Shared venting

The water heater and furnace are both fueled by natural gas. They are sharing the same flue and the y-adapter is slightly bigger than furnace flue. My client is getting a weird smell at the top of the basement steps recently around the corner on rear side of block chimney. I’m thinking blowback because of the shared flues and both appliances are about 20 years old. Her HVAC guy has been there and says there are no issue with anything. I am thinking the water heater flue NEEDS to be separated from the Y-Adapter and installed higher up. Any recommendations?

The water heater is at least half the btu output as the furnace and should enter after .
Call it out.

at aproximatley 12 inches above