Theory Of Operation - PHP Heat Wagon LVF90A Service Manual

High Pressure Theory of Operation
When the thermostat is turned up power is sent to the flame/safety control
and to the fan switch. This brings power to the motor relay and the trans-
former. The transformer raises the voltage to a high level and sends the high
voltage to a spark plug or electrode(s). When the coil in the motor relay is
energized, a magnetic pull draws in the contacts in the motor relay and allows
power to reach the motor.
When the motor reaches full speed, a centrifugal switch inside the motor
allows power to open the solenoid valve located in the fuel line before the
nozzle. The motor turns a fan blade at the front and drives a fuel pump
attached to the rear of the motor. The pump draws fuel up from the tank thru
an inlet port on the pump. A fuel filter is located in the draw line or inside
the pump itself. The pump pressurizes the fuel and sends fuel to the nozzle.
Another port returns excess fuel back to the tank. The pump has a pressure
adjustment port with a screw in it to adjust the pump pressure. The pump
also has a port that allows a pressure gauge to be attached to the pump.
With spark and fuel spray the fuel ignites. A photocell detects the presence of
flame in the combustion chamber. If flame is not detected within approxi-
mately 45 seconds, the flame/safety control trips and power to the motor and
transformer is removed. When flame is established and the photocell senses
the flame, the heater runs and heats the contacts in the fan switch so that
power for the motor relay is now obtained from the power cord and not from
the flame/safety control.
When the desired temperature is reached, the thermostat cuts power to the
flame/safety control. This causes the transformer and solenoid valve to lose
power, thereby cutting off the spark and fuel spray. The heater is now running
the cool down phase. The motor will continue to run the fan blade to cool
down the combustion chamber. The pump is still driven by the motor but the
fuel blocked by the closed solenoid is returned to the tank by the pump's
return line. When the fan cools the combustion chamber enough, the fan
switch contacts also cool until power through the fan switch to the motor
relay is cut and the motor stops. When the surrounding temperature drops
enough, the thermostat control will send power to the flame/safety control
and the cycle will begin again.
Always allow the cool down phase to run before turning off power to the
heater. To start the cool down phase when the heater is burning simply turn
the thermostat control down to its lowest setting.
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