LOW MEDIA DIFF. PRESS. indicator light (Fig. 2)
An indicator light that denotes pressure/flow status of thermal fluid in the helical coil. It is controlled by a normally open switch (Fig. 12) that detects abnormally low differential pressure of thermal fluid across the helical coil. If the indicator light is off the switch senses that the differential pressure at the outlet is too low and remains open. When the light is on there is enough differential pressure to close the switch. See Pressure switch herein for additional information.

Inadequate pressure/flow of thermal fluid is probably the most common cause of heater malfunctions. A clogged strainer is the most common cause of inadequate flow of thermal fluid.

HIGH MEDIA DIFF. PRESS. indicator light (Fig. 2)
An indicator light that denotes pressure/flow status of thermal fluid in the helical coil. It is controlled by a normally closed switch (Fig. 12) that detects abnormally high differential pressure of thermal fluid across the helical coil. When the indicator light is off the switch senses that the differential pressure at the inlet is too high and opens. When the light is on the differential pressure is low enough to close the switch. See Pressure switch herein for additional information.

COMB. AIR AUX. CONTACTS indicator light (Fig. 2)
An indicator light that denotes the status of a set of auxiliary contacts (Fig. 4) on the motor controller that connects electrical power to the burner fan motor. When the indicator light is on electrical power is connected to the fan motor. When the light is off the contacts are open because electrical power is not connected to the motor.

The contacts are also part of the limit circuit and will shut down the heater when opened. The contacts automatically close when power is restored to the motor.

LIMITS MADE indicator light (Fig. 2)
An indicator light that denotes the status of the limit circuit, which has several switches connected in series with each other. This circuit is also connected to the Fireye flame monitor so that its current flows through the limit circuit of the Flame monitor and to solenoids of the fuel control valves.

When this light is on all of the switches in the limit circuit are closed and operating conditions are normal.

When this light is off one or more of the switches in the limit circuit is open because of an abnormal operating condition. An open switch disconnects current flowing to solenoids of the fuel control valves. This closes the fuel valves, shutting off fuel to the burner. Consequently, the flame monitor will display the message: FLAME FAILURE 3 TO P INTERLOCK OPEN. The flame monitor must be manually reset after the abnormal condition has been cleared.

A separate indicator light is also provided for each switch in the limit circuit, except for the low combustion air switch, which is the last switch in the series. The indicator lights identify the status of each individual limit switch. All indicator lights in the limit circuit have clear lenses, except for the LIMITS MADE light, which has a green lens.

As already noted, there is no indicator light labeled LOW COMBUSTION AIR. However, if the low combustion air switch opens due to low combustion air, the green LIMITS MADE light will go out, but none of the clear lights will go out. Thus, the limits made light in effect functions as a low combustion air light in addition to a limits made light.

PILOT GAS VALVE ON indicator light (Fig. 2)
This indicator light denotes the status of two ignition circuits: ignition spark transformer and pilot gas solenoid valve (Fig. 13). The light has an amber lens. When the light is on, power is connected to the ignition spark voltage transformer and to the pilot gas solenoid valve. As a result the pilot valve opens and high voltage is generated in the secondary winding of the spark ignition transformer. The high voltage produces a spark across the gaps in the points of the igniter to ignite the pilot gas. When the light is off, power is not connected to the transformer or solenoid valve. Hence, there is no spark and the pilot gas solenoid is de-energized. Power to the circuit is controlled by the Fireye flame monitor. This light was previously labeled IGNITION ON.

GAS VALVE ON indicator light (Fig. 2)
This indicator light denotes the status of the main gas shutoff valve, which is an actuator-operated valve. When the light is on, the valve is open. When the light is off, the valve is closed. This light has an amber lens.

OIL VALVE ON indicator light (Fig. 2)
This indicator light denotes the status of the main oil valve and the auxiliary oil valve. Both are solenoid-operated valves. When the light is on, the valves are open. When the light is off, the valves are closed. This light has an amber lens.

HEATER ALARM indicator light (Fig. 2)
This indicator light denotes the status of the heater alarm system. When the indicator light is on the heater is in an alarm state. The light has a red lens. The alarm is controlled by the flame monitor, which turns on the alarm when it detects a variety of abnormal conditions. The monitor displays one of several messages to report the condition that caused the alarm.