Air Conditioning - Can I install a HVAC plenum in an unused Chimney Flue?

We just removed our old oil-fired boiler and will be replacing it with a new gas fired direct vent boiler (thru wall), so we have an unsed flue that was connected to the old boiler. I would like to use that flue to run the plenum from a new central air handler between levels.

IRC R1003.11.6 states that a relined flue that doesn't comply with the requirements for general residential use must be permanently labeled for it's new specific use. Does anyone know if it'd be permissible to use the relined flue as a HVAC plenum? This is the only (easy) path I have from my attic to the lower level of my house for our new central air handler.

The chimney also has a second flue for our woodburning fireplace (which we'll likely replace with a gas insert and its inlet/outlet ducting).

Thanks,
Brian

If I understand this. You would like to use the flew as duct work to get conditioned air to the 2nd floor. If that is correct then the answer is a big NO. A 6 to 8 inch flew would only serve one small room. Also it would be un insulated. You will need to do a heat load on the home so you will know how big the duct work needs to be. If this is a 2 story home Id advise on one unit per floor.

OK, a bit more info. Our house is SMALL - Total heat gain (for both levels) is around 17,000 BTUH, so multiple units is not really an option. We're installing a 2 ton SpacePak high velocity system into the attic and the main plenum on that is only 9" in diameter. That will tee off to a plenum for upstairs and another for downstairs. The chimney flue is about 8" square clay lined to which I'd add some sort of sheet metal duct inside.

The other option is to run the small 2" supply lines individually down through existing walls - it's possible, but if I can avoid cutting into the plaster (there are fireblocks) I'd like to do that. There are no real good closets I can use to run it either.

Or you did set on the high velocity system? A regular heat pump would be better from a comfort stand point.

I don't have room for a small plenum between levels let alone what's required for a standard system.

Does anyone have an answer for the question I originally asked?

Although I can't quote the specific code the answer is still no.
You can not use a flue as ductwork as it is not approved for this purpose.
To use the space for air movement you would have to remove the chimney and install ductwork in its place.

It really doesn't make sense to do this especially with your chimney having two flues within it.............What happens if the brickwork that separates the two flues fails?

That's why I was asking. I know to run certain gasses (gas appliance exhaust, for example) you need to line the clay with a stainless (or other corrosion resistant) liner. For direct vent you can even run the inlet air down in one duct while running the exhaust from that appliance back out in another (all in the same flue).

Based on the fact that you can use the flue (when properly modified) for uses other than what it was originally intended, so long as it is permanently labeled for such use, I just wanted to know if you could or couldn't install a separate sealed steel duct inside an existing, but unused, flue for HVAC ducting.

A leak from the second flue could definitely be unsafe. Makes sense why it would be a bad idea. I was just looking to see if there were any ways to make my HVAC installation job go a bit easier. I guess I'll just have to open up the walls and run the ducts down in those cavities.