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

I've been very pleased with Vent-A-Hood. It works extremely well and is as quiet as you'll find, unless you go with a remote blower of some sort. That being said, we planned for an optimal installation. The ducting goes straight up through the attic (less than ten feet). No turns, and terminates in a properly-sized roof jack per Vent-A-Hood specifications. My current system was installed in 2005. I think that that in quite a few cases, the source of dissatisfaction by some owners has more to do with complicated installations, involving several turns in the ducting, long duct runs, or constriction at the roof jack. I cannot over-emphasize the importance of a proper roof jack. The system in my kitchen operates flawlessly and is very quiet. Even when turned on full speed, it doesn't interfere with conversation or require the television to be turned up to a higher than normal volume. Most of the time, I run at the very lowest Sensasource speed setting. Even then, nothing escapes it. I know this because I have recessed can lights throughout the kitchen, and if I had smoke or air-borne grease flying around, I'd be able to see it in the beams of downward light from the cans. As for your installation, I think that if you can forego cupboards above the hood, it would probably make more sense to go with one of the wall-mounted or chimney-style hoods. Matching shrouding is available to hide ducting on the wall-mount models. Look for a model that can be ordered in 27" depth. You'll need the extra depth over a commercial style range. Far more important than a strong blower is a large capture area. Having a range hood that extends beyond the the perimeter of the cooktop will allow you to capture more with the blower running at a lower speed. You should definitely try to install a hood at least 6" wider (3" on each side) than your cooktop, and one that extends out to the front of the burner area, If you have room to go with an even wider hood, that's even better. My range hood is 12" wider than the cooktop and extends out to the cooktop leading edge. Other manufacturers make hoods in similar dimensions, but I like Vent-A-Hood and recommend it. I've been entirely pleased. The important thing is capture area. The larger the hood, the more you'll capture, and the fewer cfm's required. Keep in mind, if you air condition in the summer and heat during the winter, all that air you're exhausting you've paid to cool or heat. You might find interesting the Vent-A-Hood video that the link below will take you to. Granted, it a piece of Vent-A-Hood marketing hype....but it does contain some useful information. Here is a link that might be useful: Vent-A-Hood video

Good advice from will2kz. A point of clarification: the 1 CFM per 100 btu-hrs is mostly a rule of thumb and not a code requirement. There are other such rules of thumb. One is 100 CFM per linear foot of cooktop. (This is often suggested for lower powered gas units and electric stoves and cooktops.) These rules are designed to help folks size the hood, but they are not the only considerations. As kaseki has often pointed out here, CFM is only a part of the story. Focusing on just CFM misses important aspects of hood design. You need to look at other things such as the capture area and whether you have a flat ventilator type hood or one with an actual canopy. Wider and deeper is better than shallow and flat. (Another rule of thumb is to have a hood that is 6 inches wider than your cooktop (3" on either side) in order to do a better job at capturing the vaopor & etc. coming from the pots close to the side. A canopy is (usually) noticeably more effective in capturing and corraling steam and smoke than a flat bottomed hood. So a taller canopy style hood may be preferrable to a shallower, flat bottomed one if you've got the space for it. Sometimes you don't. Of course, how much power/capacity you use at any given time will depend on what and how much and how you are cooking. One advantage to higher powered hoods is that the greater capacity may allow you to run them at lower settings which will be quiter. But, again, this is personal preference (and budget), not a code requirement. But, going to a larger hood may put you under code requirements because of make-up-air (MUA) provisions. Some localities have adopted MUA standards, but many have not. The best way to find out what is "required" is to do precisely what will2kz suggested and call the inspectors. If you want to search on make-up-air, the commonly used abbreviation "MUA" is good to include in a search string.

The extraction efficiency will be much better if installing the hood just 30" above the cooking surface as opposed to 36". If you're "only" 6' tall, don't worry, you won't bang your head against the hood. The 6" difference between the hood bottom and top of the head (6' - 36" - 30") will be lost when you bend over the range, especially since the hood is only 24" deep while the range is probably at least 26" deep. I installed mine at 29" above the cooktop and I'm 6'2"; I can barely touch the hood with my forehead if I lean over the pots. Moreover, in time one develops a subconscious feeling on how to move around the hood without hitting it. IOW, install the hood as low as possible and it will be much more efficient at removing grease, smoke and odors.