radiators and the relatively cooler, heavier column of
water returning from the radiators. The second type is
the forced-circulation system in which water is
circulated by a power-driven pump.
The distribution systems and piping for hot-water
heating systems and for domestic hot-water supply
systems are simpler in design than those for steam
because there are no traps, drips, or reducing valves.
Several items, such as supports, insulation, and some
valves and fittings, are the same for steam and
Gravity hot-water distribution systems operate
because of the gravitational pull on the heavier cool
water, which sinks as the heated water becomes lighter
and rises. At this point, some of the types of gravity
systems that are currently used are discussed.
Figure 4-63.A one-pipe, open-tank gravity hot-water
One-Pipe, Open-Tank System
The one-pipe, open-tank gravity distribution
system shown in figure 4-63 consists of a single
distribution pipe that carries the hot water to all of the
convectors or radiators and returns it to the boiler. This
system is easy to install and moderate in cost.
The water that flows into the radiators at the end of
the system has a lower temperature than the water
entering the first radiators. A system of this type should
be designed so the water reaching the last convector is
not too much cooler than the water reaching the first
convector. Because of this progressive temperature
drop in the distribution system, convector radiators
should be installed at the end of the system to equalize
the amount of heat radiation per radiator. It is difficult
to get enough circulation by gravity to give the system
small convector temperature drops; consequently, we
do not recommend the one-pipe, open-tank gravity
Two-Pipe, Open-Tank System
Figure 4-64.A two-pipe, open-tank gravity hot-water
Many hot-water gravity distribution systems are
two-pipe, open-tank systems, as shown in figure 4-64.
are usually made into a gravity return, which pitches
This heating system is constructed with separate water
downward to the return opening in the heating boiler.
mains for supplying hot water and returning cold
The water temperature is practically the same in all
water. The radiators are connected in parallel between
radiators, except for the allowance to be made for the
the two mains. In the two-pipe, open-tank gravity
temperature drop in the distribution supply mains
system, the distributing supply mains are either in the
occurring between the boiler and the end of the circuit.
basement with upfeed to the radiators or in the attic.
Water temperatures are the lowest at the end of the
When the system is in the attic, it has overhead
circuit. The amount of temperature drop between the
downfeed supply risers. The return mains are in the
beginning and the end of the line depends upon the
basement. Return connections for the two-pipe system
length of the main and upon the heating load.