The stack effect refers to an underpressurized house, usually due to a loss of warm air to the attic space. This can be dangerous because when the air pressure in the house is lower than in the outside environment, appliances and furnaces may no longer ventilate properly. For instance, today’s modern hot water heaters burn efficiently and generate exaust vapors that are relatively cool. If a water heater is vented through a chimney that has a cold-air downdraft problem due to the stack effect, cold air will rush down the chimney flue when the heater is off (air will naturally flow from an area of higher pressure to an area of lower pressure, and the chimney is one of many airways through which this equalization of pressure occurs). It will be impossible for the vapors to push the cold air plug out through the chimney when the heater comes on. This condition occurs regardless of whether the chimney is lined or not. Instead, the vapors will spill into the basement, potentially causing dangerous Carbonmonoxide levels.
Chimney downdraft due to the Stack Effect
Equally dangerous, but much more annoying, is when there is a fireplace or a wood-burning stove connected to a neighboring flue in the same chimney, because smoke will get drawn down through the cold flue into the basement, and the stack effect will cause the smoke to eventually seep into the living areas and spread thoughout the home. This situation can render a fireplace unusable.
The latter problem can be somewhat alleviated by installing a draft-inducing power vent that is controlled by the temperature in the vent pipe (i.e. it comes on shortly after the water heater or the furnace starts up), but while such a measure helps eliminate a symptom, it does not solve the problem. Because the draft inducer only runs when the water heater (or furnace) is running, cold air rushing down the chimney and through the water heater’s burners when it is off will cause the water in the boiler to cool off much quicker. Also, a manual override switch will be required for when there is a wood-burning stove or fireplace in the same chimney, so that the draft inducing vent can run continuously while a fire is burning, or a vent damper is installed in line with the draft inducer (as indicated in the drawing above).
Clearly, fixing the negative pressure problem is the preferred solution.