Figure 4-24.Internal view of a furnace.
The gas manifold assembly includes the gas
valves, pressure regulator, and those components that
automatically control the flow of gas to the pilot and
main burner. It is directly connected to the burner.
burner mixing tube or "venturi," where it mixes with
the gas that burns at the burner ports. The secondary air
is supplied around the base of each separate burner
flame by natural draft or is induced by a draft fan.
Gas Burners and Controls
To use natural gas, a nearly ideal fuel, requires
comparatively simple equipment and unskilled labor.
This clean gas is almost free of noncombustible and is
therefore clean. However, it is relatively dangerous
compared to coal or oil because it mixes easily with air
and burns readily. Extreme care must be exercised to
prevent or stop any leakage of gas into an unlighted
furnace or into the boiler room. All gas burners should
be approved by the American Gas Association and
installed according to the standards of the National
Board of Fire Underwriters.
The gas burners used in gas-fired furnaces usually
have a nonluminous flame and are the Bunsen type, as
shown in figures 4-25 and 4-26. Part of the air needed
for combustion is primary air that is drawn into the
The gas burner controls include the following
unitsmanual gas valve, gas pressure regulator,
solenoid gas valve, diaphragm valve, pilot light,
thermocouple, thermocouple control relay limit
control, heat exchanger, draft diverter, and humidifier
(fig. 4-27). A manual gas cock or valve must be
installed ahead of all the controls.
MANUAL GAS VALVE.The manual gas
valve is installed on the heating unit next to the gas
pressure regulator. It is used to shut off the gas to the
heating unit in case some of the controls must be
repaired or replaced.
GAS PRESSURE REGULATOR.The gas
pressure regulators used in domestic gas-heating
systems are usually of the diaphragm type, as shown in
figure 4-28. A gas pressure regulator maintains the
desired pressure in the burner as long as the gas main