Many furnaces, boilers, and hot water heaters draw their combustion air from inside the house.  If the draft inside the heating appliance’s flue is not sufficient, or if the house is under a negative pressure, then the combustion by-products could spill into the house, creating indoor air pollution, and possible fire and carbon monoxide hazards.  Running one heating appliance up a chimney originally designed for two appliances, or adding whole house, range, or bath vents could depressurize a house.  Air sealing a house to remove drafts and improve energy efficiency could also depressurize a house.  A house is a system of components.  Modifications to one area could adversely affect other areas.  Because of this, it is important to have an energy professional verify heating equipment is operating within safe limits before and after modifications to a house.