The 2009 code in many cold weather states require MUA systems for over 400 CFM M1503.4: "Exhaust hood systems capable of exhausting in excess of 400 cfm shall be provided with makeup air at a rate approximately equal to the exhaust air rate. Such makeup air systems shall be equipped with a means of closure and shall be automatically controlled to start and operate simultaneously with the exhaust system." "Makeup air required: Exhaust hood systems capable of exhausting in excess of 400 cfm (0.19 m3/s) shall be provided with makeup air at a rate approximately equal to the exhaust air rate. Such makeup air systems shall be equipped with a means of closure and shall be automatically controlled to start and operate simultaneously with the exhaust system." A2 is strictly enforcing this. I decided to go induction and stay under the 400 CFM. I was leaning toward induction and this just tipped the scale. I am probably going with a Broan evolution hood @ 350 CFM and 4 halogen bulbs using theater lighting. The zephyr genova is very discreet, but concerned that the single fan on the left might not do the trick (only 250 cfm). The ventahoods have a true HEPA filters. I figure A2 is pushing me into the future. If you go too high on CFM, there are dangers, so I plan to avoid the problems! Good luck!! PS - I plucked the Wolf induction on sale. Love the appearance of the surface and had no trouble with the controls - I used my knuckle to tap the power up and down when my fingers were slimed! I liked the large burner and the set up of the burners and only one cook as my husband only knows how to order pizza and not cook. I only tested it at the appliance dealer, so can't wait to the kitchen is done and I actually can really test it.

marthavila: sounds like a good decision based on good advice. All the cfm junkies on GW are nodding their heads. Both changes will reduce noise: the duct upsizing reduces the airflow turbulence, and as you note, the larger blower may run at a lower speed with same cfm output as a smaller unit running harder. The latter is particularly true if you do have a continuously variable fan speed control so you can "fine tune" the blower to what you're cooking. I have a somewhat different take than trailrunner however, on the issue of airflow speed and grease handling. In general, the concept of a well-designed hood is to trap the grease before it enters the ductwork. Once it's in the ductwork, all is pretty well lost, especially if the duct run is at all long. Much of the grease will end up on the duct walls, never to be removed again. As a result, different hoods use different systems to trap grease. There is the Ventahood "squirrel cage" system, which uses a centrifugal fan that theoretically flings the grease particles into the fan housing, where it is trapped, while the air continues up into the ductwork. There is the mesh filter system, which is similar to what is used in most (barely useful) recirculating hoods and UC MW/hood units. The air flows through a metal mesh that is supposed to trap the grease particles. Then there are the baffle systems. These use a system of alternating U-shaped baffles that force the air to make two abrupt 180 degree turns before heading out the duct. At each turn, the grease particles that fail to negotiate the tight turn are trapped by the baffles (and shameless bias--the polished stainless steel baffles on our new MA hood are gorgeous! Almost too pretty to hide under a hood). High airflow speeds likely do not increase grease trapping efficiency: I could imagine that overly high airflow might even facilitate more grease making it past whatever filter system is being used. The reason that high cfm are desirable is not so much to whisk grease out the ductwork, which is unlikely, but to exhaust as much of the fumes/smoke/odor/grease from the temporary confinement in the body of the hood before it can spill back out of the hood into the kitchen. The combination of large capture area, a deep hood, and a high cfm blower should give maximum f/s/o/g evacuation. But totally concur with trailrunner's advice about starting the hood early and letting it run afterwards. Definitely will help.

