Burner and Ignition System Setup and Troubleshooting
Setup
- Set manifold pressure with the burner firing and steady.
- Clock the meter: Btu/h = ft3 timed x 3,600 / seconds x heating value.
- Primary air shutters: yellow tipping means too little primary air; a lifting, blowing flame means too much.
- Verify inlet pressure under load, with other appliances running.
Ignition and proving
- Standing pilot, intermittent pilot, hot-surface ignition, and direct spark each follow a sequence: call for heat, inducer, pressure switch proved, ignition, gas valve opens, flame proved, blower.
- Flame rectification needs flame touching the sensor, a clean sensor, and a good ground. Typical flame current is 1-6 microamps; below roughly 0.5-1.0 microamp the module drops out.
- Thermocouples generate about 25-30 mV open-circuit and 10-15 mV under load.
- Clean flame sensors with steel wool, never sandpaper.
Worked example
An 80,000 Btu/h furnace clocks 1.0 ft3 in 45 seconds at 1,000 Btu/ft3: 1 x (3,600 / 45) x 1,000 = 80,000 Btu/h, correct. At 36 seconds the same meter test gives 100,000 Btu/h, 25% overfired, pointing to an oversized orifice or excessive manifold pressure rather than an ignition fault.
Traps
- Adjusting manifold pressure with the burner off.
- Jumping a pressure switch to "test" it.
- Replacing a gas valve before checking flame current and ground.
- Assuming a glowing igniter is a good igniter; check resistance and current draw.
- Ignoring the inducer and pressure-switch sequence when the fault looks like ignition.