Sequence of Operation
The
Flame Safety Control
The first system to understand is the Flame Safety Control. The FSC is there only to monitor the flame, NOT to control temperature. The FSC uses a flame rectification sensor mounted on the pilot assembly to detect the presence of flame in the burner. Flame strength and presence can be measured at the FSC by reading the rectified flame signal. This is done by removing the FSC cover and using a DC voltage meter in the test jacks. Flame is present when the DC voltage reads between 6 and 18 VDC. Ideal flame intensity produces a signal of 12 VDC or greater. The FSC is also wired into an airflow switch, which tells it whether there is proper airflow through the unit (not just any airflow, but proper airflow). Proper airflow occurs when there is a
.15 in. w.c. to .65 in. w.c. differential pressure drop across the burner. When the airflow through the heater produces a pressure drop in this range, the FSC indicates so by illuminating the AIRFLOW LED. The FSC controls the opening of the redundant solenoid gas valves and the operation of the spark igniter to initiate a pilot flame upon
Flame Safety Controller
DC Flame Signal
The OPR CTRL LED indicates that there is power to the FSC. Next, the AIRFLOW LED will come on if there is proper airflow through the
unit. Third, the unit will pause to purge any gasses or combustible vapors before attempting flame ignition. Then, there is a Pilot Trial For Ignition (PTFI) and the PTFI LED comes on. During PTFI, the FSC opens the pilot gas valve and allows gas to flow to the pilot assembly. At the same moment, the spark igniter is started, causing the spark to ignite the pilot gas. When the flamerod sensor detects the flame, it turns on the FLAME LED, turns off the PTFI LED, and powers the modulating gas system. This is the normal operating mode. The FSC continues to monitor the flame and airflow. Once this occurs, the unit is in a main flame cycle and thus powers the main gas valve and the modulating gas system. This is the normal operating mode. The FSC continues to monitor the flame and airflow. The last LED on the FSC is the ALARM LED. This will turn on when the FSC determines an unsafe condition has occurred, and will not allow the unit to recycle for heat until it has been properly reset. Anytime the FSC has gone into “Alarm” mode, the problem must be diagnosed and corrected to avoid future lockouts after resetting. To begin troubleshooting, or to reset the FSC, shut down power to the heater and restart the heater. This will clear the alarm from the flame safety.
Air Flow Switch | Air Flow Switch |
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There are both high and low airflow switches contained within one housing measuring the pressure drop across the burner. This is to insure that there is proper airflow (.15 in. w.c. to .65 in. w.c.) across the burner and proper combustion at all times. Both switches are wired in series and have single pole double throw (one common contact, one normally open contact, and one normally closed contact) switches that are ‘switched’ by air pressure. There are two airflow tubes in the heater, located near the burner and profile plate assembly (profile plates surround the burner and control air into the burner section). In the case of clogged filters, blocked intake, excessive duct static pressure, or a broken belt, the correct burner differential pressure may not be achieved, not allowing the low
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