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

1. where do they go? Well, you have options. First of all, the idea is to bring outside air into the house. So they 'go' from the outside to the inside. On the outside wall you will probably need a screen of some sort/size to keep critters and insects out. Somewhere along the path you will need a filter box like the one you have for your HVAC unit with filters that you will replace from time to time. If you live in a cold climate, you will probably need an inline heater of some sort for those cold days. Now, some folks connect the pipe to the supply (inlet) side of their HVAC. This lets the HVAC cool and dehumidify in summer and heat in winter, and do so directly. Others send the pipe to a vent in the kitchen (or nearby) in the ceiling, floor, behind or under the stove. A 'damper' is something that opens and closes to control the air flow. For MUA, this is usually something (a flapper) that is either electrically activated from the exhaust blower switch or barometrically activated by measuring a low pressure in the house when the exhaust blower is on. The second one is passive. 2. do you already have one? Maybe. Maybe not. The codes for this are fairly recent and may not have been in place where you live in 2000. 3. Who installs? That's still up in the air (pun intended) somewhat. I think of it as an HVAC thing since it deals with duct work. It's also a bit more involved than exhaust fan installation and routing. The HVAC people should be able to do it as part of new construction. They may not be interested for remodeling since there isn't that much duct work involved.

Adam with that much CFM on the blower, if you stand to close you might get sucked out hahaha, only joking.One question did they use the right size ducting material that the manufacture recommend if undersized this could be the source of your problem.

Turns and length add pressure loss and thus decrease performance for a given blower. Perhaps not very much, relatively, in view of the baffle pressure loss and the MUA potential pressure loss. Where possible sweeping turns should be made. Aim for 1000 to 2000 ft/min duct velocity, but go for the low end, even under, if the bends have to be sharp. Duct velocity follows from achieved CFM, which should match the flow rate needed for performance. Recursively, one chooses a blower based on the anticipated pressure losses to achieve the performance needed for the hood ventilation system. Choose your hood to overlap the cooking zone, and taking its entry aperture area, design for 90 CFM per square foot of entry aperture. The blower rated flow then follows from the pressure losses (usually estimated) and the fan curves of candidate blowers. You haven't addressed combustion appliances taking air from the house interior. MUA must be considered in concert with the ventilation system as it affects the overall pressure loss that the hood blower has to work against. It is also sometimes a bigger "deal" than the hood system due to having to interact with the house architecture in even less convenient ways. And, worst case, it will need a control system more complex than the hood system.