30" works for most situations. 24" is too low for normal use for the normal human being.

We've got a 36" RCS and planning a 27" insert (only about 20" depth).

I suspect VAH recommends 24" to cover their butts, while BlueStar is probably more practical. Of course the tradeoff is always going to be the higher it is, the less effective venting you'll get per CFM.

Go with Bluestar's spec's. and select another hood. I mounted my hood 36" above my Bluestar, but it's also 27" deep.

My range hood is 24" above my cooktop and I hate it! It seems that the hood is always in my way.

Ah, I had the same issue last year with VAH and our American Range, I had posted on here as well about it. We were more concerned with building code and even that we never had a clear answer. We had to sign off with the cabinet maker if we wanted 24", I had spoken to VAH and they kept saying you will lose performance if you go above 24", the gas guy was saying 30", another installer was saying do what you want, the installer who did most jobs for the store where I bought it from said he ends up installing them all at 30" to avoid dealing with issue with the gas company in case they follow up with an inspection. At the end of the day I went with 30" as I found 24" too low. Ideally I think I would have split hairs and gone to 27" although it still seems to be quite efficient at 30".

Here's the post.

http://ths.gardenweb.com/discussions/2258513/vent-a-hood-height?n=27

I would take into consideration the chance of bumping your head if you lean in to stir something etc. (depending on how far it comes out from the wall) The higher the better, in my opinion. (this is not really addressing efficiency).

Thanks for the feedback everyone. Sounds like I need to rethink the hood model or maybe even brand.

We are currently looking at Vent-a-Hood based on their claims around noise levels and the recommendation of our salesperson.

I see they offer a hood at 24" depth, but it gets all around bigger, not just in depth but also the height of the hood, chimney, and the lip of the hood.

I will poke around these forums to see what else might be recommended for my needs, but in the meantime if anyone has any advice please do share :)

This one from Best seems to be a bit less bulky than Vent-a-Hoods 24" depth option: http://www.bestrangehoods.com/store/products/productdetail.aspx?id=d660ec4e-3c83-432e-ad2b-784105680017

This one seems nicer than the one from Best, but it is even more expensive than the Vent-a-Hood option: http://www.prestige-america.com/pyramid-classic/

I do like the electronics-less design though (no display screen with buttons, just two knobs). And it can come in a 1200CFM option.

This forum is not lacking in hood advice, so a tour of hood topics will fill many gaps (and many hours).

It is unfortunate that the sales people seem to be unable to describe the relatively simple functions that the hood has to perform in a way that answers all of the sizing questions. To wit:

The hood has to capture and contain the rising effluent plumes from the cooking surfaces. These plumes expand as they rise, so the higher the hood, the larger it has to be for full capture. Capturing a ten-degree expansion half angle from all the likely the pan surfaces should be sufficient, at least in low cross-draft environments. This establishes the relationship between size and height from the cooking surface.

Height should allow direct view of all pan interiors, and have a low probability of skull impact. Hence, place the hood high enough to clear the cook's view, and then set the size per the paragraph above. This may require some recursion unless the height is over the tallest cook.

Containment means that the captured effluent stays in the hood and doesn't curl out due to insufficient air flow rate at the baffles or mesh filters. The necessary air flow rate varies with cooking temperature, and slightly with height, but aiming for an actual 90 cfm per square foot of hood aperture should suffice for most cooking. (Charcoal grills and 100 kBTU/hr wok burners would be exceptions.)

The ventilation air flows from outside, through the house to the cooktop, into the hood, through the ducting, and back outside. This circuit will have pressure drops at various points. Pressure drops (called pressure loss) reduce hood blower performance. Typically, given nominal fan performance curves, one might want to have the maximum rated flow rate 1.5 times the value calculated from the product of 90 cfm/sq.ft. and the aperture area (in square feet).

The air that comes from the outside is called make-up air (MUA). There is always MUA or there is no hood flow rate. The more restrictive the MUA path, the lower the house pressure. The house pressure cannot be allowed to get too low. Too low depends on presence or not of combustion appliances, and on local codes. Hence, the MUA path may have to be deliberately designed rather than just assumed to be sufficient from house leakage.

Noise level, lighting, duct diameter, blower control, general aesthetics, effectiveness of the capture aperture configuration, and cost are other dimensions that have to be addressed, but aren't so directly related to size and height.

kas

I know my comments will be slightly off base. But they should be a factor in your decision. The following videos can be found on YouTube.
It speaks to how effective the Vent-A-Hood system (concept) is compared
to a Best® rangehoods.

Video #1

Vent hood failure

“I have a vent hood over a grill
in my 3 season room. The hood does not work...my room fills with smoke. My
guests can't even stay in the room to eat.”

