-11-
Revision 3/F3601
SAFETY SYSTEM
The purpose of the safety system is to shut off
the flow of gas if the flame goes out. It is
comprised of the flame itself, the
thermocouple, and the flame failure gas valve.
The burner flame is lit by holding in the gas
control knob, which in turn temporarily pushes
the plunger inside the safety valve open and
allows gas to flow through. Once the burner
is lit, the thermocouple will begin to generate
millivolts (after about 10 to 30 seconds of
being heated) and will energize the
electromagnet inside the gas valve. Once
energized the electromagnet holds the
plunger inside the gas valve in the open
position. The plunger has to have been
pushed all the way in for the electromagnet to
be able to hold it in place. If the burner flame
goes out for some reason, the thermocouple
will cool after about 10 to 30 seconds and
stop generating millivolts. The electromagnet
will then de-energize, and the plunger will
snap shut, cutting off the flow of gas.
Detail of each component in the safety system
is explained below.
THERMOCOUPLE
The thermocouple is a device that generates
electricity when heat is applied to the tip.
The tip of the thermocouple is located in the
burner flame, and the nut at the other end of
the thermocouple screws into the back of the
gas valve. Inside the copper tubing is a wire
which is joined at the tip but insulated from the
rest of the tubing. These two parts (the
copper tubing and wire) make up the "wiring"
for an electrical circuit. When these two
dissimilar metals, wire and tip, are heated an
electrical voltage is produced. This type of
thermocouple generates between 7 and 30
millivolts when heated in the pilot flame.
Figure 3.1
3.2 EXPLANATION OF CONTROL
SYSTEM
Insulator
Nut
Conductor
Tip
Internal
Wire
position, stopping the flow of gas.
Millivolts are provided to the electromagnet by
the thermocouple (not shown) which
generates millivolts when heated. The
thermocouple screws into a fitting at the back
of the gas valve to make an electric
connection. By pressing in the gas control
knob, the plunger can be temporarily held
open while lighting. There's two reasons for
this; gas has to flow through the safety valve
to make it possible to light the burner, and
secondly the plunger has to be pushed all the
way in for the electromagnet to hold it in. I.e.;
the electromagnet is strong enough to hold
the plunger in once there, but is not strong
enough to pull it in by itself. Sometimes a
problem with the flame not staying lit after
releasing the button can be attributed to not
pushing the plunger all the way in.
The Troubleshooting Guide (Section 5) should
be used to identify any incorrect operation.
On correct identification of the operating fault
the Troubleshooting Guide will make
reference to the corrective action required, or
refer to the Fault Diagnosis section and/or
Inside the body of the gas valve is an
electromagnet connected to a spring loaded
plunger. When the electromagnet is
energized, it holds the plunger in, allowing gas
to flow through the valve. When the
electromagnet
is
de-energized, the plunger snaps to the closed
Figure 3.2
Thermocouple
Gas flow
Electromagnet
Plunger
Shaft
Knob
Gas flow
Plunger
ELECTROMAGNETIC FLAME FAILURE
GAS VALVE
The purpose of the safety valve is to shut off
the flow of gas if the pilot flame goes out.
