Figure 4-33. - A thermopile.

Figure 4-34.vA typical thermocouple and valve relay assembly.

closing of the valve or switch prevents the burner from filling the combustion chamber with unburned gases.

To re-light the pilot light, push up the reset button at the bottom of the relay and allow the gas to flow to the pilot light. Since some heating units are not equipped with relays, the pilot light is not automatically shut off in case of gas supply failure.

The relay shown in figure 4-35 is an electrical switch type of relay. It is entirely electrical and can be used as a controlling unit for either the magnetic or diaphragm gas valves. This unit is actuated by the electric current generated by the thermocouple. It controls the operation of the gas valve in the magnetic and diaphragm valves. A relay of this type must also be reset manually for normal operation.

LIMIT CONTROL. - The limit control in a gas burner system is a safety device. It shuts off the gas supply when the temperature inside the heating unit becomes excessive. The limit control device can be adjusted to the desired setting. It exercises direct control on the gas or diaphragm valve.

HEAT EXCHANGER. - This unit or assembly may be either a single or sectional contoured steel shell. It extends vertically from the burner enclosure to the flue exit. Functionally, it transmits heat from the hot gases of combustion to the circulating warm air that passes the outer surfaces.

Draft Diverter. - The diverter is simply a sheet metal chamber that encircles the flue. It has an opening at the bottom to allow air to be drawn in by the flue draft. Its purpose is to reduce the downdrafts and updrafts that are objectionable to pilot and burner operation.

Humidifiers. - Humidifiers used with forced warm-air heating systems are usually of the pan type shown in figure 4-36. Unless the water is relatively free of solids, these humidifiers require frequent attention, since the float may stick in the OPEN position or the valve may clog. Overflowing of the pan may result in a cracked heating section, and a stopped-up inlet valve will make the humidifier inoperative.

The drum type of evaporative humidifier uses an evaporation pad in the shape of a wheel. The slow-turning wheel is submerged in the water in the lower pan where the spongelike plastic foam material becomes saturated with water. The wheel lifts this portion of the pad and exposes it to the warm, dry air flowing through it. The air then absorbs more moisture because of lower relative humidity at a higher temperature.

OIL-FIRED FURNACES

Oil-fired furnaces are similar to gas-fired units in physical arrangement. Internally, oil-fired units have three areas - the burner compartment, the combustion