Video #2 (Please listen for his comments on what he found
inside the duct when his installer removed the Vent-A-Hood.)

Best® Outdoor Hood

“I recently replaced my failing
Vent A Hood with this Best® hood from Broan-Nutone. This hood works great!”

Many
consumers are pulled into the concept of filter-less ventilation. The
reality is most homeowners are not up for the more than occasional squirrel
cage blower assembly removal and cleaning.
Question – How handy and comfortable are you with climbing into the hood and using an
allen wrench? You’ll need to be in order
to clean the “Magic Lung” blower wheels.

For
my money give me an easy to remove filter that can be tossed into a dishwasher. No matter what the mounting height and hood depth is.

Thanks Kas, I see your name come up in a lot of these hood discussions. Much Appreciated.

Thankfully MUA unit is already in the budget because it is required building code on all hoods over 400CFM in North Carolina.

I am currently looking at this hood from BlueStar: http://www.bluestarcooking.com/products/hoods/pyramid-hoods

From what I can gather, it is made by Prizer, which also makes the Prestige hood that I posted earlier in this thread.

This seems to have a more appropriate aperture size for the height I want to install it at. It also comes in a 1200CFM blower which would give me a lot more power. One downside that I am currently wrangling with is that the 1200CFM blower is made to fit 10" ducting, and my ducts are currently 8".

That leaves me with the options of either getting an 10" > 8" adapter and dealing with some performance loss; or asking my contract what it is going to take $$ to replace the duct work with 10".

I wonder if anyone has an opinion on the performance loss or other downsides of using an adapter would be?

Or if there is another good option I am missing?

@1_ohio_bald_eagle

Thanks those are some great videos comparing the two hoods under some real heavy testing.

I wonder how the Best KEX4236SS would compare to the BlueStar pyramid hood with 1200CFM blower.

I notice that the Best is 'flat' on the bottom, whereas I believe the BlueStar is open at the bottom with the baffles further up, like with the Prestige. I am not sure if this makes for much performance difference?

The Best seems to natively hook up to 8" ductwork which is nice since that is what I already have.

Base on Pi r squared.
10" round duct is to 78.6 square inches. 8” round duct is equal to 50.4 square
inches.

Caution: A reduction in duct sizes below the recommended
cross section will reduce air delivery and increase the noise level.

If at all possible do not reduce the duct work. Stay with 10” round. When you lose the 36% of your potential
recommended space to vent out you will not be happy with the result.

You can alway check with Best tech support for the official guidance 800-637-1453.

Well that would push me in the direction of the Best hood, as it is designed for the 8" ductwork I already have. I am going to see if I can find one installed somewhere locally to go look at it.

Duct size should be established to allow the air VELOCITY in the duct to fall into the 1000 ft/min to 2000 ft/min range at full blower power. (Tend toward the higher value if the ducting is normally cold, the lower value if normally warm.) Pending a careful analysis using parameters that may not be available, a 1200 cfm blower should be assumed to move only 800 cfm in the system. A hood/blower combination that is rated at 1200 cfm together might eke out 900 cfm in the system. So, divide whichever CFM number applies by 1000 and separately by 2000 and you have the range of duct sectional areas to strive for (in square feet).

Example: 800 cubic feet/minute divided by (for editorial convenience) 1600 ft/min is 0.5 square feet. At 144 square inches per square foot, this is 72 square inches. Hence for this example application, 10-inch duct is most suitable.

Use this process to determine compatibility of blower flow rate and ducting. Note that there is a lot of tolerance here, and operating at the high end of the velocity range will be slightly noisier than at the low end, but turbulence noise from baffles and blower blade tips will generally dominate turbulence noise from duct surfaces.

As described above, the capture area has to be established first, then the flow rate to get the velocity at the baffles where it belongs, then the fudge factor for pressure losses to get a fan rating. But it is the actual flow rate at the aperture and the duct size that determines the duct flow velocity, and this velocity is intended to minimize grease particle deposits on the ducts.

kas

Thanks Kas, I had to read that twice, but it was helpful :). I have asked my contractor to quote me for upgrading my ductwork to 10".

I drove around today to different showrooms in the area to look at range hoods and get a feel for how their size and mounting heights effect the feeling of standing in front of the range.

I brought a tape measure and I found that hoods which were mounted at 33-36" above the range felt really good to me.

I liked the look of many of the commercial style hoods with the baffles, although it is not yet clear to me how/where most of them collect grease.

Ah, you have encountered the very subtle baffle grease collection method.

As the air is rotated around the baffles, the larger end of the grease particle spectrum is expelled and collected into the upper side of the lower baffle of each pair, where it may then drain to a low spot where it collects. In commercial hoods, the baffles are required to be steeper than 45-degrees, and this allows grease to drain to the side channels and thence into a container. In vaguely-commercial-looking hoods, like my Wolf Pro Island hood, the baffle angle is slight, and the grease tends to partly slide to the end channels, and partly dry on the baffles. Hence all those parts need to meet the dishwasher at some interval.

kas

Nice, that sounds like a pretty elegant solution.

