This requirement is to ensure that minimum life safety and health requirements are maintained when the operation and/or design is modified on existing heating, ventilating or air-conditioning systems. For example, this provision would apply to such cases as
(a)
the conversion of a heating appliance from oil to gas where venting and clearance requirements differ for chimneys and
(b)
the branching of a new duct from a main supply duct in which the new duct now penetrates a fire separation.