling, walls, cabinets etc.
Summary: A 40"or 42" wide hood would likely better contain the escape I'm getting in those extreme cases, and 1400 cfm and 10" duct may have done a better job of removing it? I went with 36" because our space is quite small, and did not want to have the hood be out of proportion and over power the room, and because of roof framing constraints, I was limited to 8" duct.
Hope this helps...

Oh and I don't know how tall you are, but 24" above the range would hit me in the chin, and 27" would hit me in the nose! How would you see what you're cooking? Those measurements sound extremely low, and awkward feeling...

This post was edited by ctycdm on Sat, Feb 15, 14 at 16:57

"Oh and I don't know how tall you are, but 24" above the range would hit me in the chin, and 27" would hit me in the nose! How would you see what you're cooking? Those measurements sound extremely low, and awkward feeling..."

I'm not that tall, 5'8" so I would be able to deal with 27".

I'm still debating what to do, all the inspectors and insurance companies I have spoken with say I need to go with the suggested specs of the range.

Any other recommended hoods for not more than $1000?

why would you have to go with the recommendation of the range.. whatever hood manufacturer you chose is who I would be following in terms of specs.

You do realize, you are going off "recommended" mounting heights. When I was researching what to buy for my hood, there was a recommended height and then there was a minimum height

basically what your concern is now, is that you think you shouldn't go over 27" with the hood you have selected yet your range is saying it needs to be 30"... post model numbers of your hood and range

American Range Performer (ARROB-430) & Vent a Hood PR9 under cabinet model.

If I am looking a the right specs for the PRH9 under cabinet is only 21" deep, which is why they want it so close to the range top. Also looking at vent a hoods own website, you will see the ranges they show under the PRH9 hoods are completely different to yours, even vent a hood don't feel confident enough to show this hood above a real pro style range (again just MO).

This depth is inadequate for the range you have, I would suggest one of the following (from OK to excellent).

30" x 24" set 30" off the range top
36" x 24" set 30" off the range top
30" x 27" set 32" off the range top
36" x 27" set 32" off the range top

what specific PRH9 are you looking at?

stepping up to a model that is at least 24" deep is what I would recommend as trevor explains.

I would be looking more along the lines of the JPH236/C1 or SLH30-236. these are more appropriate models for your range choice

I think I need to give up on the Vent A Hood, 27 inches is too low and I'm not too sure I wanted mounted at 30" if they are recommending 27" as a max. What other recommended hoods are out there in the $1000 price range, I had one salesmen pushing the Broan Elite Toscana and a Zephyr model. Anyone have feedback on the Broan?

I think you are reading way too much into this. that specific model is not really designed to handle your range choice. Look at the couple I linked. the VAH is going to be a better choice than the broan or the zephyr based on the research I did when shopping

Hi Zivman, thanks for the recs. I did look into them but they are way over my budget, those hoods are listed at $3500+. I spoke to someone at American Range they claim 21" depth will suffice.

Its clear that American range don't make hoods with such poor advice they are giving you.

if you actually want something that is going to work for your range, and given your height concerns, you are going to have to up your budget... not saying you need to spend 3500+, but you will be spending more. You can look into the zephyr and others, but to match that hood, you are looking at VAH, ModernAire, Prizer, etc

Example of one hood that you could get noted below for $1409.00

36" Wide
27" Deep
10" high
600 cfm internal blower

Trevor, which brand?

The 24" deep VAH can be 30" above the range and still work just fine. The PRH9 model is 21" deep and is designed to go under a cabinet and needs to be closer to properly capture the cooking vapors so that is where the 24" to 27" comes from. It looks like you are going with a pro style range so you should go with the 24" deep hood. It does not matter what brand hood that you choose if it does not properly cover the appliance it will not properly vent the appliance. You cannot make up for a undersized canopy with more CFM. If you try this the blower would need to be so large to pull in the cooking vapor it would be unbearably loud and you would not use the hood or you would turn the blower to a lower setting and thus defeat the reason you have a super high CFM blower in the hood.

Exactly! A modest jet engine with a roughly 1 sq. ft. intake duct area mounted above your ceiling would have a large effective aperture at the height of the cooktop -- perhaps the entire kitchen area. But the blade tip turbulence noise alone would give new meaning to the concept of loud kitchen ventilation, at least for the several seconds your hearing lasted.

kas

Vent a hoods all have specific height recommendations based on the principle the contaminates rising off the cooktop begin to have more lateral movement as they cool off. Therefore the hood depth in hinged on the height recommendation. Here is a websight that illustrates these concepts with video tutorials

Here is a link that might be useful: vent a hood purchasing tutorials

One comment among those above that was passed over needs some expansion. Range manufacturers can only make requirements for distances to flammable materials (typically 30 inches), so their advice to the AHJ applies to cabinets and hood surrounds. It would wisely be considered relevant to hood motors also.

Hood manufacturers provide (or should provide) height requirements based on capture and containment, along with a minimum safe height for fire resistance of any exposed grease on meshes or baffles (probably where the 24 inches comes from). The meshes and baffles are supposed to provide a firestop for interior grease. Most hood materials are stainless steel (very high melting point) except aluminum meshes (not so high) and wiring (degrades at fairly low temps).

So one should think about having a grease fire on the stove top and what it means for all the materials within range of the heat and flame.

kas