My vendor is still recommending the VAH as being better than the commercial style hood I am looking at now ( BlueStar BS-PC36240 ). However looking at the differences between the two online, there are a number of things I like better about the BlueStar hood over the VAH

• rheostat style fan and light controls

• closer to the size I want

• I like the commercial looking baffles

• I can get a 1200 CFM blower, which might be more than I need, but the 600 CFM VAH seems like it might be less than I need

Unfortunately I can't find anywhere in my area that I can actually see the BlueStar hood in person (or a Prizer, or Prestige for that matter, which are very similar).

I did find this video review, although annoyingly they never turn on the fan.

I am still waiting to hear back about the ductwork pricing to go up to 10" ducting, but assuming that it is not overly expensive I think I will go with the BlueStar hood.

I would want 900 CFM VAH if going with VAH. If the BS uses internal blower, I am pretty sure you won't use more than 600 or 900 normally anyway because of the noise. I can use 900 every time on VAH. Make sure you know how loud it is for both.

It is unfortunate that all manufacturers do not publish noise specifications based on measurements done to some accepted standard.

Where noise is to be avoided, and space is available, such as an attic, the blower should be roof mounted and an in-line silencer used. This will minimize all higher frequency noise except the baffle turbulence noise. Even without the silencer the blade tip turbulence noise will be somewhat reduced at the hood end of the duct.

kas

My hood vents directly out the roof in a straight shot vertically above the hood. I have read that remote blowers should be at least 10 feet away from the hood to be effective at reducing noise, I am not positive if mine is that far away or not.

Do the inline silencers reduce the effective CFM much?

"Do the inline silencers reduce the effective CFM much?" Pressure loss is a small fraction of an inch of water column; significantly less than the baffles. You can contact Fantech for values for particular silencers at specific flow rates. Pressure loss effects on CFM require a copy of the fan curve to determine, but I would expect it to be slight. Note that the 10-inch duct silencer is fairly large (see Fantech's web site for details).

kas

Kas, using the formula for duct size above, I calculate that a 1,500 CFM fan would require a 14" duct. Is that correct? I'm getting a 1,500 CFM roof-mounted fan ( I live in a one-story house) and am planning on a 10" duct. Will this be too small?

The key fact is that with the system* pressure losses, even with reasonable MUA, the actual flow rate of the 1500 CFM fan on maximum will be a lot lower -- perhaps only 1000 CFM. I once calculated for my 1500 CFM-rated Wolf/Broan roof fan, with silencer, hood baffles, hood transitions, duct, bends, and some slight house negative pressure would intersect the fan curve at about 900 CFM. (This calculation was done in 2007 and I'm a bit weak on the details
now, but active MUA is likely easier done controlling for a slight
negative value than trying for exactly zero.)

I use 10-inch duct, and aimed for the higher end of the 1000 - 2000 ft/min range due to the duct being in a cold attic (at least in winter).

You can find representative fan curves somewhere on the Broan/NuTone web site. Wolf will send you some if you ask the engineer you eventually get to. Fantech has curve tables on their web sit for their various in-line blowers. All commercial up-blast systems have fan curves somewhere.

If you want to get an idea of how fan curves work, try this exercise for the student. Take a piece of graph paper (if it still exists) or print one from a blank Excel plot. On the abscissa (representing CFM) establish a linear scale of 0 to 2000 and put a point at 1500 CFM. On the ordinate (representing pressure across the fan, i.e., system pressure loss) establish a linear scale of 0 to 2 inches, w.c., such that the two axes are the same length. Put a point at 1.5 inches. Draw a diagonal straight line between those two axis points. Note that these values do not represent any particular fan, only the nature of fan curves.

Take a protractor and draw a quarter circle, centered at the origin, between those two points. Real fan curves for blowers designed to be operated at various speeds between min and max without hunting will typically fall between those lines and be monotonic everywhere (no dips). You will see for any curve between those lines that as the pressure loss increases, the CFM realized will fairly rapidly decrease.

The fan curve typically represents full power operation. At reduced power the curve shrinks toward the origin.

Pressure loss is a function of actual flow, so the effective resistance of the system loop is generally approximated by a straight line intersecting the origin and a CFM value point where the pressure loss was calculated. Because the baffles may be the biggest source of pressure loss and the least documented, this whole process is more of an estimation effort than an exact calculation, even when real fan curves are used and real duct calculations are performed.

kas

*System means the air-path loop from the cooktop to the outside and back to the cooktop.