It's hard to know if that price is high without knowing the details on the install (length, turns, etc.). It is a one story house or a 3? Can you vent it out a sidewall instead (go into the ceiling and then run the ducting between the joists)? The longer the run, the more turns, the move power the vent needs to have. Make sure they don't use flex duct. Mine was installed by my husband and BIL. Between them they know all the skills involved. My HVAC guy would have done it, I had him exclude it from his bid. Bathroom and kitchen exhausts are not required here. Only bathrooms with no window are required to have one.

Thank you both. Let's see if I can be more clear. Our kitchen is decades old in design, cabinets and some wall ovens. When kitchen Ventilation was undertaken, it appears all exterior vented HVAC included not only the over the range or cooktop area vent hood and run but (at least in our house) a 6" diameter vent over the wall double ovens. There are a few GW comments of others who have this arrangement in their older kitchen, some of whom choose not to upgrade from 24" ovens nor change layout because of this vented wall oven arrangement. In my kitchen, the tall double oven wall cabinet with 6" vent on top sticks out like a sore thumb in its location. Very ugly and makes the kitchen smaller in many ways. By removing the soffit above and this tall cabinet, I will open up the width of the kitchen dramatically and make it much more aesthetically appealing. That is why I started this thread to see if the 6" HVAC vent can be capped. I don't want to keep the wall oven cabinet (nor move it to the side a bit) as it affronts me so badly where it is. Regarding the current over the cooktop HVAC arrangement, I have a decades old Thermador 42" under-cabinet vent hood, back draft damper and then a 3 1/4" x 20" w x 47" h enclosed galvanized steel rectangle box running up behind the shallow 36" width cabinet to a 14" soffit. I'm still trying to figure out the very top of the venting directly behind the soffit itself and under the ceiling. It's a maze of galvanized steel as I crane my head up through those 3 1/4" by 20" holes with a LED flashlight. As I see a curvature of the galvanized metal on the back of the cabinet side, I suspect they may have installed a 3 1/4" x 20" 90 degree Elbow Round. There appears to be a box like opening on opposite this, on the back kitchen wall at this height/location. Problem is, I can't measure this opening, can't probe it with my pull out tape measure to see if it's 90 degrees, and can't fully visualize it. All I know is this is where the ventilation moves through, and that eventually a long run of 10" round duct goes into the 1000 cfm Thermador centrifugal fan on the exterior side roof (I climbed up to the top of the addition and removed screen venting to peak in and yep, there was the round duct leading to the exterior fan). When the soffit comes down I will see the truth but in the interim I am planning a new vent hood and replacement of all the kitchen visible galvanized steel that can be done. Still, I'm not going to tear out walls to change the run: that's what I inferred about using the run as is, exiting the kitchen proper onward but to a new exterior 1000 or 1200 cfm blower. And modifying the electric turn switch of course from the current infinite 6 amp to whatever the new specs on the selected blower require. Our house is not overly tight and yet I am planning on the Broan MUA adaptor (can't recall model number off hand). I have a goodly amount of electrical panel open (200 Amp total) which is a benefit. Thanks for your help. I hope it's clearer now. This post was edited by SparklingWater on Sun, Jan 27, 13 at 16:45

Obviously you could cut a vent in the top of the wall. Here are some of the issues. The furnace is presumably combusting a fuel - the result of this is carbon dioxide and some amount of carbon monoxide. Now most of this is carried away by a chimney of some fashion. But some of it may not be. I would suspect that in most jurisdictions, this would be a code violation. Now the flip side of all this is that the same thing is done all the time to provide air for combustion in the furnace. I'm not sure that it is ever done in a bedroom however. My immediate thought is that this is not an ideal situation with a problem room so close to the furnace. So I doubt the furnace is ideal - so I'd be concerned about the products of combustion. The right answer of course is to get more forced air into the room. And another thing... A furnace room shouldn't be particularly hot. It is another red flag that there is a problem that shouldn't be ignored. I live in a new house and my furnace room in a basement is really not warm. It is a 90% eff NG furnace. One would think that it would be warm but it really isn't. I suspect you have duct issues creating a large leak. That makes your room cold and the furnace room hot.

I think the key word above is "appear" because to me these OTR microwave ovens look like they are not flush by two or three inches. From an earlier post: "If I can use the top vent [to] go up through the cabinet and resolve the ductwork above that cabinet, that vent will care for the stove venting and the magnatron venting, and I would not need a MW with a front-facing louver." In my view, neither this nor any other microwave oven venting scheme I'm even vaguely aware of will "care for the stove venting" because they move far too little air per unit time and the units do not overhang the burners sufficiently. Perhaps if a supplementary blower designed for higher pressure loss is used in the path taken by the this so-called ventilation to augment the flow rate, barely adequate air flow will be possible. But even then, capture area will be too small to have much effect on front burner plume capture. You can design for style and give up on having adequate ventilation, or design for adequate ventilation and give up on OTR microwave ovens. Your statement "For what it's worth, most of the cooking on our stove is boiling water or soup, scrambling eggs, or simmering an occasional kettle of sauce." suggests that you can get away with doing the former. In that case, provide manufacturer specified cooling paths for the magnetron, and ignore limitations of the mediocre or worse ventilation. And don't sear steaks on the cooktop before baking them in the oven.