Thermistor; Flame Rectification - Rinnai RHFE-551FA 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.
Flame Rod
104
Alternation current
Direction of
AC current
Main Burner
Rinnai Direct Vent Service Manual
Resistance decreases as
the temperature increases.
Temperature
AC 200 volts
PC Board
Flame Guard Amplifier
Gas Valve Circuit Solenoid