What could be causing this gas smell from vents?

We rent an apartment and everytime we have the furnace or air conditioning on AND run the water is starts smelling like gas and our carbon monoxide detector goes up to about 90. Its only when we have the air or heat on and run water at the same time. Landlord replaced the furnace but it didn't help. She says the hot water heater doesn't need replaced because its only nine years old. Why does it do this?