I was very happy with my Vent-A-Hood. It moved lots of air, never had an issue, the price was not bad. It was one of the best looking hoods for what we were looking for. Plus on lower settings it was not obnoxiously loud.

27-inches is better than 24 inches, all else being equal. (Equality in this case has to include air velocity at the hood aperture, so larger requires more CFM.)

VaH CFM values are a topic in themselves, but with any blower you will not achieve the rated CFM levels, which are established at zero static pressure across the blower. However, you may achieve the needed CFM levels. Actual/achieved CFM is reduced by pressure losses, including those of the ducting, duct cap, and particularly reduced house pressure due to imperfect make-up air (MUA) supply.

For a 27 x 54 inch hood (pretending that is the aperture size), and a desirable 90 ft/min across the aperture (90 CFM per square foot), the actual full power flow rate should be ten square feet x 90 cfm/sq. ft. or 900 CFM. So the 900 CFM blower is inadequate to start with if this rule is followed. (As it should be in my opinion for gas grilling.)

VaH fan curves (implied from their published point data) are somewhat more steeply sloped to the left from the zero pressure loss flow rate point than (e.g.) Broan blowers, and hence pressure losses of the installation are even more critical to achieved flow. I would suggest upwards of 1500 CFM rated flow rate to achieve the 900 CFM required.

On the other hand, if a 24-inch hood (9 square feet aperture) is used, then only 800 CFM actual is needed, so the 1200 CFM blower may be adequate. If the MUA is too restricted, none of these blowers will be sufficient. MUA must be part of your planning from inception. A 24-inch hood will likely not capture all the expanding-as-it-rises effluent toward the front of the stove because it may not overlap the hot surface by the ten-degrees minimum that is needed.

As for head bumping, this can be evaluated by a side view stick drawing of the cooking layout and hood layout with the stick-figure cook bending at the waist. Also check sight lines into pots.

My Wolf Pro Island Hood (largest size) covers a 36-inch induction cooktop and a Cooktek induction wok, feeds a Wolf (Broan) 1500 CFM roof blower and an in-line Fantech silencer to yield a relatively low noise level and excellent capture of cooking plume effluent, so long as there are no significant cross drafts.

(Keep in mind that the 90 ft/min desired air velocity is at the hood aperture, and if achieved still drops like a rock the farther from the aperture one measures it. Capture with a hood that doesn't sound like a jet engine is only possible by virtue of the hot cooking plume having its own upward velocity -- for gas grilling this velocity is above a meter per second.)

(The 90 ft/min value is for conventional baffled hood filters used in commercial and most Pro-type residential hoods. Your candidate VaH hood may have different requirements for "Magic Lung" designs, for which I have no good information on flow patterns at the hood aperture. Baffle systems have relatively uniform flow if properly designed.)

kas

[edited penultimate paragraph for better readability]

I'd get bigger on all 3 categories. (I know extra height reduces sucking goodness, but it feels more comfortable to work under.) If the cost makes you balk, you need to downgrade the rangetop or find a cheaper hood.

Rethink the grill and you may be able to do the 900CFM. Grilling indoors is filthy dirty smoky. It truly belongs outside. I've grilled outside even in the dead of winter, so it's certainly possible to do without it indoors.

Whatever you do, you cannot skip the Reuired Make Up Air. That requirement is not optional. The national building codes have all adopted it becaus if the dangers of back drafting. It is for the safety and health of your family. In a cold climate, heated make up air may be required

If you find the total package price for doing your cooking zone too cost prohibitive, then downgrade the whole package. Starting with your choice of pro style cooking appliance. You cannot just simply downgrade the vent. It is a package as a whole that includes ventilation and MUA. If you want the cooking performance, you need the full package venting performance as well.

Thank you all for your comments. I really appreciate it all.

I'd still like to shoot for the 54" x 27" hood design as that would seem optimal for capture area (assuming other variables reasonably attended to) and we have the space. We need to be at ~ 34" for height from range top to bottom of hood (ceiling is ~112").

