Rinnai RHFE-431WTA, Service Manual

Page: 64 / 68

64 Rinnai Direct Vent Service Manual
Thermistor
The thermistor is a semiconductor which
reacts to changes in the temperature by
altering it resistance. The relationship
between the temperature and the
resistance is outlined in the graph. The
resistance value is detected
electronically, and activates the
amplification circuit, which in turn
operates the gas input control circuit
(temperature control).
Flame Rectification
Flame rectification is utilized to check flame presence. A flame sensing electrode consisting of a stainless steel
rod supported in a ceramic insulator (flame rod) is positioned above the main burner. An alternating current is
applied to the flame rod by the flame guard amplifier. The body of the burner and the flame guard amplifier (FGA)
are both grounded.
The principle of flame rectification relies on the ability of the flame to conduct positively charged ions more easily
than negatively charged ions. This characteristic is called flame rectification, as the current leaving the burner,
having passed through the flame, is no longer an alternating current. The signal returning to the FGA has been
rectified by the flame, and is used to determine flame presence.
The signal can only be produced by the flame. Short circuits, dirty contacts and other fault situations cannot
simulate the rectified signal. The FGA section of the PC board sends an amplified signal to the main PC board to
indicate flame presence, allowing the gas valves to remain open.
Temperature
R
e
si
sta
n
ce
Resistance decreases as
the temperature increases.
Flame Rod
Alternation current
AC 200 volts
Direction of
AC current
Check
A
m
plif
y
Flame Guard Amplifier
Main Burner
Solenoid PC Board Gas Valve Circuit
1. Plug unit up to the power supply circuit.
2. Now with the unit in the
“Off”
mode press the
“UP”
button first and then press the
“Test Switch”
. This will
enter the unit into the programming mode.
3. To begin programming your desired temperature limit set point, press the
“Up”
or
“Down”
temperature
adjustment button on the control panel until you have achieved your set point. Once you have select your
maximum temperature set point, press the
“Test Switch”
once, this locks in your maximum operating
temperature at the temperature you selected.
Temperature set point limits are - (H) high fire, 80, 78, 76, 74, 72, 70. The unit will limit its operating
temperature to one of the set points above, based on your selection.
Setting a Maximum Temperature
RHFE-263FA/FAII, RHFE431WTA,
RHFE-556WTA, RHFE-1004FA