Does the 3-2-10 rule apply to a masonry chimney venting a gas fireplace insert? The fireplace has its own 4" vent that runs (almost) the length of the lined chimney.
Forget the height rules, you never terminate a metal liner half-way into a clay liner, especially from a gas-fired appliance. And why are there two of them? When this is corrected, let them figure out the proper height. Looks like an improper conversion.