I'm leaning toward the VAH primarily due to public comments about their quietness (will use 12" duct) and what seems to me might be a simpler installation (blowers in hood rather than in-line or external). Contractor says we can do a 12" duct. Wolf may still be in play but I do need to flesh out costs,etc. Wolf does seem to have more options for internal, in-line or external blower options. I imagine silencer can be used if needed for either installation (assuming it fits!).

Make up air is the most confusing part, for me. Assuming one of the above (VAH or Wolf) and the Wolf SRT446CG as noted earlier, I'd like to know if anyone had any recommendations for what type of MUA system to deploy. I realize, through my research, that there are many variables to consider and so am looking for general types of solutions to csondier, keeping costs as low as possible.

Current design has large windows on opposite wall that could be open (about 10' across from cooking area) but I'd like to avoid opening them since there is a small island (with seating) in between and it could get cold, or drafts to cooking area might occur. We do live in San Francisco, so year long temperature is pretty mild, which I know is a consideration. There are windows in other rooms on the same level (1st floor) that could be opened too but I'd like to consider an all windows closed option, therefore some sort of MUA unit, hopefully at minimal cost. We also have a fireplace about 15' from the cooking area in an open floor plan and I know that is a real consideration.

One floor below, in the garage, is a gas forced furnace which was installed 2 yrs ago. It's ducted and heats the 1st and 2nd floors of the house. Venting will need to change a bit on the 1st floor since currently there are a couple of large vents only. Contractor stated that we'd probably use some sort of under cabinet venting in the kitchen. Reason that I bring that up is that I read that there are options for integrating make up air systems with your furnace which I will with him as well.

A quick product that I saw that is expensive but may be worthy to consider is this from Fantech. You think this type of unit is worth considering? I picked the 1600 CFM unit just only because it's greater than the 1200 CFM of the VAH.

https://www.amazon.com/Fantech-MUAS-1600-Makeup-System/dp/B015YW0918/ref=sr_1_2?ie=UTF8&qid=1475614937&sr=8-2&keywords=makeup+air++fantech

Also noted that Broan has a complete system including a motorized damper and exhaust flow sensor. Could that work for the VAH or Wolf? Maybe I should look at their hoods if not.

So, thanks again for all of your inputs so far. I've read a lot about all this but being a first timer, so many questions do arise. I do believe in the KISS (keep it simple stupid) approach to most things and would like to think I could apply some of that here. Time will tell :-) Hard to believe with all my rambling!

If you have any comments please do let me know. And again, thanks to all of you for your earlier replies.

-Bob

Why a 12" duct? Doesn't the hood require a 10" duct? Going too big can be as bad as going too small on ductwork. I forget the physics of why, but perhaps Kaseki will explain why a duct that is larger than specs is not good either.

Ventahood is regarded highly for performance. It is the cleaning issue of the squirrel cage system that is frequently mentioned for why people may not like VAH. Instead of baffle filters that go in DW, the VAH involves dismantling and cleaning of the squirrel cage casing and the squirrel cages themselves. More parts, more surface area, and more time needed to clean. The VAH hoods work great though.

Freidajune,

The newer VAH units have a slide out pan that is a bit easier to clean than the older models, details here: https://www.youtube.com/watch?v=KgW5OkqnQXg. In either case, I don't think either is a big deal.

As for 12" ducts being a problem, that I do not know. The 1200 CFM double B200 dual blowers requires a 12" duct and that is what I am looking at (due to issues raised earlier to support the Wolf range top SRT484CG). The 900 CFM model requires a 10" duct. I certainly would be interested if anyone has any 12" duct experiences and know of any problems or why this might not be the best option for me, assuming the range top will be as stated. I still need to review this all with my contractor.

VAH spec: http://www.ventahood.com/installpdfs/ept18nept18sept18%20eurostyle%20wall%20mount%20range%20hood%20specifications.pdf

Follow whatever the directions say for the ducting. Ours had 3 fans and 2 ducts. One was 8" and the other was 10" or 12". Whatever it was we simply followed the installation instructions.

Hmmm. My long answer to several questions above just evaporated into the aether when I submitted it. I'll try again later.

In parts...

