MODERN BURNER UNIT (MBU)
THEORY OF OPERATION
THEORY OF OPERATION (Refer to Figure 1-3) .
Electrical Supply. A 24V DC supply is connected to the power connector (1). Voltage is then supplied to the
main control board (2), which contains all vital electrical circuits.
Fuel Supply . The fuel tank (3) is filled either through the fuel tank fill plug (4) before the MBU is installed into an
appliance, or after installation into an appliance by connecting a quick disconnect hose to the fuel fill nipple (5)
and the feed adaptor on a fuel can, then pressing the FUEL button (6). This energizes the air compressor (7) and
opens the tank fill valve (8) (requires electrical power connection). A full tank of fuel allows up to four hours of
operation before the MBU shuts down. A check valve (9) prevents fuel from entering the air line during shipping
Power-Up. MBU power-up is initiated by pressing the power switch (10). This tests the indicator lights (11) for 3
seconds. The hour meter (12) back light illuminates to indicate the power-up condition. The digital numbers will
sequence before setting on actual accumulated operating hours.
Start-Up. With burner control (13) in HIGH (START) position, MBU startup is initiated by pressing the START
button (14), which energizes the ignitor (15). After two seconds the compressor (7) starts, and the fuel valve (17)
on the fuel delivery block assembly (18) opens, allowing fuel to flow to the nozzle (19). The vent valve (20) on the
fuel tank (3) opens, allowing air to enter the fuel tank. The compressor (7) draws air through the air filter (21), on
the fuel delivery block assembly (18), pressurizing the fuel nozzle (19). Simultaneously, fuel is drawn from the fuel
tank (3), through a 30 micron filter (22) and a zero pressure regulator (23) to the burner control valve (24), which
regulates the flow to the nozzle (19). This fuel-air mixture is sprayed into the burner (25) in a fine mist and ignited.
The ignitor circuit remains energized for 30 seconds.
Fault Detection Override
Holding down the START button will override the fault detection circuitry, preventing shut down for
2 minutes. This allows purging the fuel lines on new units, and units that have been completely
drained. It also facilitates troubleshooting.
Actions during Operation (After warm up is complete). Adjust the burner control (13), as necessary.
Automatic Shutdown. The following are the usual causes of an automatic shutdown:
Loss of combustion. If the flame sensor (26) fails to verify combustion, the MBU shuts down immediately
and the red SERVICE indicator (27) lights.
Low fuel. When the fuel tank level float switch (28) reaches a minimum set point, a 30 minute shutdown
timer is activated. Control Panel indicators will light as follows: LOW FUEL indicator (29) blinks green for
15 minutes, changes to blinking amber for 7.5 minutes, and then changes to blinking red for 7.5 minutes.
The indicator (29) lights steady red at shut-down.
Voltage. If input voltage increases to 29.5 V DC or greater or decreases to 22 V DC or less, the MBU
will shut down and the VOLTAGE indicator (30) will light red.