XLT TS Series Service Manual 7
Burner Subsys em
The Burner subsystem consists of a forced air burner and gas train a pilot valve
(V2) a main valve (V1) a burner control (BC) a temperature control (TC1) a
thermocouple (T/C) a modulating valve (V3) and signal conditioner (SC1) and a 24V
transformer (XR1). When the burner switch S2 is turned on power is applied from S1 to
the common terminal of the output relay of the temperature control. The output 2 relay is
closed by the temperature controller (high alarm auto reset programmed) and provides
power to the primary of the 24V transformer and also to the burner blower motor (M4).
The transformer provides the necessary voltage for both the signal conditioner and the
burner control. The burner blower motor (M4) must be spinning for the internal
centrifugal switch (S4) to complete the circuit to the burner control.
The thermocouple provides a temperature feedback to the temperature control.
The temperature control provides a 4-20 MA output to the signal conditioner. The signal
conditioner in turn proportionately provides a 2-20 VDC 400 MA power output to the
modulating valve. If the thermocouple either breaks or becomes out of range the
temperature controller will sense and force the output off.
The input of the temperature controller is protected by fuse F2 and the output is
protected by fuse F10. The transformer and burner blower motor are protected by fuse
F3. Fuse F9 protects the input of the signal conditioner while fuse F6 protects the output
to the modulating valve. Fuse F4 protects the pilot valve (V2) and fuse F5 protects the
main valve (V1). The combination gas valve has the pilot valve the main valve and the
pressure regulation contained within it. The knob on top of the combination valve must
be left in the ON position. See Figure 1. The lamp (L1) on the control box if present
indicates whether the flame sensor in the burner pilot is hot.
When the burner control receives power the pilot valve opens and the spark
generator operates. If the pilot burner lights; the burner control senses flame current
through the spark wire the spark generator then shuts off and the main valve opens. The
burner control continues to monitor the pilot flame current. If the pilot burner does not
light after 90 seconds the burner control locks out and must be reset by switching the
burner switch off and then back on. If the power is interrupted the burner restarts when
power is restored. If there is a pilot flame failure (no current sensed in the spark wire) the
burner control starts trial for ignition again.
The modulating gas valve is plumbed in series with the main gas valve. See
Figure 3. In normal operation the main gas valve stays on all the time while the
modulating valve is controlled by the temperature control. If there is a large difference
between the actual temperature measured by the thermocouple and the set point
temperature the temperature controller has a high output value to the signal conditioner.
The signal conditioner in turn produces a corresponding high voltage output to the
modulating valve which opens up to the maximum and causes the burner to operate at
maximum. As the temperature difference narrows the outputs similarly decrease causing
the burner to fire at a reduced rate. There is enough heat loss even in an idle oven for the
modulating valve to always be open somewhat. Therefore the main valve is also always
open and the burner control only needs to operate the ignition cycle once.
The modulating valve usually requires no adjustment. There are two bypass
needle valves on the opposite sides of the valve. The one that is inaccessible is always
closed by default. The accessible one is also usually closed but may be slightly open
depending on the model of oven. Another adjustment is the low fire bias screw which is
