Basically, codes no longer permit running a vent duct through a sectioned off part of a chimney. I'm sure you can imagine the furnace by products being sucked backed through a contiguous vent duct up the other portion of the chimney.

If you don't physically touch your current set up, at least in my historic town with lots of tight codes, they can't force you to change out what is there. But if you do, then you have to go with current codes, all of which are designed to potentially save a life from venting mishaps.

Please check your local and state codes first though before any purchases of cooktop etc. Most do require make up air if any vent hood you install draws more than 400 cfms out which sounds like it would double your trouble.

Hopefully others will come along with some bright solutions.

Yeah, I think you've got potential problems if the vent stack that contains your kitchen duct runs in the same chimney chase that is also used for a fireplace below, or even just other peoples' vented stuff.

Couple of issues: the air inside a fireplace stack is really hot - and needs to be in order for it to draft. Perhaps that's not for the fireplace, but for another kitchen exhaust stack? Another kitchen vent stack would be the least worrying because of what is carried up by a kitchen fan: cooking heat, steam, odors and some grease, annoyances but not really dangerous. Any burning appliance or fireplace vented to the stack would be a major worry for me.

However if it is a fireplace stack (and except for woodstoves, or fake gas-fired fireplaces) the chimney on a wood-burning fireplace is pretty big, perhaps 15 X 15 inches square.

The other issue is that the exhaust from the chimney (or from any other combustion appliance) could be sucked back down into your kitchen under certain atmospheric and draft circumstances, which depending on what is being ventilated could be dangerous or fatal to you. You really need to investigate what's being sent up in the other flues! Espcially dangerous is a vent stack from a gas or oil-fired water heater, furnace or boiler.

Leaks among the vent stacks are a small issue, the main issue would be backdrafting which could be drawing stuff from one stack back down into the living space of another unit. That's scary.

Do you know the symptoms of carbon monoxide poisoning? It's colorless, odorless, and deadly. The early warning of slight, persistent carbon monoxide poisoning is a feeling of malaise, tiredness, sometimes with cracking headaches that disappear pretty promptly when you leave the building. And with higher levels a bright red coloration to your face, like you get when you're out of doors in really cold weather. You can get a carbon monoxide detector where smoke detectors are sold.

Now about your own venting: can you get the vent to the exterior of the building and just vent to a wall?

Your tile roof doesn't preclude having the exhaust there. I have a slate roof and with proper installation of the roof jack, with careful flashing, etc., there should be no problems with it.

You will need a skilled and competent tile-roof worker doing that part of the job (opening the roof, installing the roof jack, flashing like a maniac around the opening and re-installing the tile field around it.) I would only trust a tile roof expert with that part because tile is so different from regular asphalt or shingle roofs.

The ducting itself could be done by an HVAC guy, but probably the guy who installs the fan will be able to do that part.

As for how they work on the non-step roof surface - that's easy - they use roof ladder brackets that hook the ladder over peaks. Then they only step on the ladders which avoids breaking the tiles. I do all my own slate roof repairs and that's how I do it.

If the roof is particularly high and challenging, they will bring in a bucket platform truck to work from. That's a pricier option. The roof may not be as daunting as it appears to someone who doesn't do roof work regularly. I work on my three story tall hay barn, using only roof ladders, and a fall-arrest safety harness, of course.

Howver, as noted the MUA (make up air) may be more of a problem than the exhaust stack pathway.

It can be done - solving these issues is what the GC does to earns his whack. Since you're keeping that part in your bank account, you'll have to find the right guys yourself.

But don't skip figuring out what's running through that chase from other units, even if you decide to vent through the roof.

HTH

L.

Sorry, I should've clarified there's no working fireplace now. The chimney is divided into at least two... chimneys, basically (one chimney with separate stacks), but the one for the fireplace downstairs is (1) not functional because the fireplace has been bricked up and (2) not being used for anything else. To my knowledge the only thing using that chimney--any stack of that chimney--is the little retro fan for this kitchen.

Thanks for the explanation of how the roof guys would work. It seems super challenging to me because of the height and the material (tile), but maybe it's just a day's work for a roof guy.

I'm glad to hear what you report about the vent stack.

Since your vent is the only one there, then up through the chimney will probably be the easiest.

You might look into a remote fan if the pathway is long. There are published specs for length, diameter, number of right angled bends, etc. for the need vent capacity. The fan manufacturer will be able to help you with that.

Do you needs pic of what a roof peak hooked ladder would look like so you know if it's being done properly? It's basically unusually long, non-extension, ladders laid flat on the roof. From the uppermost two rungs of the ladder, brackets - a pair for each ladder is best - arch up and over the peak. They have little moveable feet on their farther end so they lie flat on the other side and spread the load. At the bottom of the ladder where it's lying pretty flat against the roof, I usually tuck a 30" piece of 1 X 10, laid at right angles, under the whole width of the ladder. This serves to spread the load and protect against breakage. (It also catches the odd tool that gets loose and slides down the roof from where I'm working.) My roof ladder brackets have casters on their backs so I turn the ladder over and just roll that puppy up to the peak, flip it over again and set in down very gently against the slate on the other side. I am a small, older, woman and there's no way I could muscle the 16 to 18 foot long ladder sections up the roof without those rolling wheels.

I"ve attached a link to pics of the ladder hooks in use.

HTH

L.

Here is a link that might be useful: Link to pics and Youtube of the ladder hooks I use