The potential for back drafting of combustion appliances such as hot water heaters, furnaces, fireplaces, etc. is a very important health and safety consideration. Carbon monoxide from these appliances builds up into a toxic gas when the gases cannot escape through flues or are not mixed with sufficient fresh air.
An ASD system can pull significant air out of the house through cracks and openings can cause additional depressurization of the structure, similar to the way an exhaust fan creates more negative pressures in a building. If a vented combustion appliance is located in the space, the negative pressure created by the draft in the flue will compete with the negative building pressure. If the draft pressure in the flue is weak (due, for example, to wind, an improperly-sized flue, or an obstruction in the flue) compared to the negative building pressure, the building can pull air back down the flue–or back-draft the appliance.
It is important to check combustion appliances back-drafting before leaving the job-site after a mitigation system has been installed.