What can cause a lack of draft in a vent? One of the main culprits is lack of combustion air. The operation of the burners and the vent system can remove air from the room faster than it is being supplied, causing a negative pressure in the room. In older, leakier homes we often relied on infiltration for combustion air. Today, you really should provide outside air to the appliances. I have seen the operation of a furnace pull smoke out of a burning fireplace. There was enough air for the fireplace, but not both the fireplace and the furnace. The room went so negative that the fireplace vent was not lower than the room pressure. If the vent gasses cool off too much in the vent pressure will increase because the gasses are heavier, decreasing the draft. This can be a particular problem when replacing an older furnace with a newer one. Often, the vent is too big for the new furnace, causing the flue gas to cool off too much as it travels through. Another possibility is a plugged vent or vent cap. The flue gasses back up in the vent and then start to spill out of the appliance.
Although most Category I furnaces with draft inducer fans have draft pressure switches to shut off the burners if the draft pressure is not at the minimum setting for the switch, I have seen furnaces operating with a positive vent pressure continue to run. Draft switches are, after all, switches and can fail. So don’t assume that because the draft switch is closed, the vent pressure is OK – measure it so you know.