@ Fred S: 1.) We manufacture range hoods, we're not HVAC / ventilation contractors. If a local building code requires a make-up air system, one can be installed in conjunction with the range hood. However, just as Lamborghini sells cars but will not get involved with decorating your garage, we don't get involved with make-up air systems - leave HVAC matters to HVAC professionals. We're aware of code requirements, which are stricter in some places (Minnessota, Vancouver, and Miami come to mind), and we do mention the possible need for make-up air systems during sales consultations when one of the "known restricted" areas is involved. However, the final decision to use an MUA or not rests with the design/build professionals *at the location* - there is NO possible way for us to know every code requirement in the 50 US states + 1 protectorate, 10 Canadian provinces + 3 territories, and the 31 states + 1 federal district of Mexico. Again, if the topic does come up in the conversation, any of our sales representatives will recommend the customer to consult with their local HVAC professional. 2.) A make-up air system (or, for that matter, any other electric HVAC machinery) can be connected to a range hood by the means of an airflow sensor (installed in the duct), or a current-sensing switch attached to one of the wires between the blower and the electronic control module. When the hood is operational, the airflow sensor / current sensor detects the moving air / voltage in the wires, trips the relay, and activates the make-up air system. This way, neither the wiring of the range hood nor the wiring of the MUA is affected, so the warranty is not voided, and the possibility of damaging either component is minimized. In addition, having a separate "connector" between 2 mechanical items allows for easier troubleshooting & service. 3.) Regarding the actual CFM: range hood blowers are rated under zero static pressure conditions - there is no hood body, no filters, no ductwork, no dampers, no duct cap, etc. Once the blower is installed in the hood, and the filters & ductwork are added, the *final* CFM is going to be a lot less than the "laboratory conditions" rating. Besides that, for the majority of cooking conditions, the user will not activate the highest speed - according to customer feedback, most people use the 2nd speed while cooking, sometimes the 3rd speed, which is far below the maximum rating of 940 CFM, and, combined with the power loss from the ductwork, ends up being below the MUA requirements in most places. The *occasional* increase to 3rd or 4th speed, for a minute or two while something is smoking up (i.e. fresh veggies tossed into a pre-heated wok = 1 minute of heavy smoke, then it settles down), does not unbalance the air pressure nearly enough to warrant freaking out & kicking the MUA pump into high gear. ----- Tl;dr version: 1.) There is a need for make-up air systems, but MUA requirements depend on local building codes - consult your local HVAC professional. 2.) Due to friction loss in the filters & ductwork, effective CFM is going to be less than the rated CFM, on ANY range hood, regardless of brand & type. 3.) You're not going to always use the highest speed, so make-up air requirement may not be as critical in the real world as it sounds on paper.

@salmonmaki: the manual's statement of a 90-deg elbow being equivalent to only 5 linear feet is very, *very* optimistic. We prefer to err on the side of caution (with 15-ft equivalent), but, this being the Internet, and you should never believe a *single* source for anything ;) - here are some alternative sites that also show numbers in the 10-12-foot range: Inspectapedia - http://inspectapedia.com/BestPractices/Kitchen_Vent_Fan_Specifications.htm Emerson Swan - http://emersonswan.blogspot.com/2013/02/static-pressure-bathroom-ventilation.html Wood Magazine - http://www.woodmagazine.com/woodworking-tips/techniques/dust-collection/figure-dust-collection-needs/?page=3

Thank you all for your comments @wsea: Thee installation instructions say that the safety clearance between the cooktop and the bottom of the hood may not be LESS THAN 24" ... no other constraint. @deborabutler: I have extra tile, so replacing a row of tile is not a big deal. thanks for your suggestion re height ... appreciated! @ patriciacolwellconsulting: With the contrast tile as a given, any suggestion? To your eye, would rehanging the hood a few inches higher make a bad situation better, and if so ... do you vote for 33.5" (1 row of tile), 36.5" (2 rows of tile) between the cooktop and the hood, or some other height??

A downdraft is an exhaust fan that is either integral to a cook top or located along a strip behind the cooktop surface. There is a large metal frame under cabinet that houses this unit along with the duct work needed to pipe out exhaust. It performs same as a exhaust fan above a range, but not quite as well. Generally, above vents are located on exterior walls and the duct work and exit vents are therefore easy to install. If it is a "downdraft" for new construction, you engineer the duct work into the slab before you pour that slab/foundation, all the way out to an exit point. It can be done on an inside wall going having the duct work (made specially for going between 2x4 framing) going up through the wall on up to a roof exist point or some other outside surface. The longer the run, the poorer the performance. You can get a "booster" fan installed along this run to improve the flow. More money, etc. If you have a basement, the duct work can be run down (with properly rated fan) to an exist point. There are lots of other technical details like number of elbows in the duct run etc., but not important to this discussion. It is easier "retrofit" with a basement.