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.

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. 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.

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 units - manual 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