Delayed ignition check

What is going on

Delayed ignition is when the gas-air mixture sits in the chamber a moment too long before it lights. When it finally catches, it can go off with a bang or roll out and burn in the burner venturi. A properly built furnace is not likely to do this unless someone has bypassed controls, so if you hear a thump at startup, look for what has changed or gone out of shape. It is something you want fixed right away, since a rolling flame can damage parts and is a hazard.

Likely causes

• Improper gas pressure
• Burners not seated in locating slots or not level
• Carry-over (lighter tube or cross lighter) obstructed
• Main burner orifice deformed or out of alignment

Check in this order

1. Check and adjust gas pressure to rating plate values. Gas work belongs to a licensed gas technician
2. Confirm burners sit in the locating slots and are level front to rear and left to right
3. Clean the carry-over (lighter tube or cross lighter) so flame can travel burner to burner
4. Replace any deformed or misaligned main burner orifice
5. Watch a full ignition after repairs and look for flame lighting quickly and evenly across all burners
6. If anything was bypassed in the controls, restore it to factory configuration

More furnace guides

• Airflow lower than demanded, furnace runs on low stage when high stage is expected
• Alternate combustion air opening (upflow or horizontal)
• Alternate vent/flue location (upflow or horizontal left)
• Annual inspection checklist
• Auxiliary limit control (location and purpose)
• Auxiliary limit switch open (blower compartment)