(a) The silencer has to be between the source of noise and the hood; a hood with an internal blower will not benefit from a silencer. If you choose a configuration that can use a silencer, be sure to check its dimensions to ensure that it will fit where you want it.

kas

Part (b)

Duct velocity should fall into the 1000 ft/min to 2000 ft/min range. The lower value region is best where the ducts are cold, while the higher achieves lower grease accumulation where the ducts are warm. I think SF is in the warm category.

For conventional blowers, e.g., Broan blowers that Wolf also uses, a 10-inch duct is sufficient up to 1500 CFM blower rating for all but the longest most tortuous duct runs. For VaH magic lung® blowers, follow whatever directions they provide. Their flow rate drops faster with back pressure than the conventional high flow blower designs. (I count Fantech in-line axial blowers as conventional in this sense.) It is likely you won't be moving more than 2/3 the rated zero static pressure flow rate in real life, so use that value in calculating duct air velocity.

kas

Part (c)

Make-up air (MUA) is a major topic of its own, but it can't be treated without considering the entire ventilation system. Here are a few areas to start with:

(1) Safety. Combustion appliances (gas and oil, but pellet stoves and other heaters are also included) are often passive draft; that is, hot combustion air rises is the principle of effluent removal. Furnaces of modern design attempt to extract as much heat as possible, so the draft is small. Blown combustion designs, such as oil burners, are less susceptible to back-drafting, but it can still occur. Back-drafting means combustion effluent, including carbon monoxide, can be pulled into the house.

So for passive combustion, the goal is to not exceed negative 0.03 inches of water column of house pressure (relative to the outside). Oil burners might tolerate -0.07 inches. These negative pressures are easy to exceed with a big blower hood system. In some cases, they can be exceeded with an open screened window. To ameliorate this risk, as well as allow the hood blower to actually flow even 2/3 its rated flow rate, the house pressure has to be kept near zero pressure difference with the outside.

(2) Configuration Alternatives. Low pressure difference is the task of MUA. Passive MUA, such as Broan supplies, is good when there is no need to heat the air, the filtration, if any, is very low pressure loss, and the risk of back-drafting is minimal. Active MUA, such as Fantech and all the commercial sources such as Greenheck supply, use a blower in the MUA path to correct the house pressure, and to make up for the pressure loss of decent filtration and any obstruction of the air flow due to a heat exchanger or electric heater. These systems can have complex control schemes where necessary.

(3) An Option. If the furnace is the only combustion appliance (other than the stove) and is in a room that can be sealed from internal air paths to the kitchen while open (or ducted) to an outside supply of air, then it can be made safe without MUA. In that case, passive MUA can be used at much lower cost and the hood performance will be whatever the fan curve of the blower dictates at the pressure loss of the baffles, ducts, and MUA path.

kas

Part (d), more on MUA

A review of MUA related topics on this forum may be useful.

A CO detector is a good idea.

Integrating MUA with the furnace hot air ducting, or even the cold return ducting, will likely entail some significant pressure loss at the flow rates needed to match the hood. In such a case an active MUA design is called for. Furnace system ducts and diffusers or registers are designed to deliver air at low pressure loss at the flow rate needed to heat the house, plus making up for any air that it leaks from the house under normal conditions. These conditions are not the orders of magnitude higher level of of 1000 CFM or greater.

No air will leave the house via the hood ducting that hasn't made it into the house first.

Big active MUA systems may need a silencer of their own.

kas

Part (e) Miscellaneous.

I believe the Broan MUA system senses air flow in the hood duct, so it should work with VaH as well as Broan and others.

Fireplaces can back-draft with very little differential pressure, depending on the fire size. If the goal is to run the hood at grilling level with the windows closed (or maybe even with the windows open), active MUA may be necessary.

Commercial MUA systems can be easier than residential to configure. This is because for kitchens, the flow rates are all fixed and the MUA can be tuned to establish a slight positive pressure in the eating area relative to the kitchen, while the kitchen pressure is set stufficient to keep the hood blowers (actually on the roof) in an efficient range of operation.

