pressure-relief valve is shown in figure 4-61. This
valve is usually on the top of the boiler. It contains a
spring-loaded valve that unseats when the pressure in
the system increases to a predetermined value, thereby
allowing water to escape until the pressure drops to a
safe point. A valve of this type can be adjusted for
Pressure-relief valves may eventually corrode and
stick if they are not forced to operate occasionally. It is
a good practice, once each month, to increase the
pressure to a point that operates the valve. When the
relief pressure on the gauge exceeds the setting of the
valve, check the valve pressure with an accurate gauge
and adjust it to the required amount. However, do not
exceed the maximum safe pressure of the boiler.
The operator must know the water pressure in the
boiler at all times. A gauge is connected to the top of
the boiler. It shows the water pressure in the boiler and
in the system in pounds per square inch. This gauge is
usually a combination gauge that also indicates boiler
water temperature and altitude. The type shown in
figure 4-62, however, indicates pressure only.
Little maintenance is required for this unit other
than to clean the glass so the gauge can be read. Some
types of pressure gauges are constructed so they can be
re-calibrated. However, the proper equipment to do
this is not always available in the heating shop. To
calibrate a pressure gauge properly, you must have
either a master gauge set or a deadweight tester.
Water-Level Control Valve
Figure 4-62.A typical water pressure gauge.
Water is added to a hot-water heating system by
either a manually operated water valve or an automatic
valve, which is controlled by a float mechanism. Both
valves are nearly identical to those used in the
free-water system of a steam boiler.
The airflow switch, or "sail switch" as it is
sometimes called, is in the stack, breeching, or the air
inlet to the boiler. This switch shuts down the firing
equipment in the event of an induced or forced draft
failure. To check the operation of this switch, you
restrict or shut off the draft. When you have done this,
the switch should shut off the burning equipment.
It is a good practice to test the pressure relief
valve by what method and what frequency?
Figure 4-61.A typical pressure-relief valve.
What is the name sometime used to refer to an
Learning Objective: Identify types of hot-water
distribution systems and components. Understand the
operation and maintenance of these systems.
In hot-water heating systems, the water is heated at
a central source and circulated through pipes to
radiators, convectors, or unit heaters. There are two
general types of low-temperature, hot-water heating
systems. The first type is a gravity system in which
water circulation depends upon the weight difference
between the hot column of water leading to the