Furnace Delayed Ignition: Why It Booms on Startup and What to Check

Delayed ignition is gas and air collecting in the burner area before it lights. When it finally catches, it burns all at once: a bang, a whoomp, or flame rolling out of the burner opening. Goodman's service instructions describe exactly that. When the mixture ignites late it can explode or roll out and burn in the burner venturi.

Mild cases sound like a soft "poof." Bad ones shake the cabinet. Either way, it's a safety problem, not a noise complaint. Repeated delayed ignition can trip the rollout switch, scorch wiring and stress the heat exchanger.

Safety: If the boom is loud or flame is rolling out, shut the furnace off and leave it off until it's repaired. Leak-test any gas connection you disturb.

Watch it light first

Pull the burner door (with the door switch held closed) and watch a few starts.

• Which burner lights first, and how fast does flame travel across? Flame should carry burner to burner within a second or two. A gap where it pauses points at a blocked or misaligned carryover.
• Does the igniter glow (or spark) well before the gas valve opens? On hot surface systems, a weak igniter may only light gas after it has built up.
• Do the flames look right once lit? Lifting, lazy or yellow flames point at pressure, orifice or air problems.

What to check, in order

Goodman's service manual (S-312, Checking for Delayed Ignition) lists four checks. They match what most furnace manufacturers say.

York's troubleshooting chart for its intermittent pilot furnaces adds the pilot side. Make sure the pilot is aligned with the carryover, and the carryovers on adjoining burners are fastened and level with each other.

On hot surface ignition furnaces, add the igniter

• Position: the igniter tip should sit where the manufacturer specifies at the first burner's gas stream. A bracket that has shifted leaves it lighting the edge of the gas instead of the stream.
• Condition: a cracked or weakened igniter may glow but not hot enough until gas has collected. See hot surface igniter testing.
• Voltage: low voltage to the igniter (low line voltage or a poor neutral) weakens it.

Other causes worth a look

• Burners that are rusty or dirty inside, especially on furnaces that sat through a humid summer or near a water heater or dryer vent. Pull and brush them.
• Wrong orifices after a propane or high-altitude conversion.
• Vent and combustion air problems that change how the flame behaves. See furnace pressure switch troubleshooting.
• A gas valve that opens slowly or overfires at startup on the first stage.

After the fix

1. Check the rollout switch. If it tripped, find out why before resetting. See furnace rollout switch tripped.
2. Look for scorching at the burner opening and on wiring near it.
3. Inspect the heat exchanger where you can see it. Repeated booms can crack it.
4. Watch several complete starts, cold and warm. Delayed ignition is often worst on the first start after the furnace has sat.
5. Check combustion with an analyzer if you have one.

Common mistakes

• Treating it as a noise problem. It's unburned gas igniting late.
• Drilling out burner or carryover ports. Clean them; don't change them.
• Turning down the gas to quiet it. Low manifold pressure is a cause of delayed ignition, not a fix.
• Only cleaning the burner the igniter lights. The delay is often at the far end, where flame has to cross.
• Putting the burners back crooked. They have to sit in their slots and line up with the orifices.

FAQ

Sources

• Goodman AMH8 service instructions, S-312 Checking for Delayed Ignition
• Amana AMVM97 service manual, S-312 Checking for Delayed Ignition
• York D1CE installation instructions, troubleshooting (delayed ignition and carryover alignment)