Preface This is the 7th edition of ANSI Z83.18, Recirculation direct gas-fired heating and forced ventilation appliances for commercial and industrial applications. It supersedes the previous editions published in 2015, 2012, 2004, 2000, 1990 and 1987. Scope 1.1 This Standard applies to newly produced, recirculating direct gas-fired heating and forced ventilation appliances, hereinafter referred to as heaters, whose purpose is to offset building heat loss and provide outside air ventilation to address the ventilation requirements of the building. Ventilation air to the heater is ducted directly from outdoors and the products of combustion generated by the heater are released into the air stream being heated. Heaters covered by this Standard are intended for use in industrial and commercial applications. This Standard applies to heaters designed for a heated discharge air temperature of 160 ?F (71 ?C) or less. This Standard is intended to cover only complete packaged heaters with integral air moving components, i.e., those which are designed by, cataloged by, and built on a repetitive basis by the manufacturer. 1.2 Inside air may be introduced before or after the combustion zone. 1.3 Outside ventilation air is required to the heater. See Clauses 4.21.1-f), 4.21.1-n), and 4.22.2-u). 1.4 If a value for measurement as given in this Standard is followed by an equivalent value in other units, the first stated value is to be regarded as the specification. 1.5 All references to "psi" throughout this Standard are to be considered gage pressure unless otherwise specified. 1.6 Clause 2, Reference publications, contains a list of standards specifically referenced in this Standard, and sources from which they may be obtained. 1.7 In this Standard, "shall" is used to express a requirement, i.e., a provision that the user is obliged to satisfy in order to comply with the standard; "should" is used to express a recommendation or that which is advised but not required; "may" is used to express an option or that which is permissible within the limits of the standard. Notes accompanying clauses do not include requirements or alternative requirements; the purpose of a note accompanying a clause is to separate from the text explanatory or informative material. Notes to tables and figures are considered part of the table or figure and may be written as requirements. Annexes are designated normative (mandatory) or informative (non-mandatory) to define their application.