Figure 4-33.A thermopile.
Figure 4-34.A typical thermocouple and valve relay
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
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
Oil-fired furnaces are similar to gas-fired units in
physical arrangement. Internally, oil-fired units have
three areasthe burner compartment, the combustion