Figure 4-48.Schematic of a remote-bulb device.
Figure 4-49.A typical Bourdon spring tube.
hollow, partially flattened, bent tube, called a Bourdon
spring tube. The pressure inside this tube tends to
straighten it, and in so doing, it moves the lever
mechanism that turns the pointer. The pressure gauge
measures the pressure in pounds per square inch (psi).
Humidity-responsive devices open or close
solenoid or motorized valves, which control the flow
of water or steam to humidifying equipment. The
sensitive element, which actuates the motion in this
device, consists of a group of human hairs. These hairs
lengthen when the humidity is high and shorten when
the humidity is low.
Accumulation of dust and grease on these hairs,
while not damaging, may decrease the sensitivity of
the controller. Consequently, the element should be
cleaned periodically with a camel's-hair brush and
clean ether. A complete wetting with distilled water
should follow this cleaning.
Electrical Switches.Electrical switches in
heating control equipment operate electrical circuits in
response to signals from automatic control units. In
other words, the actions initiated by devices
responsive to temperature, pressure, and humidity
changes open or close switch contacts. These, in turn,
control the operation of the heating plant through
electrical circuits. Switches may be either the
snap-action type or the mercury type.
Snap-action switches vary in their designs.-Some
are constructed so they have an over-center spring
arrangement that is designed so the movement of the
actuating lever engages the spring and causes the
switch to move with snap-action. The snap-action type
of switch is shown in figure 4-50, view A.
Another snap-action switch shown in figure 4-50,
view A, has a small magnet that causes the electrical
contacts to remain firmly closed. It also provides the
switch with the snap-action effect. The contacts of this
switch must open or close quickly to avoid excessive
arcing across the points. Arcing burns the contacting
surfaces, which eventually causes switch failure.
A mercury switch has the electrical contacts and a
small amount of mercury in a hermetically sealed short
glass tube, as shown in figure 4-50, view B. Tilting the
switch causes the mercury inside the tube to cover or
uncover the contacts. When the contacts are covered,
the electrical circuit is completed.
Every electrical switch is designed so it has a
specific rated capacity in amperes and volts; for
example, a capacity of 8 amperes at 110 volts. An
electrical switch should never be overloaded because
overloading causes overheating, which eventually
results in switch failure that can create a fire hazard.
The standard controls furnished for automatic
fuel-burning equipment come in sets designed for
warm-air, hot-water, and steam-heating systems. A
standard set usually consists of a thermostat, limit
control, primary control, and electric motor. Auxiliary
controls are those designed for a specific function in a
warm-air, hot-water, or steam-heating system. They
are in addition to the standard controls.
Thermostat. The thermostat is the nerve center
of the heating-control system. It is the sensitive unit
that responds to changes in room temperature. It
indicates whether more or less heat is required from the