IMO Ventahood is saying that their 600cfm is equivilent in performance to a 900cfm using baffels, mesh screens, or any other obstructions (filters) in the path of the exhaust air. They do not claim that their 600cfm=900cfm. VAH has no obstructions in its path to exhaust. 600 will be 600. However, take a 900cfm and put some filters in the way and you no longer have 900cfm.

a VAH unit w/600cfm will consist of two squirrel cage blowers in a single housing. Each motor/blower/cage=300cfm.

With a two blower setup you will have 4 availible flow rates of 150,300,450,600 cfm.

As with any other exhaust system the unit will work harder(with less cfm) if it can't find replacement air easily, hence the need for make up air.

>>And while I have your expert attention, what if I decided to do a 48" CC rangetop with a 6 burner and grill combo? How many cfms or squirrel cages?

OK no expert here but (assuming you stick w/VAH) I would do a 900cfm three blower unit(one 300 single, one double 300 unit, in two housings. IIRC that puts you into a 54"ish wide hood. Make sure your duct work will meet the requirements as provided by VAH.

Check your local building code pertaining to make up air and don't rely on the builder.

soooo,

if I have a 36" CC range-top....is the 2 squirrel cage set up 600 cfm good?

thanks

While a 600 cfm VAH dumped immediately outside may well pull 600 cfm (with all the house windows open), to claim the 900 value as an equivalent presupposes that all 900 cfm fans operated with more conventional hoods have the same fan curve and that all the conventional hoods have the same pressure loss versus flow rate.

The OP can be assured that if the house is closed up and there is no deliberate MUA, the actual flow rate will be less than 600 cfm. Does VAH publish their fan curves? If so, then the actual flow rate for any given negative house pressure could be estimated.

Just because a location doesn't impose an MUA requirement doesn't mean that there isn't an implied requirement. The need to avoid back-flow in combustion appliance exhausts is independent of politics. One way or the other natural MUA will happen. It will be supplied by open windows, or leaky house walls, or back-flowing chimneys if present and not isolated from the kitchen.

kas

OK, so I have no answers for easy peasy questions, but have a question of my own since we are talking vent hoods. We are going to put in a vent hood where we currently have none, so that means adding the venting. In my pollyanna dreams, it shouldn't be too big of a deal, but beekeeperswife says one word that gives me pause....expensive. We'll just need to vent to the ceiling and then make a 90 degree turn for about 5-6 feet to go out the side (behind is the laundry room so can't go out there, above is the master bedroom). Then I just pick a vent hood and voila, easy peasy, right? This should be a pretty inexpensive project, right (I am quickly realizing there is no such thing in the remodel world)? Who do you contact to install the venting and will understand all of cfms, MUAs and squirrel cages? Is that a plumber or HVAC guy?

>>soooo,
if I have a 36" CC range-top....is the 2 squirrel cage set up 600 cfm good?

IMO yes it is good. The best? We will never know.

bee... May I ask why you want a VAH instead of something similar to what you currently have?

FYI, I'm still so confused on what vent to get. :)

thanks john.com.

tncraft--I don't need the external mount motor this time, run is very short, practically none since it's on an external wall. Last one ran up into garage's attic and out. Plus, currently have the range and not just a rangetop.

Yes, it's strong, but it's loud. Would like something quieter this time. The Viking has great capture area too. But we are looking for a v.a.h. that has great capture area as well.

kellie, you should copy and paste your question into a new thread so you get some responses. Asking a question in the middle of a thread might not yield you the results you are looking for.

I am sure that you are right, but I feel like I have asked so much already of strangers on the internet so don't want to start too many threads. A search hasn't really revealed what I am looking for. I'll poke around other areas on the internet to see if I can find anything. Good luck with your vent though.

bee... Thanks! Hmmm... I did not even think about distance to roof/external wall. DUH! I don't want to hijack your discussion, so I may create a new post (effect of distance in the selection of blower setup) if I can't find satisfactory info in previous discussions. Thanks, again! :)

It would help knowing the total BTU's of the range. A rough ratio I was told once is to use 100 BTU to 1 CFM when venting outside. This is for gas only.

So for my 30" RCS BS, BTU's total 54k, so I have a 650 CFM fan in total. This would more than cover the total amount of BTU's if I was cooking everything on high seat, simmer and having the oven turned on.

As always check local code and seek professional advice.

Electric and Induction are more efficent, so the rating would be less.

I'm HIGHLY biased but I'm going to reiterate tncraft's question. I can understand the decision not to use a remote blower if the duct run is short. But why oh why a VAH??? Why not a hood with an internal conventional blower and baffles, rather than the (IMHO) miserable "squirrel cage" blower system in the VAH?

I speak from at least some experience, as we have a "two-squirrel" 600 cfm VAH at our lake house, as well as a Modern Aire hood with a remote Fantech blower in the attic at our home. I hate the VAH. It's incredibly noisy and the ventilation performance is highly underwhelming. And no, it's not my duct run (a straight run of 10'' duct, vertically directly to the roof, about a 10' run). If I had the wherewithal, I'd rip it out and replace it in a minute.

