I have seen it done many times. Is it right? Is it legal? Without filling this screen with 8 what ifs and sounding like a "what if" guy, I would say do it, no problem. If i put my salesman hat on, I could tell you 8 reasons why you should not do it and if you did do it i would.....there I go. Do it and remove it. Seal it where it is secured to the attic fan duct work. You may burn up your attic fan early but I doubt it. I am confused though about installing a new fan and using the existing space duct for this? Now that is only an opinion!!

This post was edited by hvacajun on Sat, Jan 26, 13 at 23:57

My first thought is: if you're installing a 1000-1200 cfm exhaust fan I sure hope you have plans of providing at least that amount of make-up air. Creating that much negative pressure within an occupied space is not a good thing. That exhausted air needs to be replaced with conditioned outdoor air.

By your post I'm confused on the purpose of this 6" vent serving the oven. If the oven manufacturer requires it, I would not remove it. You say it does not currently serve to vent the oven but is connected to the oven?

Sounds like you do a lot of cooking. :-)

This post was edited by ryanhughes on Sun, Jan 27, 13 at 10:41

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

can you get a better idea from the attic side
rather than from inside the kitchen?

best of luck.

Thanks for that suggestion energy rater la.

The blower squirrel cage is on a brick addition to the back of the house and has a false attic, meaning an enclosed triangular space with wood beams and insulation. I removed the addition's vent screen via climbing a ladder and there was the 10" (not small) diameter steel duct leading to the Thermador 1000 cfm blower on the side. I'm still trying to figure out where the 10" duct exits from the original house.

I think your suggestion is good. I haven't looked for ducts in the attic and it may be fruitful. I want to put in a bathroom fan through (not into) the attic roof anyway someday, so I can scout that out also. I'm also going to look for some house old blue prints.

Thanks. Great suggestion!