All fuel-burning appliances require oxygen to burn and sufficient oxygen to burn clean and safe, without soot and CO (Carbon Monoxide).
I live and work in Florida where most of our fuel-burning appliances are 80% efficient with open combustion that utilizes air and oxygen from the space for combustion.
With these low-efficiency appliances whether the appliance is forced vented or natural draft that combustion air is leaving the space, and exiting the flue.
This causes negative pressure that must be allowed to equalize as well as consumes oxygen from the space. It is because of this that these open combustion appliances must either be in a sufficiently large space or communicate with (be open to) a larger space or outdoors.
When you consider that other gas appliances also need to use oxygen and need to vent to outside you can see that without sufficient communication to outdoors that negative drafts can occur on natural draft appliances like water heaters.
This is why all open combustion appliances that utilize combustion air from inside the space must be in an “unconfined space” or connected to an unconfined space or the outdoors using an approved method.
I see many furnaces jammed into tight closets and mechanical rooms with little thought or planning regarding combustion air.
According to NFPA 31, 54 & 58 an unconfined space is a space that has at least 50 cubic feet of open area for every 1,000 Btu of input. This means that a 100,000 Btu furnace must be in a 5,000 cubic ft space to be considered unconfined.
If the appliance is not unconfined then additional combustion air must be made available to the space with one opening at the ceiling level and one near the floor.
If the air is coming from another unconfined space then the openings should be at least 1 square inch per 1,000 BTU and 1 square inch per 5,000 BTU if it is connnected to the outdoors.
While these openings and are needed in many cases to allow for proper combustion and venting it helps illustrate why modern sealed combustion “direct vent” appliances that take all of their combustion air from outdoors make so much sense.
Not only are direct vent appliances more efficient on the fuel utilization side, they also prevent the negative home pressures and/or thermal losses associated with having vents in walls and ceilings.
So either make sure you have an unconfined space, you are bringing air in from an unconfined space or outdoors or you have a direct vented appliance.