Need recommendation for high CFM vent hood for 6"duct

fran415December 13, 2012

I live in a condo with, sadly, only 6" ducting.

I have a pro style range and the zephyr cyclone hood (650CFM) I have is just not cutting it. I am trying to find the highest possible CFM hood that will be compatible with 6" ducts, as I can't change the diameter.

So few sites allow sorting by duct size, and some don't even mention it...

I'm trying to get as close to 1000 CFM as possible...any insights?

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Not going to happen. It's physically impossible (well almost) to move that much air through a 6" pipe.


get a straw for sipping smoothies or milkshakes and one for stirring cocktails. Now blow through both. Which one is easier and can move the most air?

    Bookmark   December 13, 2012 at 6:06PM
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I'm surprised that 650 CFMs isn't sufficient. I have a 600 CFM hood with 6" ducting and I find it more than adequate - in fact I mostly run it on low, occasionally on medium, and rarely on high. Are you sure your fan is operating correctly? Kobe makes some hoods that use 6" ducts but I think they only go up to 700 or 750 CFMs.

    Bookmark   December 13, 2012 at 6:36PM
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The zephyr is a big improvement over the microwave/vent combo that came with the place!

The fans are working properly, but we're replacing the cabinets so I have a chance to fix a number of things to improve performance:

1) what I have is only 30" wide above a 30" range-I'll increase to 36"
2) what I have is only counter depth-I'll increase to full depth
3) i can't change the vent size, nor the distance from the roof-we're on the 3rd floor of a 4 story building and the vent does make a right angle which impacts performance. So the best I can do is get the highest CFM possible.

All these factors might explain why I don't get the same results as you with my current CFM. Thanks for the reference to Kobe. Highest CFM I have found so far...

    Bookmark   December 13, 2012 at 7:21PM
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A fan designed to produce a desired cfm at a specified pressure will achieve what you want. Whether it is available from residential kitchen venting suppliers is a question I can't answer. Possibly Fantech can answer it.

In the commercial world, fans such as the one with the fan curve at the link can push 600 cfm through a restrictive path having nearly 2 inches of water column pressure drop. I anticipate that this is more drop than a 6-inch duct at 600 cfm will cause, even including baffle loss. But I haven't tried to calculate it. This fan might be a tad noisy.

It is a centrifugal fan and not the shape you might want for inline use, so this is more an example of flow vs. pressure vs. input power. An axial fan might be a better fit.


Here is a link that might be useful: Ametek/Rotron example fan curve

    Bookmark   December 13, 2012 at 8:02PM
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There is no limit as to how much air you can push thru a fixed diameter duct. It all depends on how much pressure a fan can develop.

Typically a hood uses induction AC motors, which by definition are synchronous frequency dependent fixed speed motors, regardless of HP power. With a fixed speed motor there is not much you can do to increase pressure.

If you have the electrical skills, you can replace the induction motor with a universal motor, which has a much higher RPM and power. This will give you much higher pressure to suck air and push air out.

Another strategy is to install an in-line (in duct) booster fan.


    Bookmark   December 13, 2012 at 8:04PM
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I have just been doing some tweaking of my ventilation setup which caused me to have a very useful conversation with a knowledgeable person at Prestige yesterday. The Prestige hoods have fabulous capture area. Their 600 CFM hood is rated for a 6 inch duct. If it were me that is the hood I would go for as I don't believe there are many higher end/pro type hoods that specify 6 inch duct. Certainly even at the same CFMs I am sure it is better than any Zephyr or similar hood.

Good luck!


Here is a link that might be useful: Prestige Hoods

    Bookmark   December 13, 2012 at 9:50PM
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Zephyr cyclones are thin, flat bottomed vents which are good for tight spaces because they are only about 5-inches tall. I have a 36" model over a 30" pro-style gas stove venting through 6" ducting. With twin fans, it has been adequate. However, the lack of height and the flat bottom constrain its effectiveness. With you having only a 30" wide hood, I would imagine the constraints would be more significant.

You mentioned that you will be getting new cabinets. I suggest you check further on the Kobe hoods you mentioned. There are many favorable postings about them here. I believe the largest-capacity ones in 30" and 36" models have a a 700 CFM fan. That might well prove fine for your space for most of the time. If you can adjust the new cabinetry to allow for a 36" wide hood, it would be even better. Of course, the 36" hood would be a little bit off-center for a straight run into your existing ducting. If you can mask the ducting with cabinets or a some kind of drywall box, the angle could be concealed.

If you really want a 1000CFM venting, I think Kas is right that you will probably have to go with something custom-built.

One thing you might check is how freely the baffles move. There will be one immediately above the Zephyr and another one where the Ducting emerges to the outside. Over time, these things can warp and stick, which really impedes the effectiveness of the venting. With you being on the third floor, it might be hard to get a clear view of the outside vent. Maybe binoculars? RUn the hood and see how well the outer baffle opens up.

Another thing to check is make-up air. No hood is going to wrk very well without sufficient air. If you open a window on the other side of your condo and put that Cyclone on high, does it vent better? If so, a make-up air system may be advisable. Depending on where you live, regulations and codes might require make-up air for any hood drawing more than 400 CFM.

More to think about.

This post was edited by JWVideo on Thu, Dec 13, 12 at 23:54

    Bookmark   December 13, 2012 at 11:53PM
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"Typically a hood uses induction AC motors, which by definition are synchronous frequency dependent fixed speed motors, regardless of HP power. With a fixed speed motor there is not much you can do to increase pressure. "

Actually, an induction motor by definition runs SLOWER than synchronous speed. At synchronous speed it does not develop any torque and slows down. The slower it runs from synchronous speed, however, the higher the torque, down to the point of not moving at all which gives the highest torque.

With 6", you're stuck with what you have. I think increasing the capture area is a good idea and the simplest way to go.

    Bookmark   December 14, 2012 at 12:18PM
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In addition to weedmeister's correction, let me add the following:

Induction motor unloaded speeds are set by design features such as the number of poles.

Flow vs. pressure loss is a function of blade design and motor power. In the extreme, a Rootes type blower can push high flow against several atmospheres of pressure. Examples of Rootes blower use are large Ingersol-Rand compressors and drag racer engines.

Don't use a universal motor. They are noisy in about every way it can be defined. They are good for power saws, though.

In any case, 1000 cfm through 10 ft of 6-inch duct should have a pressure loss less than 2 inches of water column, so it is a matter of finding a fan that can achieve that capability. It won't be inexpensive, me thinks, and it won't be very quiet moving all that air through a small duct, although it might be tolerable on the roof.

As pointed out earlier, typically one wants to use 8 to 10 inch ducts for serious cooking ventilation with affordable fans.


    Bookmark   December 14, 2012 at 6:37PM
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Kaseki is absolutely right, an 8" or 10" duct is the best choice for cooking ventilation.

However, in a condo you don't really get to decide. As a matter of fact, you're one of the lucky few that actually has a duct, many condo buildings ignore kitchen ventilation completely. Even though the duct is undersized, it could be worse, you may not have had a duct option at all.

Considering the duct size restriction, take a look at the brand Futuro Futuro. They're designed for 6" ducts and rated at 940 CFM. Kobe has a lot of favorable feedback on this forum as well.

Regarding make-up air, in a condo situation specifically, it's a good idea to find out how much air the building's ventilation system is rated to supply, and how that relates to the need for a make-up air system.

    Bookmark   December 27, 2012 at 2:06PM
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