Preface This is the second edition of ANSI Z21.22-2015 / CSA 4.4-2015, Relief valves for hot water supply systems. Scope 1.1 This Standard applies to the following types of relief valves constructed entirely of new, unused parts and materials: a) combination temperature and pressure relief valves for use on storage tanks of hot water supply systems without heater input limitation, hereinafter called "T&P" valves; b) valves having only pressure relief features for use on storage tanks of hot water supply systems without heater input limitation; and c) vacuum relief valves. 1.2 A T&P valve operates when the temperature sensing element is at 210 °F (99 °C) or less. A device with a higher operating temperature is not acceptable under this Standard. 1.3 A vacuum relief valve is a single-purpose type. 1.4 All references to psi throughout this Standard are to be considered gage pressures unless otherwise specified. 1.5 Clause 9 contains provisions that are unique to Canada. 1.6 In this Standard, "shall" is used to express a requirement, i.e., a provision that the user shall 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 (non-mandatory) to define their application.