Burn Cycle
The operational cycle of a burner from startup to shutdown.
This document describes the operational behavior. For the serial protocol commands and telemetry used to control and monitor these operations, see Fusain Protocol.
Ignition
When the user requests heat (via STATE_COMMAND with mode=HEAT),
the system begins the ignition sequence. The appliance transitions through
PREHEAT and PREHEAT_STAGE_2 states (reported in STATE_DATA).
The motor starts first, spinning up to the ignition RPM (monitored via MOTOR_DATA). This serves two purposes: airflow prevents the glow plug from overheating, and verifies the motor is functioning before committing to ignition. Most air heaters use a single motor that drives both the combustion intake fan and heat exchanger fan on a common shaft. The system waits for the motor to stabilize within 5% of target RPM. If the motor fails to reach this speed, it indicates a hardware failure requiring maintenance.
Once airflow is established, the glow plug lights (status reported via GLOW_DATA). The glow plug heats a metal mesh inside the combustion chamber. This mesh will vaporize and ignite the fuel when it arrives.
After the glow plug warmup period, the fuel pump begins pulsing at the ignition pump rate (events reported via PUMP_DATA). This rich mixture improves ignition reliability. Fuel travels through the lines, reaches the combustion chamber, contacts the hot mesh, vaporizes, and ignites.
The system has a configured timeout to reach the combustion establishing temperature (monitored via TEMPERATURE_DATA). This generous window allows time for the fuel pump to prime empty lines on a cold start, which can take a minute or more. If the temperature is not reached within this timeout, ignition has failed—typically due to an empty tank, air in the fuel lines, a blocked filter, or a faulty glow plug. The appliance reports error code IGNITION_FAIL in STATE_DATA.
Preheating
When the heat exchanger reaches the combustion establishing temperature, combustion is starting but not yet stable (PREHEAT_STAGE_2 state in STATE_DATA). The system adjusts the fuel/air mixture to optimize the burn.
Motor RPM increases to the preheating RPM, providing more airflow for complete combustion. The pump pulse rate decreases to the preheating rate, creating a leaner mixture that produces less smoke and unburned fuel. The richer mixture during ignition ensures the flame catches; the leaner mixture now ensures it burns cleanly.
Temperature continues rising. When the heat exchanger reaches the combustion stable temperature, combustion is self-sustaining. The glow plug extinguishes—it is no longer needed to maintain the flame.
Heating
With stable combustion established, the system enters normal operation (HEATING state in STATE_DATA).
The user controls heat output by setting the pump pulse rate (via the
argument field of STATE_COMMAND with mode=HEAT, or directly
via PUMP_COMMAND). More fuel means more combustion, which requires
higher fan speed to maintain the target temperature. The faster fan pushes more
hot air into the heated space. A PID controller (configured via
TEMPERATURE_CONFIG) automatically adjusts motor RPM to maintain a
constant heat exchanger temperature regardless of the pump rate.
The PID uses inverted control because the motor drives the heat exchanger fan. More airflow over the heat exchanger removes more heat, lowering its temperature. So when temperature rises above the target, motor RPM increases to push more air and bring temperature back down. When temperature drops below the target, motor RPM decreases to reduce cooling.
Motor RPM range is learned dynamically based on the heater’s characteristics.
If the PID cannot maintain temperature, additional protection engages. At the pump reduction threshold, the fuel pump rate is reduced by 50% until temperature drops back to the pump recovery threshold. At the pump disable threshold, the fuel pump is disabled entirely until temperature drops below that threshold. These measures reduce heat input when the fan alone cannot keep up.
The system monitors for flame-out. If temperature drops below the combustion establishing threshold during normal heating, combustion has failed and the system transitions to an error state (error code FLAME_OUT in STATE_DATA).
The heater continues in this state until the user requests shutdown.
Cooldown
When the user requests shutdown (via STATE_COMMAND with mode=IDLE),
the fuel pump stops immediately but the motor keeps running (COOLING state in
STATE_DATA). The combustion chamber is hot and contains residual fuel
that must be handled safely.
While the heat exchanger is above the cooldown glow start temperature, the motor runs at the cooldown RPM to dissipate heat. The glow plug remains off during this phase.
Between the cooldown glow start and cooldown complete temperatures, the glow plug lights again. This burns off residual fuel and carbon deposits in the combustion chamber. Without this step, unburned fuel would form carbon deposits as the chamber cools, eventually clogging the system.
Once the heat exchanger drops below the cooldown complete temperature, the glow plug extinguishes. The motor continues running for the fan cooldown period to fully cool the system before stopping. After this timer completes, the system returns to Idle.
The heater should remain powered until the glow plug extinguishes. For safety, wait for the full fan cooldown to complete before moving or unplugging the heater.
Note
If the heater is being automatically controlled, its power source or relay should not be disconnected until the glow plug extinguishes.
This cooldown sequence runs whenever the heater has reached operating temperature, even after an error condition.
Fault Conditions
Two abnormal conditions can interrupt the burn cycle.
Ignition Failure
If the heat exchanger does not reach the combustion establishing temperature within the ignition timeout, ignition has failed. If the temperature rose above the cooldown complete threshold, the system performs the cooldown sequence before entering the error state. The user should check the fuel supply, prime the lines if necessary, and clear the fault before attempting another start.
Flame-Out
If temperature drops below the combustion establishing threshold during normal heating, combustion has been lost. The system performs the normal cooldown sequence (if temperature is above the cooldown complete threshold) before entering the error state.