If the cooktop or range is against a wall, then the amount that the hood aperture extends past the burners front-to-back should be commensurate with the amount the hood overlaps the burners side-to-side. This is because the rising effluent expands equally in all directions as it rises. (A powerful hood will cause some distortion, but nominally and for the most part practically, the hood air flow does not "reach out" and suck in air that already has a velocity that dominates the air velocity of the hood intake air away from the actual aperture.) The best insertion of make-up air (MUA) is any method that doesn't cause any cross-drafts at the hood. In test kitchens, this is done by introducing MUA through porous walls. We can generally exclude this approach for residential kitchens. Floor, wall, or ceiling registers (diffusers are what you want for air flowing out, but are not really suitable for floors) are all potentially suitable options. Mine is a ceiling diffuser located down a hall that leads to the kitchen. This provides some flow straightening and area expansion as a means to minimize cross drafts. Cost of heating MUA is not high, but the assembly of parts needed may be high in cost. Unless you operate your hood 24/7 boiling off maple sap, you likely will only be running the hood at partial to full power for short durations. I have a heat exchanger hooked to my hydronic heating furnace that can utilize all of its (forget the exact number) probably 180 kBTU/hr output if needed for MUA heating. I think the furnace at this rate uses 1 point something gallons of oil per hour, so this high cost is around a few dollars an hour for usually less than an hour when the exterior temperature requires the maximum temperature correction. In any case, I believe that an electric heater in the MUA path will be an easier thing to integrate. Cost per hour may be higher in proportion to your electric rate's higher cost (per BTU) over oil. You may have to run significant power from breaker box to heater if you are going to make-up very high flow rates through the hood at -20F. (Fireplaces can have high flow rates that have to be made up also, but I don't know anything relevant about the flow rates of gas fired fire places, so that should be something you can find out for planning purposes.) If you are rugged enough, the MUA heating doesn't need to be perfect in temperature control, just good enough to keep the introduced air tolerable during the cooking. Your code requirement doesn't provide for different conditions needed for safety, it just mandates MUA. The safety requirement that should be driving a more sophisticated MUA requirement, applicable where combustion appliances are used without their own separate MUA sources (ducts to outside, operation in a closed room ventilated to outside, or whatever), is to keep the internal house air pressure high enough to not back-draft the appliances, and thus avoid pulling carbon monoxide into the house. For unblown gas appliances, the house negative pressure limit is 0.03 inches of water pressure, and requires very low pressure loss in the MUA system with a powerful hood. I think this is necessary for fireplaces also, as they depend on the draft for flow, and the draft depends on the pressure difference between hearth and top of chimney, given hot flames to provide upward momentum. All this means is that you can either provide a code-compliant MUA system that is rated for a given flow rate, and for safety do whatever is really needed to safety proof your combustion sources, or provide an MUA that is sophisticated enough to keep the house pressure up over the range of ventilation flow rates possible with the hood, fireplace use (potentially high flow rates themselves), bathroom fans, etc. While I am working toward a closed-loop pressure controlling MUA system for myself, I am certain that for most needing MUA, isolating appliances is the better cost approach, allowing the MUA to aid hood blower performance, and not be so critical for safety. My oil burning furnace does have its own MUA, but I wouldn't operate my fireplace with my kitchen ventilation running at full power, even with windows open (due to pressure loss in the screening), without a powered MUA path. Can't help with an educated HVAC company, but FanTech has a line of powered MUA systems (don't recall if they are heated). If not, a suitable heater can be found and inserted into the duct path. Someone on this forum back in the 2008 to 2010 period, possibly clinresga, provided an image of his basement ducting with coil electric heater in the path. My [not yet completed] system is in my attic and draws air from above the roof. A blower will push this air through a filter, through the heat exchanger (think car radiator), and into the diffuser. HVAC companies dealing with restaurants will understand what you have to do, but often their setups are not compensating -- flowing a fixed MUA flow rate against fixed vent hood flow rates. Large building MUA is usually compensated, but the schemes used are better suited (as far as I've been able to learn any details) for continuous pressurizing MUA systems that have to compensate for outside door openings. They can do this with passive air pressure control of dampers that recirculate MUA around the blower. The blower motor runs continuously. The relatively recent of introduction of residential MUA requirements has provided some impetus to development of residential MUA systems, so availability of systems and components should be expected to increase. Keep reading; be sure to check out the Greenheck paper available at their website: KVSApplDesign_catalog.pdf, as well as CKV_Design_Guide_2_031504.pdf, available somewhere on the Internet. These are commercially oriented, but should provide useful insight. kas

I don't have equipment to measure the pressure differences but I have done some qualitative tests to evaluate the velocity of air that comes through a known opening (small window) for H/M/L blower settings as well as the velocity of air exiting the blower exhaust. What I found is that the pressure difference between the kitchen/dinette/family room space is greatest on the LOW blower setting and less on HIGH and MED settings even though the volume of air exiting the house from the exhaust is H/M/L as you would expect. I attribute this to the lack of MUA on the LOW setting due to the closed MUA damper. The hood flow I am perfectly satisfied with although it bothers me that the low setting is lower than designed because of the lack of MUA. I believe replacing the airflow MUA switch with a simple current sensing switch will be a good solution. We live in the PNW with a fairly mild climate with low temperatures seldom below freezing and never below 0 degree F and our furnace does not seem to have any trouble keeping up with the colder MUA inflow so far (although we have not had many cold days yet). opaone: Our MUA damper is a Broan MD10TU with a motorized damper triggered by a remote flow sensor switch. Thanks for the input on interlocking the MUA with the blower - I had thought that would be a good idea for MUA performance but not considered the static buildup aspect. The 6" MUA damper originally installed with the furnace is interlocked with the blower so I guess I better research how that works. And thanks for the kitchen compliment.