10 January 2002
2-1. GENERAL. When designing a heating or cooling distribution system, the designer must
first select two critical items: media type and system type.
2-2. DISTRIBUTED MEDIA SELECTION.
a. Connecting to an existing system. Almost all heating and cooling distribution systems will be
connected to an existing central distribution system. In this case, the designer most often designs for the
media to which it is being connected-HTHW, LTHW, steam/condensate, or chilled water.
b. Installation of new system. When no existing system is present, the designer must select the
system that is most appropriate for the end user. High temperature hot water and steam/condensate
systems are the most common types of distribution systems currently used on military installations.
However, a new system should only use the temperatures and pressures necessary to meet the
requirements of the installation. For example, the use of high pressure steam sterilizers or steam kettles at
several facilities may require the use of a high pressure steam or HTHW system. However, it is usually
much more cost effective (on a first cost and life cycle cost basis) to use a low or medium temperature hot
water distribution systems whenever possible and to incorporate stand alone high pressure/temperature
systems where required. The lower maintenance costs, safer operation, longer life of systems, and simpler
system controls for hot water systems often offset the costs of larger piping required. For further assistance
for selecting the system type, refer to ASHRAE Handbook, "HVAC Systems and Equipment."
2-3. SYSTEM TYPES. When selecting a distribution system, the designer must determine which system
types apply to a particular medium. The designer must also exclude systems which are not appropriate for
a particular site or for which the customer has no interest. Examples of this are locating aboveground
systems in non-industrial areas where the installation is sensitive to the aesthetic appearance of the area or
routing concrete shallow trench systems through drainage swales or flood plains.
a. Heat Distribution Systems in Concrete Trenches (chapter 4). This system is a buried system with
its removable concrete cover installed at grade and will typically be used for HTHW and steam/condensate
systems. In rare instances, it may also be used for chilled water and LTHW in the event no plastic piping
is installed in the same trench as high temperature (greater than 250 degrees F) piping systems.
Experience has shown that if insulation of a high temperature system is compromised, temperatures can
increase to such a level and cause damage to the plastic piping.
b. Pre-engineered Underground Heat Distribution Systems (chapter 5). This system is designed for
higher pressure and temperature applications. The two types of pre-engineered systems are the drainable-
dryable-testable (DDT) type which is used for high pressure steam/condensate and HTHW at all sites, and
high temperature hot water at any type of site, and the water spread limiting (WSL) type which is used only
for steam/condensate systems in bad and moderate sites. HTHW supply and return lines may be provided
in a single casing; however, steam and condensate lines must always be provided in separate casings
because condensate lines typically last less than half as long as the steam line and are easier to replace
when in a separate casing.
c. Prefabricated Underground Heating/Cooling Distribution System (chapter 6). This system is
designed for lower temperature and pressure applications. It is typically used for LTHW, chilled water, or
combination LTHW/chilled water systems.
d. Aboveground Heat Distribution System (chapter 7). This system may be used for HTHW,
steam/condensate, and LTHW systems, and for chilled water systems where freezing is not a concern.