Gas and Oil Home Heating Furnaces - Return necessary in basement?

I'm finishing my basement and have a gas forced hot air furnace. My HVAC guy is installing a zone dampler so I'll have a separate zone for the basement. But he told me it's not recommended to put a cold air return in the basement as it could suck bad fumes into the living space. I presume he means carbon monoxide from the furnace or water heater. He claims it's fine to :thinker: just use the returns from the first floor. I'm skeptical about whether the basement zone will heat properly without a return. Anybody else run into this issue and have any suggestions?
Thanks.

I encounter this problem frequently. The system will work far better if there is a return in each conditioned space. I certainly hope he knows what he's doing when zoning. Improperly zoned systems can do severe damage to a furnace. The return should be located as far from the furnace as is practical.

if the furnace and water heater is in a room by itself, there should be NO return in that room itself. In other living area in the basement, suggest having returns near the floor for comfort.

Im with Grady . If the HVAC guy didnt rework all the duct that was in the home and put in a bypass damper forget the zone damper for down there. Yes you need a cold air return down there for sure but not so it can pull down on the furnace flue.

ED ;)