S15 -- Medium- and High-Temperature Water Heating (SI)

Medium-temperature water (MTW) systems have operating temperatures ranging from 250F to 350F (120C to 175C) and are designed to a pressure rating of 125 to 150 psig (860 to 1030 kPa [gage]). High-temperature water (HTW) systems are classified as those operating with supply water temperatures above 350F (175C) and designed to a pressure rating of 300 psig (2000 kPa [gage]). The usual practical temperature limit is about 450F (230C) because of pressure limitations on pipe fittings, equipment, and accessories. The rapid pressure rise that occurs as the temperature rises above 450F (230C) increases cost because components rated for higher pressures are required. The design principles for both medium- and high-temperature systems are basically the same. This chapter presents the general principles and practices that apply to MTW/HTW systems and distinguishes them from low-temperature water systems operating below 250F (120C). See Chapter 13 of the 2024 ASHRAE Handbook -- HVAC Systems and Equipment for basic design considerations applicable to all hot-water systems. System Characteristics Basic System Design Considerations Distribution Piping Design Heat Exchangers Air-Heating Coils Space-Heating Equipment Instrumentation and Controls Water Treatment Heat Storage Safety Considerations