Gas/Propane Basics...
Since the available head or force producing
circulation in a gravity system is limited, the piping
must be of such size that the friction loss at the
desired flow is not in excess of the available head.
Gravity circulation heads can be obtained with various
supply and return temperatures. The basic principles
determining design in gravity and forced systems
are the same but since the former has less total head
available to produce flow, greater precautions are
necessary in design to secure even distribution of heat
with gravity systems. Gravity systems are no longer of
commercial or residential importance. A typical system
using forced hot water as its concept might look like
Figure 3.
On older systems, an air cushion tank (Figure 4)
was used to absorb the expansion process of the boiler
heating water. Diaphragm expansion type tanks are
replacing many of these tanks. On many of the older
systems, it was typical to see the pumps on the return
side of the boiler. These systems had a larger water
Figure 4: Closed Expansion Tank. This type of expansion
tank is sealed against free venting to the atmosphere.
volume than we see with boilers today. There are still
some of these older systems working in homes today.
Later on in this section, there will be a discussion
about the concept of pumping away from the point of no
pressure change.
It really does not matter whether it is an oil-or gasforced
hot water boiler; the controls used are the same.
In Figure 5 we see an example of a typical oil boiler
with controls for domestic hot water, circulator control,
low limit and high limit control.
In Figure 6, we see what would be called a typical
“Package Boiler.” This idea was the result of the need to
simplify the shipping and installation of boilers. In the
past, boilers came in sections and the installer put them
together at the job site. This included the trim packages
for steam called “S” Trim and the trim for boilers called
a “W” Trim. This package boiler included the circulator
Figure 5: Interior view of a typical boiler used in a
hot-water heat system.
on the return side of the boiler, expansion tank and
other associated controls. The draft hood above the
boiler was fixed at a certain height to insure some selfventing
of the unit. The draft hood location should not
be altered in any way.
There are many of these package boilers still in
operation today. This was typically a boiler that
operated in the range of efficiency around 65% to 70%.
It was an atmospheric boiler and in many cases was a
powerpile self-generating system, wired as an isolated
burner circuit from a 24-volt aquastat relay. The relay
would have an “Auto” or “Manual” switch to allow
gravity operation in the event of a power failure. ICM
Figure 6: Typical Gas Package Boiler
22 ICM/September/October 2017