10 January 2002
PREFABRICATED UNDERGROUND HEATING/COOLING DISTRIBUTION SYSTEM
6-1. GENERAL. This system is similar to Pre-engineered Underground Heat Distribution System (chapter
5) because it is factory fabricated in lengths, which are transported to the site for field assembly. However,
the system is not allowed for any high temperature water (greater than 200 deg. F) or steam/condensate
systems. The project designer is also responsible for more of the overall design.
6-2. SYSTEM DESIGN.
a. Site information. As with all heat distribution systems, the site investigation is the responsibility
of the designer. The designer will obtain soil borings, be responsible for designing all grading in the area,
and investigate all utilities for possible conflicts with the system. The designer will provide detailed design
plans and profiles of the distribution system routings. The design will ensure a minimum slope of 1 inch in
20 feet is maintained between valve manholes. The site information requirements are covered in detail in
chapter 3 of this manual.
b. Valve manholes. The designer will design all valve manholes for the system as covered in chapter
3 of this manual. As with valve manholes for the Pre-engineered Underground Heat Distribution System,
manhole spacing will not exceed 500 feet and all manholes will have drainage capabilities. Also, all valves,
flanges, unions, and couplings shall be located within the manholes.
c. System Material Selections. Although this system is manufactured in sections in a factory, the
designer will specify all materials.
(1) Piping. The piping materials allowed are steel, copper tubing, reinforced thermosetting resin
plastic (RTRP), or polyvinyl chloride (PVC). RTRP piping can not be routed through valve manholes with
heating systems that could damage the RTRP. PVC piping can not be routed in any valve manhole with any
other heating system piping due to its comparatively low temperature tolerance.
(2) Casing. Allowed casing materials are PVC, polyethylene (PE), or RTRP. Because these
casing materials are susceptible to damage from high temperatures, they must be installed a minimum of
15 feet from buried HTHW or steam systems to avoid plastic deformation and failures of the casing
(3) Insulation. Insulation type for these systems is typically polyurethane foam. Open cell type
insulations, such as fiberglass, mineral wool or calcium silicate, are unacceptable for use with chilled water
systems due to the tendency of condensation forming in these insulations. Insulation thickness will be
specified in the guide specification.
d. Expansion compensation. The designer will perform expansion compensation calculations as
covered in chapter 3 of this manual. When required, based on these calculations, sizes and locations of all
expansion loops and bends, and any other expansion-compensating device, will be clearly shown on the
contract drawings. The designer shall provide expansion loop details.