fantech.net sells blowers that do 600 CFM and less. It is possible for you to buy from a distributor who has a business relationship with fantech, but not from fantech directly. As luck would have it, the company that bill102 linked you to above is a fantech distributor, and can get the fan you want for your custom hood. Hope you understand that this solves all your problems.

Actually hood ventilation noise consists primarily of two components - blower or blower mechanical noise and airflow noise (ignoring vibration caused from a cheap and poorly constructed hoods). A low quality blower will introduce mechanical noise hence one should always strive to install the best quality blower you can - each of those blower options below have specific areas of use - an inline blower mounted remotely will reduce immediate mechanical noise versus an internal blower in the hood HOWEVER an inline blower brings its own risks in terms of resonance (air waves from blower oscillations and minute vibrations) in a confined area. An external blower mounted close to the hood is about the same as an internal blower in the hood - one typically uses an external blower if you have a difficult duct run - sucking air from the end of the duct run along a convoluted and long duct run is far more efficient than trying to PUSH air from inside the hood via an internal blower along the same duct run. At the end of the day noise has mainly two components - poor quality blower that introduces excessive mechanical noise (there will always be some but this can be mitigated by using a good quality blower) and most importantly the QUALITY of the duct run - a duct run that is too small or multiple 90 elbows or has been crimped(dented badly at installation) OR using the ribbed aluminum stuff versus smooth HVAC galvanized ducting introduces a lot of air flow noise - of course the type and design of the filtration system also introduces airflow noise in the hood. By far the biggest problem we have found in over 20 years of custom range hood sales and installations is ducts that are far to small for the airflow generated by the internal blower - modern blowers (good quality ones) sense duct restrictions and run faster (more revolutions and therefore more mechanical noise) to compensate and delivery the requested volume of airflow - a simple mathematical calculation will illustrate this - a 6 " duct which everyone believes is adequate provides 28.28 sq inches of airflow capacity whereas an 8" duct supports 50.28 sq inches - nearly double the volume of airflow - so of course an 8" duct is far more efficient and is why we recommend an 8" duct over a 6" for 600cfm and 10" duct over 8" for 900cfm and 1200cfm. An added tip - another benefit of external blowers is that because they are far more efficient at moving air over distances and bad duct runs is an 8" duct is sufficient for say a 900cfm or 1200cfm external blower - qualification of course being a high quality external blower system such as ours - cant comment on the others out in the market - as always you get what you pay for......at Custom Range Hoods we are always willing to help and will assist anyone with technical questions - placing an order with us will of course open the door to a mountain of experience and good advice - feel free to email us at allenl@customrangehoods.ca.

In principle, any hood with an internal blower could be gutted of the blower and operated with an in-line or roof mounted blower. (GreenDesigns may have meant that using it while leaving the blower inside would be a poor plan.) I can imagine construction details that might cause non-optimal internal hood flow patterns, but I have no detailed knowledge of the internal construction of myriad hoods to know whether such are prevalent. There may be nanny jurisdictions that would claim that code approval of the hood was violated when it had its blower removed. Any hood available for order without a blower likely is fully compatible with an external blower. This, of course, can be confirmed with the manufacturer. The most important adaptation requirement is that the blower be continuously controllable, and that the hood control be a continuous motor control. A hood with just a few motor speed positions on a switch might require a compatibly-wired motor. While a multi-position hood control could in principle be replaced by a continuous control, par. 2 may apply. When I was doing my kitchen reno, I chose a compatible hood/roof blower combination (supplied by Wolf) because the entire scope of the project (gutting and reconstructing part of a house) made playing with the details of the blower circuit a side project of tertiary importance that I couldn't afford the time to deal with. As it happens, inside my Wolf (Independent) hood is a diac/triac motor control circuit, about the simplest design available that goes back to the '70s. The compatible Wolf (Broan) roof blower uses a typical induction motor that is easily controlled by this type of circuit. The cost versus time value trade-off needs to be considered when dealing with issues like this. kas

Generally, internal blowers that are capable of high flow rates will not be quiet, although they might be tolerable. The issue is turbulence from the fan blades. The closer the turbulence is to the ear, the louder it will sound. When the duct is short, little of that noise will be attenuated if the blower is moved out of the hood into the duct or onto the roof. Also, the air moving through the baffles will make noise due to turbulence, although in my experience (with a Wolf hood made by Independent) the noise is a hiss and easily tolerated. And yes, once you have determined the actual required flow rate (90 CFM per square foot of hood entry aperture recommended) then the rated flow rate of the blower can be estimated or analyzed. If the blower chosen has a large diameter blade design and is run at lower-than-maximum speed, there can be plenty of flow at the pressure loss that results from the flow. I was recommending an up-blast style blower (which would be mounted on a roof pedestal) because such blowers can be tailored to the user's needs. A catalog cover from Greenheck is shown below to clarify what an up-blast blower looks like. (You wouldn't likely need one the size the average restaurant needs.) A company like Greenheck could advise on model selection given a desired flow rate, likely pressure loss (also a factor of MUA design), and the goal of low noise. My only concern is whether there is enough room for a pedestal mount at the edge of your roof. Ultimately, you will need (a) a HVAC person who can provide the estimate of pressure loss and also aid your selection of a solution to providing make-up air, and (b) an intelligent installer who can read and follow directions, and has some skill at adapting installations to the existing architecture. I recommend reading at least the first dozen or so pages of the Greenheck guide: http://www.greenheck.com/media/pdf/otherinfo/KVSApplDesign_catalog.pdf