What's going on with this chimney?
Normal settling cracks, would not be worried about it structurally.
But...
Is it sleeved? Is there a metal pipe inside the chimney?
Most municipalities now require you to sleeve a masonry chimney that is carrying CO2 or any other potentially dangerous gas that might leak into your house. Even if you can’t see the cracks, they are there. By running a metal pipe from the furnace or water heater to above the roof line, you ensure that gasses can’t leak out and circulate inside the structure.