No internal blower is going to be near as quiet as our remote setup, but at least a conventional fan and baffles would offer the potential of better performance and MUCH easier cleanup. Why not something like this:

PS-15 hood

with an internal 1200 cfm blower? Now that would be a nice hood!

And to further solidify my reputation as a hood eccentric, here's an old post

vent hoods and noise

While a relationship between the thermal plume volume and BTU input into the plume can be measured and associated, the implied rule of thumb is more applicable to commercial griddles and such where the plume flow rate is relatively constant and the vent hood flow rate is relatively constant, with the hood and ducting sized for the cooking unit below it.

When considering residential cooktops, it is not valid to assume that some tiny BTU input into a tiny frying pan implies that a proportionately tiny hood flow rate will suffice. It would only suffice if the hood size, its flow rate, and the duct diameter could shrink and expand with the plume size and thermal input. This would in any case be a non-linear function of BTU input.

A residential hood requires a flow rate that is sufficient that plumes from any burner using any reasonable pan at any available BTU rate be captured and contained. This requires a sufficient air velocity at the hood interior aperture. Further, the velocity of the effluent in the ducting has to be high enough to minimize grease precipitation on the duct wall when passing through a cold air space.

Hence, rules of thumb that are hood size related and type of cooking related are better than those that are only BTU related. (Greenheck used to have some documentation on this at their web site.) As noted in previous commentary, I prefer to do the analysis on the basis of thermal plume velocity and hood aperture, based on the Finnish researchers study reports I've referenced in the past.

And once the required cfm is estimated, the pressure losses have to be estimated to determine what zero static pressure capability is needed from the fan to yield the desired cfm with the duct, hood, and baffle pressure losses and insufficient MUA that may be present.

To paraphrase tax advisor columnist Bonnie Lee: It is easier to untangle 5 miles of Christmas tree lights than answer an easy peasy kitchen ventilation related question.

kas

thank you

When you add up all the circumstantial evidence against VAH, it amounts to a lot of evidence. Why why go VAH?

The MUA will necessarily come from "somewhere" but no one on the internet has seen your house so no one will know where that somewhere is, in reality. Plan for MUA to happen. Plan a flow path. Air must come from somewhere to go into that canopy that is sucking greasy air out of your kitchen.

The canopy shape is independent of the motor which blows or sucks. Manufacturers like to sell a single combined product that they can ship in one box, and that you can buy and carry out in one box. Some manufacturers will sell canopies separately. Hint hint. Look into this please.

Blower motors can be purchased for a very small price. Small compared to the price of "combined" products that include a blower and canopy ("hood") fitting in one box. Canopies can be made by anyone who can bend sheet metal. A common skill, not rocket science. Hint hint. Look into this please.

Hth

bee -- I looked at VAH and after reading what it took to clean the squirrel cage I demurred. Have you read that? Can you deal? I didn't think I could and DH isn't handy.

My hood is Viking, 600 cfm with an electric smoothie. It's just ok and quiet isn't a descriptor I'd use. But, took the baffles out last night, put them in the DW and replaced them in a second. Snap.

There may be other brands that will do a great job with a high powered range top and be easy to clean. Plenty of brands cited, few complaints.

I agree totally with rococo: baffles are the way to go. A load in the dishwasher and you're good to go. As opposed to hand washing the "squirrel cages" (hand washed because of multiple posts complaining of the finish peeling off when they were put through dishwasher). It's one of my least favorite jobs, and thus it doesn't get done as often as it should. No question that any internal blower will be noisy at high flow rates: quiet is only really achievable with remote blowers, but from your perspective, baffles are the only way to go!

With all the disparaging comments about Vent A Hood ( presented as fact) one might think it is the incarnation of the Devil Himself.

You may even consider it foolish to purchase such a unit.

I find the Vent A Hood to be more than adequate.

I have had and cleaned both the VAH and baffle filters. I agree that putting the baffles in the dishwasher is easy but there is more to cleaning the baffle type hoods. The filter does not get all of the grease, in fact you might be suprised how much gets past it. Take out the filters and look up behind them and see how much grease is inside the hood and if you have the internal blower is on the motor and the wiring plus try to get the blower wheel off the motor. This is because most people run the motor on the low setting to keep the noise down and when they do this the filters do not work as well as they should. The air must be moving fast for the filter to remove the grease.I currently have the VAH and it is a snap to clean with the new easy clean tray they now have. This is my second VAH and the blower wheels come off easy enough and I just put them in the dishwasher, if the paint comes off them who cares it does not hurt anything.

If you are going to have the 48" range with a grill make sure you go with the 1200 CFM hood or you will be sorry. Indoor grills must have this amount of CFM.

Interior of my hood behind the baffles is immaculate. I get less grease accumulation in the hood and ductwork than I do in the ductwork above our VAH hood. Airflow is necessary for baffles to function optimally, but the same can be said for the purported centrifugal effect that flings the oil particles into the catch housing on a VAH.

