Do you believe in magic?

Can this salesman explain why 600 cfm from one manufacturer is "equivalent" to 900 cfm elsewhere? For that matter, can the manufacturer explain this, or just assert it?

Some things just don't make no sense. One might argue that if there were a bona-fide explanation for this, we might have heard it by now.

I have owned two vent a hoods...current one is 1200 cfm....I really don't think its more efficient than any other 1200 cfm.

I don't think there are any standard ways of measuring how much airflow any of these hoods generate. I don't think they're giving you blower curves so that you can calculate how much air you're actually moving based on pressure losses in the ductwork, etc.

The things you can do that I think are important are to oversize. You're already putting a 36" wide hood over a 30" stove, so that's good. If there's an option to go deeper (i.e. wall to front lip), that's also good.

I think an external blower may be quieter. That said, the only Broan products I've ever been around have been awful bath fans. I know their more recent and higher end stuff is likely of higher quality.

FWIW, we have a VAH over a 36" range (6 years now), and it does the trick. That's with the 2 blower (one single speed, one dual speed), 600 cfm setup. The hood is 42" wide and 27" deep (no upcharge over 24" deep).

It does make noise. We don't do tons of high-heat searing, but we do break out the wok occasionally. It has handled that without any issues. Regardless of any marketing double speak in their blower airflow ratings.

This has been debated to death in this forum over the years. Here's my take:

vent hoods and noise: the real scoop on VAH"

Thanks for posting that link again, clinresga. For some reason, that thread cannot be found by doing a search on gardenweb, but can be found with a Google search. I noticed that when I replied to the thread, and it didn't show up as recently appended to.

The problem, besides using the term magic (a usage that should send everyone running away) is that all fans have a varying flow rate (cfm) with pressure drop across the fan. Most fans sold separately are rated for the cfm they move hanging in the air (zero pressure drop). A quality fan manufacturer will measure and supply the "fan curve," a plot showing the relationship between cfm and "pressure loss".

When a hood is sold with its fan, the flow rate will necessarily be less than for the fan hanging in the air. (A factor of 1.5 is a bit bold, though, in my opinion.) Very likely, if they aren't lying, VAH is claiming that if the fan were unrestricted, it would achieve 900 cfm flow.

Of course, a 900 cfm Broan fan, put in series with a hood and reasonable ducting, might only pull 600 cfm also. However, I don't think VAH is counting any ducting, so comparisons are difficult. If VAH are tying to claim that their fan design has a more "robust" fan curve due to the blade design, then they should supply a fan curve that supports that intent. I doubt that a squirrel cage design is as good for that purpose as a backward curved blade axial fan, but I've been hopeful that VAH would some day provide useful information. (I'm also hoping to find a winning Megabucks ticket.)

I think it best to consider other factors such as noise, cleaning ease, etc. VAH is often touted by appliance sales persons, and one wonders whether the mark-up is higher for VAH than for competing suppliers.

kas

(as always) I agree with kas. There is nothing objective that I can see with my VAH that would in any way justify the kind of fawning that appliance sales guys always seem to do over VAH. It was only with kitchen redo #2 and the advice of GWebbers like kas that I realized that there were far superior options. I'd love to discover the secret of how VAH gets all these dealers to tout them so highly, but like kas says, it's likely an issue of $$$.

VAH does publish their specifications to some degree. The 600 CFM rating is SP@ 0.0, SP@0.1 =531 and SP@0.2 = 480 and SP@0.3 = 430. They use an 8" duct.

Looking at a Kobe premium hood, they only give the SP@ 0.0 rating of 720CFM for their 30" wall mount. However it is feeding into a 6" duct so that drops the effective CFM by about 20% and the baffle filters drop the CFM by about 20% Thus effective CFM could be as little as 432 CFM.

Looking at Imperial, they feed into a 7" duct and claim 750 CFM but also state that the baffle will drop the CFM by 20% meaning an effective rating of 600CFM

As can readily be seen, there are many factors that will influence the actual CFM achieved. The size of the ducting is a big one. How far it has to run is another. How many turns and at what angle is yet a third factor. Every 90 degree bend will drop the flow significantly. It would appear that VAH's claim of about a 30% effective improvement over baffles or screens is probably not too far off when combined with their use of an 8" duct.

Thanks barjohng for the VAH data. I have fan curves from Wolf (for what I believe are Broan fans, maybe Wolf spec) and from Broan's web site. Looking for something as "wimpy" :) as 600 cfm among these curves leads me to the RM325H, which is 610 cfm at 0 inches. Its fan curve reaches zero flow at 2 inches. At -0.3 inches, the flow is about 575 cfm. There are other examples with different curve shapes. Some are rather "sloped" and some are more vertical. The latter are best for tolerating pressure loss. The fan curve shape is related to blade design and leakage around the tips.

Contrary to one's intuition, working against pressure loss doesn't necessarily further load the motor; fan motors actually do less work moving less air. How much less varies with blade design and which side of the fan is being restricted, because blades still move air even if it doesn't get out of the duct.

I'm not sure 0.3 inches is a good place to compare, as it won't take much restriction on either side of the fan to cause that much pressure loss.

kas

I've never heard anything as quiet as a VAH Magic Lung, and they are EASY to clean. This comment is after USING mine daily for the last 7 years. Definitely worth the money if you like to socialize with your guests while cooking. If you don't want to be able to hear yourself think, and don't care about efficiency and cleaning ease, then you'll be happy with a box fan.

If one can configure a VaH Magic Lung hood such that it has low pressure loss ducting on the output side, and low pressure loss make-up air on the intake side, then an equivalent actual CFM baffled hood will require a higher zero static pressure blower rating, and if this blower is installed into the hood rather than on the roof with an intermediate silencer, then I would expect it to be noisier.

Where pressure losses are higher, the VaH is disadvantaged, and conventional designs can be fitted with better performance.