The Broan device will close a damper when the exhaust fan is off and open it when the fan is turned on. When the damper opens you'll have the exact same outside air supply to the living space that you have now, I don't see why there would be any change insofar as the backdraft condition is concerned. I'm just a fellow homeowner and not an expert, but it seems to me that If you had better ventilation to and closed up the mechanical room (to isolate it from the rest of the house), the issue would be over. It's good you were alert enough to recognize the problem, you need a competent person to end your conundrum. I don't think the solution is all that complicated, It may be that a carpenter or handyman is more useful to you than an HVAC guy. Find out what your local codes require before you do anything. Remember that air is dumb - it doesn't necessarily follow the path you want it to, it follows the one that is shortest and offers the least resistance.

This is an issue that requires some measurements and some analysis. To even know where you are you need to measure the pressure in the house relative to outside the house as a function of different settings of vent air flow and MUA air flow. Either hire someone to make the measurements or buy (cost is fairly modest, as I recall) a differential air pressure device. In test kitchens, the means of introducing MUA is via a perforated wall some distance from the test stove/vent air handling equipment (hood). What is desirable is that the MUA, when it gets to the air volume between the cooktop and the hood entry aperture, be relatively non-turbulent. Introducing MUA at one's feet next to the stove may or may not achieve this. Often the toe kick spaces there are too small, and may, depending on configuration, aim at some other structure (wall, island, whatever) that will force the air flow up. Now there is turbulent air all around the cook and the hood. I think the MUA exit area(s) should not be too much smaller than the hood entrance area unless the MUA injection point is fairly far from the hood. In a (completely burger odor free) burger joint in Concord NH there is a CaptiveAire system in which the MUA is expelled downward from the ceiling in front of the hood. (This aperture is actually part of the hood assembly.) I would estimate that the MUA aperture is roughly half that of the typical very large commercial hood aperture, although I cannot see all of the hood aperture from the customer seating area to be very exact about this. In my residential configuration, a 3 x 3 foot ceiling diffuser is used about 20 ft down a hall from my 10 sq. ft. aperture hood. This hall delivery should slow down the air velocity to about 37 ft/min, which is a slight breeze -- less than a half mph. I haven't pressurized the MUA yet, but I have seen the effect of an interior split cycle air conditioner head spilling air toward the hood, there is significant plume displacement. Is your MUA source via a basement, or via the roof? It might be better if you dumped the MUA at the ceiling directed away from the hood such that the air takes a longer path to the hood aperture. Or dump it into diffusers at various locations in ceilings connected to the kitchen. (This might be a casus belli if not heated.) Or use a lot more toe kick area. Besides measurement, I would check that with windows open, MUA off, and hood on, that you can cook something and see it fully captured and contained by the hood system. Then as you close up windows, and do whatever you are doing (manually?) to set MUA flow, does this start interrupting the capture efficiency? And yes, if you don't have a closed loop controller then both hood and MUA should have continuously variable controls. By analysis I meant making a gross estimation from the hood system fan curve and the properties of your vent system of what flow rate you may be achieving. (There are contractors who can measure this by temporarily replacing a door with a measurement device.) Ditto for the MUA system. What is its fan curve vs. ducting, filtering, diffuser pressure loss, and heating scheme, if present? If your actual replacement air is 200 CFM, then ideally you want the MUA to be 200 CFM less than the hood can pull when the house pressure relative to outside is zero. One expects that if the house pressure is positive, more air will flow out the hood system. But in your case I suspect, as did opaone above, that there is really a lot of turbulence around the hood and this turbulence is interfering with the cooking plumes attempting to rise to the capture area (the hood entry aperture). kas

"The manufacturers of Vent Hoods are miseducating the consumers." Yes. Somewhat. Not really. They are in business to make money and they do that by making hoods as inexpensively as possible and then selling as many as they can for as much money as they can. Their goal is NOT your health or the IAQ of your home - that is up to you. Telling consumers that they'll also need to spend money on MUA if they buy hood X will negatively impact their sales (ignorant consumer will just go buy a hood from brand Y instead because it doesn't say you need MUA even though it has an identical need for MUA) so they do not want to do that. MUA is also not something that fits well within consumer hood manufacturers world except to the extent that they might offer a hood w/ integrated front curtain ducting. It is much more of a pure HVAC thing. It can be supplied to the return ducts of an HVAC system, the supply ducts or ducted directly to appropriate locations. Incoming air can be heated by gas, electric or hydronic. Incoming air may need to be humidified or dehumidified. It often needs to be electrically integrated with the HVAC system controllers to function properly. Consumer hood manufacturers - ARE NOT IAQ or HVAC people or engineers - they make something decorative that by nature must include air movement so they reluctantly include air movement in their product. Commercial hood manufacturers - ARE IAQ and HVAC people and engineers. Their customers are much more educated than consumers. The one singular purpose of the product they are selling is IAQ - so they design and sell systems that provide good IAQ. Aesthetics is quite secondary for them. Residential HVAC companies in the U.S. - ARE NOT IAQ people (they should be though). A tiny few, less than about 1%, know it well and a few more know it a very little but the vast majority know just enough, based on what they've been told at a 4 hr CEU course, to sound like they know what they're talking about but they don't really understand it. Most residential HVAC people do not understand air movement or why things are done the way they are - they only know how to use tables that tell them to do this or that and as soon as something is outside of defined parameters they're totally lost (though sometimes don't even realize that they're lost). Licensed Professional Engineers do (or most do, some don't) understand air movement and other elements. They are the people who create the tables that HVAC people use. They can think independently. Defined parameters for them are not tables but the physics of air - it's movement, temperature and components. Unfortunately, most U.S. engineers do not have a good understanding of human physiology (except perhaps for biomedical engineers). They do not, for instance, understand how CO2 functions in our bodies and how high levels of CO2 due to poor ventilation affect us. Or how poor ventilation resulting in high levels of VOC's, PM, Carcinogens and Pathogens affect us.

MUA should really be provided mechanically, not through “drafty” walls, crawl spaces or attics. The hood you are purchasing should provide you with the volume of MUA that is required to run the system efficiently, assuming it is correctly sized with your gas range. Your log stove should have its own MUA system, but if you are relying on the hood system, it needs to be sized to accommodate both the log stove AND your gas range. The International Building Code for Residential Construction should provide all of the details you need (in the mechanical code) for this system - even if your local jurisdiction or contractors are not familiar with it.