If the chimney isn't being used for a fireplace, then it's nothing but a fireproof chase, and I don't see any reason why it couldn't be used to house the duct.

Obviously I'm not familiar with your local codes, but it sounds safe, cheap and easy, so I don't see why there would be any obstacles, assuming you removed the fireplace. If you left the fireplace in, on the other hand, even if you weren't planning to use it, installing a duct that partially blocks the chimney seems like a potential safety issue.

Thanks for the input Cawaps. It seems silly to fill one hole while making another.

If I have to lose the chimney altogether, I'll need to get the roofer back since most GCs around here don't want to touch shake roofs. So this ridiculous plan appeals to my frugal side as well as being (I think) a nice way to disguise an otherwise prominent roof pimple. :)

i am not sure what size your chimney is, but a larger hole could make your house less energy efficient/leaky, i think. unless you can have some sort of damper on your hood large enough to fill in the chimney hole?

Maybe this is a dumb question but, if you remove the fireplace, what's left to support the chimney?

Not that dumb, Ci. And I don't know. It's not a tall or large chimney compared to the fireplace. I guess I should crawl (yeah crawl) up into the attic and see what is there. I've been putting that off.

Michou, I'd want the chimney capped, filled, whatever they do to abandoned chimneys with the exception of the duct. I just want the duct to exit the roof inside the cute little brick box but not actually use the chimney to funnel effluent. Fortunately we live in a mild climate. And my home is quite leaky. This could actually improve it. :P

You could probably support the chimney with a much smaller structure than the original fireplace.

My building (1840) has fireplaces on upper floors where there are none on the lower and they are built out on corbels. These are three stories of fireplaces and chimney supported by cantilever. Of course it's a masonry structure.

I think what you are proposing is a metal duct for the vent surrounded by lightweight concrete lining the existing? That should be fine, and would be a variant of chimney liner for a modern fireplace in an old chimney. It will be a rather Expensive vent for a range, but it will look nice.

You cannot duct into a chimney, but you can run ducting through the chimney as a sort of soffit that contains that ducting. Ducting for range hoods needs to be rigid smooth metal so as to avoid grease buildup. Even if you have a terrific range hood with high CFM and an excellent filtration system, the air that passes through it will still contain lots of microscopic grease particles that can attach themselves quite easily to a rough surface like masonry. Then they build up. That's a fire hazard. It's a particularly nasty fire hazard if this is an old fashioned non lined masonry chimney that already has it's own buildup of creosote from being used as a wood fireplace.

Right. No. Yes. Ummm I'm not sure of the mechanics at this point. I would use proper hood ducting from end to end and just have it terminate inside the chimney above the roofline instead of in a new roof hole with an obvious cap. So somewhere in the attic the duct would cut in and run through the remains of the chimney.

If I duct through the roof normally (I could put in a slightly longer run and put it in the rear of the house instead of front and center), I'll still have to deal with the chimney. The fireplace really does have to go...