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

Rule No. 1: Commercial ventilation is motivated by electrical cost to be the most efficient design. The more a particular hood concept deviates from commercial design the less efficient, and/or less effective the hood will be. This means that baffled hoods mounted over the cooking area and large enough for the height of their entrance aperture to capture rising expanding effluent plumes are best. Commercial ceiling collection schemes involve an entirely perforated ceiling with a plenum above that is kept clean by intense ultraviolet light. You might be able to do this by putting in a false ceiling at 8 feet. However, I don't know how all the ozone that is produced is dealt with in practice. I can guess that kitchen drudges have to clean the ceiling often. Rule No. 2: Everywhere the rising effluent plume reaches needs to have an upward air velocity commensurate with the plume velocity to achieve capture and containment. The slots can only have a small effective range of capture. Rule No. 3: Noise is proportional to air velocity when it is constrained, and using perimeter slots with sufficient air flow (at least 90 cfm per square foot of area of all the possible plumes at the ceiling) should be a real screamer. Most likely, this air flow is not reached with slots, because the pressure loss would be so high that an axial high pressure blower would be needed, and these (typically used for aerospace electronics applications) are only quiet in comparison to jet engines. Rule No. 4: Downdraft capture and containment can only be effective if the air velocity from the downdraft -- at the distance to the furthest edge of any effluent plume -- is sufficient to redirect the effluent to the downdraft screen. Generally, downdraft systems are neither high enough or great enough flow rate to achieve this when hot griddle cooking can achieve an upward plume velocity of 3 ft/s or so, and the air velocity from any aperture including a downdraft aperture drops off very rapidly with distance. You would have to only griddle close to the screen. Overarching Rule No. 5: You get your choice of only two of: Economy, Performance, and Aesthetics. If you want an island or peninsula cooktop with adequate ventilation, you will have to use a sufficiently large island hood, and accept it being a point of (possibly commanding) visual interest. This often means that some expense beyond that needed for functionality will be required to achieve an acceptable appearance. Sight lines should be considered to establish height, from which one can derive necessary aperture size. Note that commercial hood apertures are typically mounted at 7 feet above the floor, but the hoods have significant cooking zone overlap to capture the ever-widening with rise distance cooking plumes. Drafts should be considered as they can move the plumes in some direction causing loss of capture. Rule No. 6: The only air and effluent flow rate that you can move from a hood to the outside is an air flow rate that the internal pressure drop in the kitchen allows to flow into the kitchen. Normally, this is restricted by walls and windows to house leakage. House leakage is inadequate for high performance cooktop ventilation. Further, depressurizing the house can be hazardous if there are combustion appliances in use. To achieve even 2/3 of the rated flow rate of the blower used with a large hood, a make-up air system will be required. See numerous topics here and in other Internet sources for more information. kas

This forum is full of thoughts on hoods, including ceiling capture. Some searching would be advantageous to understanding the issues. Cooking plumes rise and expand. If the capture area is within about 3 ft of the pan, the hood has to extend only several inches beyond than the extent of the potential cooking pans. Nominally, most of the plume is within a cone that has sides sloping at 10 degrees to vertical. However, the higher the capture area, the larger it has to be to accommodate the expansion. Also, the plume momentum slowly drops, and becomes ever more susceptible to cross drafts and room air motion. Commercial cooking hoods are typically at 7 ft from the floor, and are quite large relative to their cooking zones. At 8 ft they would have to be even larger. At some point, perforated ceilings are used because the plume effluent can drift too far to depend on a hood cavity for capture. http://www.kitchen-ventilation.co.uk/heydal Normally in hoods, the baffles are used to collect grease, or more accurately, the larger end of the grease particle size spectrum. A register at the ceiling will pass most grease into whatever ducting is being used. In deliberate ceiling collection systems, means are provided for grease protection (fire protection). One such method is intense ultraviolet. In the range of 30 inches to four feet above the cooking zone, hood capture areas need a flow rate of about 90 CFM/sq. ft. to assure that the captured effluent will remain within the hood (be contained). While this may be relaxed slightly at 5 or more feet, the large size of the capture area required will still require a growing flow rate (with) ceiling height. On your more prosaic questions: You may use an in-line (axial) blower or an external radial blower, either on the side wall or roof. If a silencer is placed between the blower and the hood, blade tip turbulence and duct wall turbulence from the portion of the duct towards the blower will be reduced. I don't own the Miele so I can't comment on its potential other limitations or its advantages.

All of the above may be true, but are not in themselves totally limiting. In general, there is always a blower that will move a desired air flow volumetric rate through a given duct, but one might not want to operate it, listen to it, or pay for it. But in this case we are on the margin, and feasibility is not prohibited using conventional devices. For example, I have a Wolf/Broan 1500 CFM rated roof-mounted blower operating with a 10-inch duct, and given various estimated pressure losses, expect that it moves 900 - 1000 CFM. A 1000 CFM rated typical hood blower, no matter how large the duct, at a minimum will be significantly restricted by the hood baffles, and at a maximum by any added MUA pressure losses, and would be unlikely to move more than 700 CFM in use with a hood. An 8-inch duct has a sectional area of 64% of that of a 10-inch duct, so the pressure losses operating at 67% through an 8-inch duct of the air flow rate through a 10-inch duct will be similar. In general, one wants to select a blower having a fan curve that supports the desired flow rate at the pressure loss that one estimates is present. Without my looking up the specified hood, let us assume that its entrance aperture is 42 x 27 inches or just under 8 sq. ft. This calls for 8 x 90 CFM/sq. ft. or 720 actual CFM. A typical 1000 CFM blower may achieve this with a 8-inch duct, and one certainly can check the assumptions against blower fan curves (where available) to either ensure adequacy, or at least force the MUA system to be active and not cause significant pressure loss. Note that this doesn't violate your appliance guy's opinion about duct flow rates, although I am sure that his view is based on typical in-hood blowers. If you have the room in some part of the duct path, then including a silencer will help reduce noise back at the hood for only a small added pressure loss. In any case, numerous commercial kitchen ventilation blowers can be found that will pull 720 CFM against any pressure loss likely from a well designed hood/MUA setup. It is quieter to move a given flow rate when the fan blades are large and moving slowly than small and moving quickly, as the latter generates more high frequency noise from turbulence. Induction motors can be controlled by rheostat adjusted phase circuits, and these can be put into hoods. My Wolf hood has such a control, and now the technology allows for it to be performed with better techniques at low cost. These comments are meant to touch on most of your questions, but feel free to delve deeper as needed. kas