I have a very short run to my external roof blower, I'd say 30" or so.
On high, it is pretty loud, I have no way of knowing for sure, but I have a feeling an internal blower on high would be even louder with the motor just above the baffles. Although, I rarely use high, usually low or no fan at all for most general cooking.
FYI: 1000 cfm on 8" duct, 36"x27" chimney style hood. Bottom at 36" above a 36" Bluestar 4 burner with griddle.

"... but I have read that on very short runs (this is about as short as it can get) it can actually lead to more noise over the internal."

It can, but not because external is intrinsically louder than internal. The external and internal fan/blower designs from one manufacturer may be different, and for the same rating external fans may or may not be tailored to the same noise standard as the internal fans. In the case of Wolf (who in my experience specify Broan/Nutone fans), they likely strive for low noise, but large cfm blowers tend to be noisy unless allowed to be large.

We know on this forum from various submissions, that external fans at the end of large silenced ducts can be very quiet. I would also assert that a 24-inch fan running at low rpm could move 1500 cfm with less noise than the Wolf external 1500 cfm fan on a 10-inch duct. But the Wolf is quiet enough compared to most peoples' past experiences with hoods that internal or external will not likely be a driver for a short-length duct.

The real issue with answering this topic is that few will have had the experience to compare internal with external for a short duct using the same company's offerings. (I certainly haven't.) Even showrooms may not be able to make this comparison unless they deliberately built both configurations.

kas

Thanks for the replies, thank you @ctycdm. For reference, the Wolf external 1200cfm blower is a rebranded Broan model 335 which I can get for $250. And I believe the 1500cfm is a Broan 336. So I wasn't factoring price in at all since that's just a drop in the bucket - in the "grand scheme" of the kitchen.

So if I were to go external, would it be better to just get the 1500CFM (obviously overkill for the application) reasoning that if it's physically a bigger fan, it can move more air at a lower rpm? Or would it be better to stick with the 1200 and have a little bit finer motor speed control?

Looking for anyone's input really, even if you haven't compared the two. If you have a small kitchen (mine is roughly 10x11) what is your setup and do you think it's quiet or loud?

This post was edited by Terrace1919 on Thu, Sep 4, 14 at 13:51

Terrace1919: In 2014 you were looking at the Broan 335 to mount externally to your Wolf Pro hood. Did you do this? If so, how did it work and are you happy with the results? I have a Wolf Pro hood currently with the internal blower and am looking to convert it to external, so I would be very interested in any comments you may have. Thanks!

While we wait for Terrace1919 to respond, I can relate that I have a Wolf Pro Island Hood with their (Broan/Nutone) 1500 CFM (rated) blower on the roof. Duct length from hood to blower is in the 15 - 20 foot range (never measured it). A section of the 10-inch duct incorporates a Fantech silencer. At full power (with MUA path, baffle and other parts pressure losses) flow rate is likely to be around 900 CFM actual through the 10 sq. ft. of hood aperture.

One can talk normally around and across the space under the hood. There is noise, mostly baffle hiss due to turbulence, but it is not annoyingly loud for cooking. My blower has some rumble that a re-balancing could help, I suspect. Some of my ducting wears automotive damping material to suppress any conduction of blower noise along the steel duct.

This hood fully captures the cooking plume from my 3500W induction wok located under the far end of the hood. Hood height above the counter is about 35 inches.

Blower control is via a conventional (at the time) triac/diac circuit above the middle baffle area on the controls side. If you replace the internal blower with an external blower, it would be safest, without evaluating the potential power capability of the existing motor control circuit, to use the same power rating external blower. Otherwise, uprating the embedded blower controller is another possibility, if Wolf is willing or you are an electronics experimentalist.

kas

Kaseki, had you tried your system without the fantech silencer? I am in the process of adding an inline blower to get more suction and decrease the noise level, and was not sure whether the silencer was worth it.

The ducting and heavy silencer went into place once (and hopefully for all time). Tearing out the silencer for the sake of noise experimentation was not even considered at the time, and I wouldn't consider it now. It would be an expensive project, or at least a long man-hour project if I did it by myself. Fantech publishes the noise reduction vs. frequency for their silencers. There is clearly a useful reduction above the low frequency cut off.

A silencer should help in your case also, unless your axial fan inline blower is large* in diameter and low in rpm.

Another case where a silencer might not be beneficial is in a system using a large* upblast commercial blower on the roof with the motor belt drive sheave ratio set to keep the fan blades low in RPM. Then the tip turbulence noise would be low, and the silencer's reduction in remaining tip noise and duct turbulence (above the silencer) might not be needed given baffle turbulence noise.

For perspective, a Casablanca type fan moves a lot of air relatively silently.

kas

*large in this context means capable, at full speed, of 5 or more times the required flow rate.

Kaseki, thanks, I did the installation myself, and it was a lot of work, and haven't decided whether to add the silence yet. I need to get the exhaust vent upgraded, and see how that impacts the sound.