Sequence of Operation

DC Voltage

0 to 5 VDC

6 to 11 VDC

12 to 18 VDC

Flame Status

No Flame

Weak Flame

Strong Flame

The direct-fired heater is most easily understood when broken down into smaller individual systems. There are two main systems, a make-up air fan and a heater. The make-up air fan consists of a blower and motor. The heater may be further broken down into two control systems, the Flame Safety Control (FSC) and the Modulating Gas System (MGS). The burner mixes air with the gas (Natural or LP) which heats the air.

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 using a DC voltage meter attached to the test jacks on the top of the control. 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 .80 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 start-up.

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

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 .80 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

MR212 specifications

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