10 January 2002
8-4. LTHW and CW systems.
a. Pipe Sizing. The most efficient method of determining pipe size for LTHW and CW systems is to use
head loss vs. flow rate charts such as those found in ASHRAE Fundamentals, Chapter "Pipe Sizing". These tables
are based on 60 deg. F water so for chilled water pipe sizing there is little error introduced using these charts. For
LTHW systems, the use of the charts does introduce some error. However, the error is on the conservative side
(the charts overstate the pressure drop of LTHW).
b. LTHW and CW Material Selection.
(1) Valves. Typically, valves on either LTHW or CW systems will be 150-pound class and will be
located in the valve manholes. Ball valves provide a good means for line isolation. Although nonmetallic valves
are sometimes allowed for these systems, metallic valves should be used for durability.
(2) Piping. The most common piping materials used for LTHW and CW systems are steel, copper
tubing, reinforced thermosetting resin pipe (fiber- glass) and, for CW only, polyvinyl chloride and polyethylene.
However, do not include nonmetallic piping in the same valve manholes with HTHW and steam systems. Chilled
water lines using PVC piping must be installed in separate valve manholes since PVC can be thermally
damaged at relatively low temperatures. Outside the valve manholes, a separation of 15 feet (minimum) must be
maintained between pre-engineered underground HTHW and steam systems and PVC encased, prefabricated
underground heating/cooling distribution systems to avoid thermal degradation of the PVC.