I would agree with john.com that the VAH is certainly adequate. When we first installed ours, it was a huge upgrade from what we had before. It was only in comparison with the baffled hood with remote blower that we realized how much better than adequate we could achieve with a better system.

No issues with anything behind my baffles -- I checked that last night, in fact, before replacing them.

I think it's fair to tell someone to take a look at maintenance issues before purchasing any product -- that's why we're here. No disparaging or facts presented, simply a caveat.

Any demons are on/in john.com's mind/post.

Clearly you will have grease behind any filter as they are not 100% effective.

Some factoids about grease collection may be found in the link below. The limitations of filters is evident from the material starting on page 19. Compare the filter efficiencies plotted on page 20 with the hamberger grease spectrum shown on page 21. One may conclude that all the money spent on baffle filtering is really being spent on attractive fire blocking.

What isn't addressed is what happens to the grease not collected by the filter. For commercial enterprises, there may be a limit to what can be expelled from the building. This forces a multi-layered approach to grease collection, for which Greenheck steps up to the plate.

For residential systems, the question is what part of the unfiltered grease collects on the ducting? This is a function of duct temperature, air velocity, and duct turbulence (see next message). One hopes that the air velocity is high enough to avoid having to perform a duct cleaning exercise at some point in the future.

kas

Here is a link that might be useful: Greenheck catalog

For the effects of velocity on duct deposition, there is the link below.

I believe this analysis lead to some changes in NFPA 96 [?] allowing lower duct velocities in some circumstances.

kas

Here is a link that might be useful: ASHRAE analysis

To avoid being misunderstood, the fire blocking comment was partly a joke. Any filtering is good filtering.

It would be useful to know what filtering VAH achieve to see if the higher noise is accompanied by higher effectiveness.

I understand that NFPA 96 now allows 500 fpm velocity on the basis of the ASHRAE tests. However, the data in the massive test report is clear that if the ducting is cold and the air velocity low, vaporized grease will precipitate onto the duct wall faster with low velocity than with high. Particulates perform in the opposite way, so the 500 - 1500 fpm range is a bit of a wash.

In any case, the flow rate (cfm) has to be high enough for capture and containment. The duct diameter can be increased if desired to achieve the lower velocity (fpm) and resulting particulate collection on the duct.

kas

Maybe kas can answer my question about noise. I have always thought that the fan motors were very quiet, assuming the bearings are good and aligned with the spindle.

Even with the fan blades, if they are balanced the noise is minimal. But when it goes up into the housing and duct work, that is where the noise is created.

So I thought that the housing or ducts should be less flimsy. I know that double wall ducts for water heaters exists, I don't know if that would help. Maybe it would have a 'drum' effect and be worse.

I've even thought someone should try wrapping the first few feet of duct with neoprene and cable tie it in place!

Kas has some good points. It is my opinion that the VAH will have excellent fire supression even at low speeds because of the velocity of the discharge at the point of the squirrel cages. I don't think a fire could be maintained there.

After 4.5 years with the VAH I clearly have some grease buildup in the ductwork. Not excessive by any means, but a light brown coating that can be observed with the squirrel housing removed. I don"t know if a duct cleaning would serve any purpose at this point because the process (what ever that would be) would not be 100%, but again I have no idea what the degreasing process would be.

Baffle filters are used in commercial settings but they are wed with a seperate active fire suppression system not often seen in a residential setting.

In any event it's prudent to have a current, fully charged, easily accessible fire extinguisher in the kitchen.

Assuming a balanced motor and balanced fan blade assembly, and good bearings, then the fan-motor assembly will still make noise, duct or no duct, due to turbulence around the blades at the flow rates that the assembly is capable of. This is probably inevitable, even if the blades were designed and manufactured as airfoils with all the care (and cost) that goes into a turbojet engine compressor blade.

The ducting can amplify some sounds, but I would expect this to be at low frequencies with wavelengths commensurate with the duct length. One potential source would be motor rumble amplification, and our assumption above (probably optimistic) was that there wasn't any. It is also possible that the blade, duct wall, and duct transition turbulence noise could have low frequency components that are well propagated by the duct.

I have suppressed a lot of low frequency noise in my system by wrapping my 10 inch duct with the leaded plastic material used after-market in automobiles for sound damping (Evercoat Q-Pads). I supplement their adhesive side with very long tie-wraps to keep them attached. Low frequencies are less effectively removed by my Fantech silencer due to its manageable size.

My intention is to eventually (re)balance the fan assembly which I suspect is the actual source of the low frequency rumble. The Wolf (Broan) roof fan is a radial blade design and rather heavy duty in its construction (read thick blades with high inertia). I have some doubts that it was deliberately balanced, probably depending on casting tolerances to meet some balance spec.

To the question: I don't think a double wall duct would be helpful unless the gap were filled with lead wool. A magnesium duct would have high self damping, but more than a little hazard if there were a grease fire.

kas

Dynamat is another product for sound insulation.

Kas, thank you so much for your insight and answer. Not much to say because your posts speak so well for themselves.