You will also note that the radiator valves in
Although all gravity, one-pipe, air-vent systems are
alike in design, it is seldom that two installations are
alike in detail. Since the details differ with the make and
model of equipment, it is recommended that the
manufacturers installation procedures be followed.
Also, you should follow the mechanical blueprints for a
particular installation. There is some general
information in this section that applies to most heating
systems of this type.
To prevent water hammer and re-evaporation of the
water, drain all condensate from the lines. The
necessary internal drainage can be obtained by sloping
the lines down, in the direction of condensate flow, at
least one-fourth of an inch for every 10 feet of pipe. The
radiators must also be tilted, so the condensate flows out
of them into the same pipe through which the steam is
Air vents are installed in the steam lines and
radiators to eliminate air in the system. Air in the system
tends to block the flow of steam, and it consequently
acts as an insulator by preventing the emission of heat
from the heating surface. Therefore, the air must be
quickly and effectively vented from the heating
equipment and steam lines to get quick and even heating
from the steam-heating system. Most steam distribution
systems are now fitted with automatic vents that permit
the air to pass but which block the passage of steam.
Figure 3-5 shows air vents in the radiator and the
The operating instructions for gravity, one-pipe,
air-vent systems vary from one installation to another.
The manufacturer of the equipment usually furnishes
the specific operating instructions for the equipment.
Generally speaking, most steam systems have a
main steam stop valve located on the top of the boiler.
The purpose of this valve is to hold the steam in the
boiler until you are ready to let it out. When you are
ready to turn the steam into the distribution system, you
should only crack (open very little) the valve. The
reason for doing this is to allow the system to warm up
slowly and avoid any thermal shock to the lines and
fittings. After the system has warmed up, the main
steam stop valve should be opened slowly. While
opening the valve, you should check often to ensure that
the proper water level is maintained in the boiler.
one-pipe steam distribution systems should be either
completely open or completely closed. Partial opening
of the valve interferes with the proper drainage of water
from the radiator.
In this portion of the text, the common problems
you are most likely to encounter in the field when
maintaining a gravity, one-pipe distribution system are
discussed. The most probable causes of these problems
and the remedies for them are considered.
When a radiator fails to heat or water hammer
occurs, there are several probable causes. One is the
failure of the air vents to function, thereby causing the
radiator to become air bound. A second cause is that the
radiator valves are not completely open. Another cause
is that the radiators and lines are not correctly pitched.
To remedy these causes of heat failure, you should
inspect the operation of the air vents and the positions of
the radiator valves to make sure they are open. You
should then check and correct, if necessary, the pitch of
the radiators and lines when the other checks do not
correct the trouble.
A fluctuating waterline in the boiler can be caused
by an excessive pressure drop in the supply lines, which,
in turn, is usually caused by partial stoppage in the
pipes. This, of course, can only be remedied by
removing the cause of the stoppage. Uneven heat
distribution is another trouble that you may encounter.
This can be caused either by inoperative radiator vents,
improperly vented steam mains, or incorrectly pitched
mains. To eliminate this uneven heat distribution, you
should check and clean the air vents at the radiator and
those in the steam mains. Then check and correct, as
required, the pitch of the steam lines if the other
remedies have not corrected the trouble.
TWO-PIPE VAPOR SYSTEM WITH A
The two-pipe vapor system with an alternating
return trap, as shown in figure 3-6, is an improvement
over the one-pipe system. The return from the radiator
has a thermostatic trap that permits the flow of
condensate and air from the radiator. It also prevents
steam from leaving the radiator. Because the return
mains are at atmospheric pressure, or less, a mechanical
return trap is installed in the system to equalize the
condensate return pressure with the boiler pressure. The
mechanical return trap is primarily a double-valve float