Well, "any" is potentially a very small amount, and I'm sure that there is some slight amount. My attic, however, is vented all around the periphery, at the ridge, and also with an attic fan housing. Ice dams at the roof edges tend to occur when the outside air warms up during the day and heats the snow bottom up through the vents instead of top down. In my present half-completed configuration, the ~ 2 x 2 ft heat exchanger sits above a Cooley and Hart 3 x 3 diffuser in a hall ceiling. There is a furnace filter taped down on top of the heat exchanger. This style of filter (Honeywell Filtrete) needs significant pressure drop to move much air through it, and I was pleased to observe that warm air doesn't seem to rise through it in the winter, or hot attic air settle through it in the summer. The sides of the heat exchanger and its plumbing are buried in insulation. When kitchen fans are running and the house is closed up, air is pulled through and the thermostats, the larger-than-typical-size Taco pump, and the furnace controls do their thing to heat the air. (The circuit is capable of well over 100k BTU/hr depending on furnace and air temperature.) The problem, as I have pointed out a few times here, is that pulling air through a filter and heat exchanger requires a pressure drop -- a drop too large for combustion appliance safety. My only combustion appliance is an oil furnace, and it now uses its own MUA system to avoid back-drafting. Nevertheless, if I were to run both kitchen fans at once at full power expecting all flow to be made up through the heat exchanger without fan boost (possibly 1400 actual cfm with restricted MUA), the pressure drop in the house causes some furnace exhaust to be pulled into the house via tiny cracks not normally relevant when there is a positive draft, so I don't do dat without an opened window. My intention is to duct the heat exchanger to a mushroom air intake already installed on my roof via an axial in-line fan and a four-inch pleated filter caddy so that at the maximum possible flow rate (possibly 2000 cfm, say) the house pressure can be balanced and all the MUA comes from outside without mingling with the attic air. A damper may be needed, although as noted, the Filtrete coating will be pretty resistant to air passage without some deliberate pressure drop from the in-line fan. kas

IÂll share what I am doing about makeup air. I need to qualify this with the fact that we have not closed the house yet so performance is still to be determined. I researched commercial installations. If you are obsessive I recommend http://energy.ca.gov/reports/2003-06-13_500-03-034F.PDF. A recommended setup is to replace about 80% of the exhaust air with makeup air at the cooktop. Pulling the rest of the makeup air from the room seems to improve capture. We have a sealed combustion oil fired boiler and no fireplace so I am not too worried a slight negative pressure. Even with new construction there is still some infiltration. For example, the baths and laundry are vented to outside. Another thing to consider is the temperature of the makeup air. In cold climates it may be desirable to temper it. I am going to try it with no tempering, but I have made prevision to add a duct heater if it is required. My plan was to pull the makeup air from the outside; however, my builder suggested we pull it from the unheated attic. He thinks that increasing the air exchanges in that space would be a good thing, IÂm not sure. There are lots of ways to deliver the makeup air. One of the better ones is to deliver the air under the cooktop. My cooktop will be mounted in such a way to create a 6" x 40" channel under it. The end of channel is covered with a decorative grate. The air flows under and to some extent through the cook top, around the front, then up and out the hood. This pattern creates very effective containment of the plume.

Sorry I didn't see this post sooner. I've got the exact configuration that you were discussing. It works well. Cleaning isn't a big deal, either. I inherited the 36" 900 cfm Vent-A-Hood, 24" deep from the previous homeowner. I have recently installed a 36" Bluestar range top. When I renovated and replaced the old cook top with the BS, I didn't see any good reason to replace the perfectly good VAH. It works fine. While it would have been nicer to be able to install a hood with a slightly wider dimension, the 36" VAH does the job with little fuss. Practically speaking, you rarely run all six burners at full blast at the same time. If you are running things at a higher usage rate, I like to start both squirrel cages at the same time to get ahead of the ventilation draw. You are going love your Bluestar.

Think of it as a shiny tile backsplash with poor vertical alignment. :) There are some pre-patterned stainless steels that can look fairly pretty. Just keep the angle of tilt-out (from vertical) shallow enough that a rising expanding plume that hits it is still reflected upward. Three inches of tilt over 32 inches (less if the range has its own backspash) is easily sufficiently shallow in angle. Even a less observable short metal piece close to the hood could be built and be 30 degrees or less from vertical. For example, a six-inch piece of metal tilted out 3 inches at the top would be at 30 degrees. Tour http://finishedkitchens.blogspot.com/p/kitchens-alphabetically.html for ideas. In any case, a serious gas range needs some consideration of what the wall behind it is made of, and I'd start with 5/8 firecode sheetrock (or stone or brick) covered by something fire resistant, such as metal or tile. Assume a raging grease fire in your planning, because that is what the range manufacturer did to comply with UL and to specify offset distances to combustibles, and what the hood manufacturer did to comply with the hood filters performing their fire blocking function. kas