Follow a burner control through prechecks, air proving, ignition and lockout, and distinguish a detectable flame signal from its recommended working margin.
Read the sequence for the actual appliance
An electronic burner control manages checks, ignition and flame supervision in a defined order. The SIT 571 DBC family provides an example, with versions for natural-draught and fan-assisted appliances. It supports different waiting and purge times, ignition safety times, attempt counts and lockout arrangements.
A family label does not identify one universal programme. Use the full appliance and control reference to obtain the correct sequence. Do not assume that a timing, reset method or number of retries from one version applies to another.
Air proving checks both states
In the fan-assisted 571 DBC sequence, the controller first expects the air-pressure switch in its rest or no-flow state. After the fan is powered, it expects the switch to move to its flow state.
Checking the starting state helps expose a switch stuck in the proved position. Seeing the fan turn is also insufficient: if the switch never supplies the expected changed state, the firing sequence must wait.
Investigate the actual fan, air/flue route, pressure tubes, switch and wiring by the appliance procedure. A proving signal does not replace the other flue and combustion checks, and a bridge across the switch removes the intended protection. Other appliances may use a different proving method.
Waiting, purging and ignition
During the waiting or purge stage, the controller checks its circuitry and looks for an unwanted flame-presence signal. At this point the burner has not yet been instructed to light. A reported flame therefore conflicts with the expected state and needs investigation; the signal alone does not establish which component is at fault.
After the preliminary checks and prescribed waiting or purge period, the igniter and gas valve are energised. This starts the ignition safety time: the controlled opportunity to light and establish the required flame signal.
Keep the periods distinct:
- Waiting or purging takes place before the firing stage.
- The ignition safety time accompanies the attempt to establish flame.
- An interpurge separates unsuccessful attempts where the configured sequence provides retries.
Do not extend an ignition period or use repeated resets to work around a fault.
A proved flame changes the outputs
Once the required flame is detected, the 571 DBC stops the high-voltage spark and maintains the gas-valve supply for the heat demand. Sparking is for ignition; its absence during normal firing is expected.
If the permitted attempts fail, the sequence reaches lockout with the gas valve shut. Retry counts and reset behaviour are version-dependent. Investigate the failed ignition and use the appliance’s reset procedure.
Read the right quantity and scale
An ionisation flame-sensing circuit is checked by its small current, normally expressed in microamps (µA). A thermocouple is a heat-powered generator whose specified output checks commonly use millivolts (mV). Current and voltage are different quantities: the test function and connections must match the actual circuit.
For the 571 DBC data, the minimum flame current is 0.9 µA and the recommended signal is greater than three times that minimum. Multiplying 0.9 by 3 gives 2.7, so the recommendation is above 2.7 µA.
A reading of 1.2 µA exceeds the minimum detection figure but does not achieve the recommended margin. A reading of 3.2 µA exceeds that margin. A signal just large enough to register is not the same as a comfortably adequate working signal.
Those numbers belong to the stated control data. Use the exact appliance’s measurement method and acceptance criteria; do not turn them into a universal limit for every boiler or flame detector.