Technical Booklet P Sanitary Appliances Unvented Hot Water Storage and Scalding·Page 12·2.3
Relief valve restrictions for systems without automatic replenishment
Temperature and pressure relief valves cannot be used in unvented systems without automatic water replenishment; instead, two independent non self-resetting energy cut-outs must be provided.
Temperature relief valves and combined temperature and pressure relief valves should not be used in systems which have no provision to automatically replenish the stored water (e.g. unvented primary thermal storage vessels). In such cases there should be a second non self-resetting energy cut-out independent of the one provided at paragraph 2.2(a).
Source — /Users/richardhill/Documents/planning-arch-project/data/documents/regional/Technical Booklet P - Sanitary Appliances Unvented Hot Water Storage and Scalding.pdf