Preface: This is the fourth edition of CSA/ANSI Z21.91, Ventless firebox enclosures for gas-fired unvented gas log type room heaters. It supersedes the previous editions published in 2017, 2007, and 2001. Scope: 1.1 This Standard applies to newly produced ventless firebox enclosures (see Clause 3, Definitions), herein after referred to as fireboxes, for unvented gas log type room heaters. 1.2 Fireboxes covered by this Standard are intended for use with unvented room heaters that comply with ANSI Z21.11.2 for installation in solid fuel-burning fireplaces. 1.3 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. 1.4 All references to "psi" throughout this Standard are to be considered gauge pressures, unless otherwise specified. 1.5 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; and "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 (nonmandatory) to define their application.