Using old B-vent pipe as exhaust for Heat Recovery Ventilator

[dsquare1]

I was wondering if I can tie into my old B-vent stack to exhaust the contaminated air from my Heat Recovery Ventilator, providing I insulate the section of pipe in the attic space. It is not being used for anything else and is essentially a dead stack. I would of course ensure it is tagged out as a hot gas-rated vent.
Is this against code or am I ok?

[Admin]

Just fish insulated flex up and out.  It's no longer a chimney if it's not connected to a fuel or wood burning appliance.

You just have to pay attention to how you terminate the HRV exhaust.  You want a cap that closes when the HRV is not on, otherwise you could affect the defrost mode.

[dsquare1]

Many thanks, I would have never even considered the termination point and the need for a cap that closes. Do I have to fish insulated flex up or can I just insulate the section in the attic space?