Forced-circulation hot-water distribution systems
have several advantages. They permit the use of
smaller pipe sizes and allow the installation of
radiators at the same level as the boiler, or below,
without impairing water circulation. By using a
circulation pump, a positive flow of water is assured
throughout the system. In larger installations,
especially where more than one building is served,
forced circulation is almost invariably used. With the
development of a circulation pump of moderate cost,
the forced-circulation system is being used more in
small heating installations.
Even as in gravity systems, forced-circulation
systems can consist of a one-pipe or a two-pipe, upfeed
or downfeed, and can be equipped with a direct or a
reversed return. Although these systems usually have
closed expansion tanks, they may have open tanks.
One-Pipe, Closed-Tank System
closed-tank, forced-circulation system shown in figure
4-66 is similar to the one-pipe gravity system, but with
the addition of a circulating pump.
The circulation to individual radiators is improved
by special supply and return connecting tees. These
tees, by an ejecting action on the distribution supply
main and an ejecting action on the return, combine to
use a portion of the velocity head in the main to
increase circulation through the radiators. Tees of this
type also aid stratification of hot and cold water within
the distributing main. They are designed to take off the
hot-test water from the top of the main and to deposit
the colder water on the bottom of the main.
Figure 4-67.A two-pipe, closed-tank, forced-circulation
s y s t e m .
Two-Pipe, Closed-Tank System
The general arrangement of the piping and
radiators for the two-pipe, forced-circulation
distribution system is the same as that for the two-pipe
gravity system. The relative locations of the
compression tank relief valve and the circulating pump
are shown in figure 4-67.
The component parts of a hot-water distribution
system are similar to that of steam heating systems as
described in chapter 3. They include the following:
pipelines, radiators, convectors, unit heaters,
Figure 4-66.A one-pipe, closed-tank distribution system with
a circulating pump.
circulating pumps, reducing valves, flow-control
valves, and special tlow fittings.
The piping system constitutes the closed
passageway for the delivery of hot water to the points
where it is used. Pipelines are made of lengths of pipe
fastened by screwed, flanged, or welded joints. They
have valves and fittings, such as tees, unions, and
elbows, according to the needs of the installation.
Pipelines are supported by hangers and fastened by
anchors. Expansion joints or loops allow for
Mains and branches of the pipeline should be
pitched so the air in the system can be discharged
through open expansion tanks, radiators, and relief