In large buildings, the pressure is kept slightly positive by having an MUA system that always runs with a pressure controlled air bypass loop around the blower. Positive pressure not only helps clear restroom air, but allows exit opening even with adverse winds.

By now I imagine you have a good grip on the bag of HVAC worms that I discovered when I started planning my kitchen reno in 2007. A stronger embrace may be needed to establish a plan. Feel free to ask more questions, or re-ask any I failed to address.

kas

Thanks again for all the comments. Some things that I have learned in my life is to be humble about what you know and what your limitations are. Seems clear to me that I am in a bit over my head. I have a call in with my contractor this afternoon to discuss this. My gut tells me we should get an experienced HVAC guy in for consultation before I purchase anything. I'll then see what he/she recommends. I will reply with what I learn. I think it's a wonderful thing to share experiences. This forum is great.

Again, thanks everyone.

I live north of San Francisco in a home without AC or furnace just a small LP stove in living room. My 30 in. LP range in on an exterior kitchen wall and vents directly to the out side. How do I address the MUA issue. The house is about 60 yrs. old and quite drafty.

INSUFFICIENT DATA FOR MEANINGFUL ANSWER

-- Multivac (Isaac Azimov)

What we don't know is what flow rate your hood achieves (or even what it is rated at), what house pressure results from that flow (which depends on what the meaning of "quite drafty" is), and at what pressure the stove back-drafts (possibly given in its installation literature).

Or we can go with an experimental answer: If you can't detect back-drafting with a CO sensor, then you don't have a problem. If you can you do have a problem and some window opening experiments may be helpful to discover whether passive MUA will suffice.

kas

I spoke with my contractor yesterday. He's confident that we can go with the 1200 CFM VAH with 12" duct. Duct span will be ~15-20' exhausted through roof with 2-3 bends (45-90 degrees). We'll address MUA with an HVAC guy in the next few weeks as well (he is confident that we have options for that).

For duct cover height I'm thinking 24" might be ok for my 113" height ceiling , standard 36" counter height, 18" hood height. I am tallest in family at 5'10" (70"). They do have a 36" height cover but costs ~200 more. If my calculation is correct, assuming simple math (assuming duct cover flush to ceiling) height from range to bottom of hood would be ~35", which seems reasonable to at least not hit my head!

How significant do you think the extra 5" might be (recommendation is for 30" height between hood bottom and range top) as far as capture is concerned?

Thank you, Kaseki. Will check out the literature and see if I can figure this out.

Bob Grand wrote: How
significant do you think the extra 5" might be (recommendation is for
30" height between hood bottom and range top) as far as capture is
concerned?

This gets us back to the question of how well the hood overlaps the expanding cooking plume. Excepting the case where there is a cross draft, so long as a ten-degree (to vertical) nominal conical expansion from the hot parts of grills, pans, woks, and pots is intercepted by the hood aperture, then capture occurs. Otherwise, part of the plume escapes into the room.

(In reality, the plume is not emitted into a perfect cone, but into a distribution of angles, so some effluent can escape no matter what. Typically with sufficient overlap this small residual amount won't be noticeable. Further, the actual cone angle vs. velocity distribution varies with what is being cooked by what heating surface, so many answers are possible. See the Finnish authors' papers I intermittently reference.)

Capture can also be aided by the side drafts that the hood flow induces against the low velocity of the extreme angle plume components. This should not be depended upon for the main plume body unless one's hood's flow rate is so high that it can pull a m/s draft at the counter top.

So, if the hood is just big enough at 30 inches height, then there will be missed capture at 35 inches. If the hood overlaps by more than 10 degrees at 30 inches height, then raising it will not significantly increase missed capture until the 10-degree angle is met.

In cases where there are cross drafts, then higher will always result in more missed plume. (Picture a bent tornado funnel above the grill.)

kas

OK, I took the plunge, this is what I ordered:

NEPXTH18-454SS Vent-A-Hood 18" x 54" x 27" Nouveau Euroline Pro Wall Mount Hood with 2 Dual Blowers (1200 CFM)

Thank you all for your comments. It really helped, I just hope it all works out!

-Bob